AAT2505 ANALOGICTECH | Alldatasheet
Document overview
- Manufacturer or author: janekim
- PDF pages: 26
Technical content
Features
- V IN Range: 2.7V to 5.5V
- 300mA LDO — 400mV Dropout Voltage at 300mA — High Accuracy: ±1.5% — Fast Line / Load Transient Response — Power OK Output
- 600mA Step-Down Converter — Up To 98% Efficiency — 27µA No Load Quiescent Current — Shutdown Current <1µA — Low R DS(ON) Integrated Power Switches — Fast Turn-On Time (150µs Typical) — Low Dropout 100% Duty Cycle — 1.4MHz Switching Frequency — Internal Soft Start
- Over-Temperature and Current Limit Protection
- TDFN33-12 Package
- -40°C to +85°C Temperature Range
Applications
- Cellular Phones
- Digital Cameras
- Handheld Instruments
- Microprocessor/DSP Core/IO Power
- PDAs and Handheld Computers
- Portable Media Players AAT2505 Dual Channel, Step-Down Converter/Linear Regulator Typical Application Efficiency (VOUT = 2.5V; L = 6.8µH) Output Current (mA) Efficiency (%) 100 0.1 1 10 100 1000 VIN = 3.3V 6.8µH 4.7µF 2.2µF 10µFC3 R3 100kΩ PGND 1 LX 2 VP3 VCC 4 ENLDO9 EN 10 FB 11 SGND 12 VLDO5 OUT6 GND8 POK7 AAT2505 L1 Sumida CDRH3D16-4R7 C1 Murata GRM219R61A475KE19 C3 Murata GRM21BR60J106KE19 VIN = 2.7V to 5.5V 3.3V at 300mA 2.5V at 600mA 2505.2006.06.1.1 1 SystemPower™
(Top View) PGND LX VP VCC VLDO OUT SGND FB EN ENLDO GND POK Pin # Symbol Function 1 PGND Step-down converter power ground return pin. Connect to the output and input capacitor return. See section on PCB layout guidelines and evaluation board layout diagram. 2 LX Power switching node. Output switching node that connects to the output inductor. 3 VP Step-down converter power stage supply voltage. Must be closely decoupled to PGND. 4 VCC Step-down converter bias supply. Connect to VP. 5 VLDO LDO input voltage; should be decoupled with 1µF or greater capacitor. 6 OUT 300mA LDO output pin. A 2.2µF or greater output low-ESR ceramic capacitor is required for stability. 7 POK Power-OK output for the LDO. This open drain output is low when the OUT is out of regu- lation. Connect a pull-up resistor from POK to OUT or VLDO. When LDO is in shutdown (ENLDO = 0V), POK is pulled low. 8 GND LDO ground connection pin. 9 ENLDO Enable pin for LDO. When connected low, LDO is disabled and consumes less than 1µA of current. 10 EN Step-down converter enable. When connected low, the step-down converter is disabled and consumes less than 1µA. 11 FB Step-down converter feedback input pin. For fixed output voltage versions, this pin is con- nected to the converter output, forcing the converter to regulate to the specific voltage. For adjustable output versions, an external resistive divider ties to this point and programs the output voltage to the desired value. 12 SGND Step-down converter signal ground. For external feedback, return the feedback resistive divider to this ground. For internal fixed version, tie to the point of load return. See sec- tion on PCB layout guidelines and evaluation board layout diagram. EP Exposed paddle (bottom). Use properly sized vias for thermal coupling to the ground plane. See section on PCB layout guidelines. AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2 2505.2006.06.1.1
Symbol Description Value Units PD Maximum Power Dissipation 2.0 W θJA Thermal Resistance2 50 °C/W Symbol Description Value Units VP, VLDO Input Voltages to GND 6.0 V VLX Lx to GND -0.3 to V P + 0.3 V VFB FB to GND -0.3 to V P + 0.3 V VEN EN to GND -0.3 to 6.0 V TJ Operating Junction Temperature Range -40 to 150 °C TLEAD Maximum Soldering Temperature (at leads, 10 sec) 300 °C AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 3 1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at condi- tions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Mounted on an FR4 board with exposed paddle connected to ground plane.
Electrical Characteristics1 COUT = 2.2µF, CIN = 1µF, TA = -40°C to +85°C, unless otherwise noted. Typical values are T A = 25°C. Symbol Description Conditions Min Typ Max Units LDO VOUT Output Voltage Tolerance IOUT = 1mA TA = 25°C -1.5 1.5 %to 300mA T A = -40°C to +85°C -2.5 2.5 VIN Input Voltage VOUT + 5.5 VVDO VDO Dropout Voltage3, 4 IOUT = 300mA 400 600 mV ΔVOUT/ Line Regulation V IN = VOUT + 1V to 5V 0.09 %/VVOUT* VIN ΔVOUT(Line) Dynamic Line Regulation IOUT = 300mA, VIN = VOUT + 1V 2.5 mVto VOUT + 2V, TR/TF = 2µs ΔVOUT(Load) Dynamic Load Regulation I OUT = 1mA to 300mA, TR <5µs 60 mV IOUT Output Current V OUT > 1.3V 300 mA ISC Short-Circuit Current V OUT < 0.4V 1 A IQLDO LDO Quiescent Current V IN = 5V, No Load, ENLDO = VIN 70 125 µA VPOK POK Trip Threshold V OUT Rising, TA = 25°C 90 94 98 % of V OUT VPOKHYS POK Hysteresis 1.0 % of V OUT VPOK(OL) POK Output Voltage Low I SINK = 1mA 0.4 V IPOK POK Output Leakage Current V POK < 5.5V, VOUT in Regulation 1.0 µA ISHDN Shutdown Current VIN = 5V; ENLDO = GND, 1.0 µAEN = SGND = PGND IOUT = 10mA 1kHz 65 PSRR Power Supply Rejection Ratio 10kHz 45 dB 1MHz 42 TSD Over-Temperature Shutdown 145 °CThreshold THYS Over-Temperature Shutdown 15 °CHysteresis eN Output Noise 250 µV RMS TC Output Voltage Temperature 22 ppm/°CCoefficient AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 4 2505.2006.06.1.1 1. The AAT2505 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assu red by design, characterization, and correlation with statistical process controls. 2. To calculate the minimum LDO input voltage, use the following equation: V IN(MIN) = VOUT(MAX) + VDO(MAX), as long as VIN ≥ 2.5V. 3. For VOUT < 2.1V, VDO = 2.5 - VOUT. 4. VDO is defined as VIN - VOUT when VOUT is 98% of nominal.
Electrical Characteristics1 IOUT = 600mA; typical values are TA = 25°C, VIN = VCC = VP = 3.6V. Symbol Description Conditions Min Typ Max Units Buck Converter VIN Input Voltage 2.7 5.5 V VIN Rising 2.7 V VUVLO UVLO Threshold Hysteresis 100 mV VIN Falling 1.8 V VOUT Output Voltage Tolerance IOUT = 0 to 600mA, -3.0 3.0 %VIN = 2.7V to 5.5V VOUT Output Voltage Range 0.6 V IN V IQBUCK Step-Down Converter ENLDO = GND, No Load, 27 70 µAQuiescent Current 0.6V Adjustable Version ISHDN Shutdown Current EN = SGND = PGND, 1.0 µAENLDO = GND ILIM P-Channel Current Limit 800 mA RDS(ON)H High Side Switch On Resistance 0.45 Ω RDS(ON)L Low Side Switch On Resistance 0.40 Ω ILXLK LX Leakage Current VIN = 5.5V, VLX = 0 to VIN, 1.0 µAEN = SGND = PGND ΔVLinereg Line Regulation V IN = 2.7V to 5.5V 0.1 %/V VFB FB Threshold Voltage Accuracy 0.6V Output, No Load, 591 600 609 mVTA = 25°C IFB FB Leakage Current 0.6V Output 0.2 µA RFB FB Impedance > 0.6V Output 250 k Ω TS Start-Up Time From Enable to Output 150 µsRegulation FOSC Oscillator Frequency T A = 25°C 1.0 1.4 2.0 MHz TSD Over-Temperature Shutdown 140 °CThreshold THYS Over-Temperature Shutdown 15 °CHysteresis Logic Signals VEN(L) Enable Threshold Low 0.6 V VEN(H) Enable Threshold High 1.4 V IEN(H) Leakage Current -1.0 1.0 µA AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 5 1. The AAT2505 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assu red by design, characterization, and correlation with statistical process controls.
Unless otherwise noted, VIN = 5V, TA = 25°C. LDO Initial Power-Up Response Time (EN = GND; ENLDO = VIN) Time (400µs/div) VENLDO (5V/div) VOUT (1V/div) LDO Output Voltage vs. Temperature (EN = GND; ENLDO = VIN) Temperature (°°C) Output Voltage Variation (%)-0.45 -0.40 -0.35 -0.30-0.25 -0.20 -0.15-0.10-0.05 0.00 0.05 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 LDO Ground Current vs. Input Voltage (EN = GND; ENLDO = VIN) 0.00 10.00 20.00 30.0040.00 50.00 60.0070.00 80.00 90.00 2 2.5 3 3.5 4.5 45 Input Voltage (V) Ground Current (μA) IOUT=0mA IOUT=10mA IOUT=50mA IOUT=150mA IOUT=300mA LDO Dropout Voltage vs. Output Current (EN = GND; ENLDO = VIN) 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 Output Current (mA) Dropout Voltage (mV) 85°C 25°C -40°C LDO Dropout Characteristics (EN = GND; ENLDO = VIN) 2.00 2.20 2.40 2.602.80 3.00 3.20 Input Voltage (V) Output Voltage (V) IOUT = 300mA IOUT = 150mA IOUT = 100mA IOUT = 50mAIOUT = 10mA IOUT = 0mA LDO Dropout Voltage vs. Temperature (EN = GND; ENLDO = VIN) 120 180 240 300 360420 480 540 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120Temperature (°C) Dropout Voltage (mV) IL = 300mA IL = 150mA IL = 100mA IL = 50mA AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 6 2505.2006.06.1.1
Unless otherwise noted, VIN = 5V, TA = 25°C. LDO Load Transient Response 300mA (EN = GND; ENLDO = VIN) 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 2.90 3.00 Time (10µs/div) Output Voltage (V) -100 100 200 300 400 500600 700 800 Output Current (mA) VOUT IOUT LDO Load Transient Response (EN = GND; ENLDO = VIN) 2.60 2.65 2.702.75 2.80 2.852.90 Time (100µs/div) Output Voltage (V) -100 100 200 300 400500 Output Current (mA) VOUT IOUT LDO Line Transient Response (EN = GND; ENLDO = VIN) 2.98 2.99 3.00 3.01 3.023.033.04 Time (100µs/div) Input Voltage (V) 456 Output Voltage (V) VIN VOUT LDO Turn-On Time From Enable (VIN present) (EN = GND; ENLDO = VIN) Time (5µs/div) VIN = 4VVOUT = 1V/div VENLDO = 5V/div LDO Turn-Off Response Time (EN = GND; ENLDO = VIN) Time (50µs/div) VENLDO (5V/div) VOUT (1V/div) AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 7
Unless otherwise noted, VIN = 5V, TA = 25°C. Step-Down Converter DC Regulation (VOUT = 2.5V; L = 6.8µH; ENLDO = GND) Output Current (mA) Output Error (%) -1.0 -0.5 0.0 0.51.0 0.1 1 10 100 1000 VIN = 4.2V VIN = 3.6V VIN = 3.0V VIN = 5.0V Step-Down Converter Efficiency vs. Load (VOUT = 2.5V; L = 10μμH; ENLDO = GND) Output Current (mA) Efficiency (%) 100 0.1 1 10 100 1000 VIN = 3.0V VIN = 3.6V VIN = 3.3V Step-Down Converter DC Regulation (VOUT = 3.3V; L = 6.8µH; ENLDO = GND) Output Current (mA) Output Error (%) -1.0 -0.5 0.0 0.5 1.0 0.1 1 10 100 1000 VIN = 5.0V VIN = 4.2V Step-Down Converter Efficiency vs. Load (VOUT = 3.3V; L = 10μμH; ENLDO = GND) Output Current (mA) Efficiency (%) 100 0.1 1 10 100 1000 VIN = 3.9V VIN = 4.2V LDO ENLDO vs. VIN 1.050 1.075 1.100 1.125 1.150 1.175 1.200 1.2251.250 VIH VIL LDO Over-Current Protection (EN = GND; ENLDO = VIN) Time (50ms/div) Output Current (mA) -200 200 400 600800 10001200 AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 8 2505.2006.06.1.1
Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 9 Typical Characteristics Unless otherwise noted, VIN = 5V, TA = 25°C. Step-Down Converter Input Current vs. Input Voltage (VO = 1.8V; EN = VIN; ENLDO = GND) Input Voltage (V) Input Current (μμA) 85°C 25°C -40°C Step-Down Converter Switching Frequency vs. Temperature (VIN = 3.6V; VO = 1.5V; EN = VIN; ENLDO = GND) Temperature (°°C) Frequency Variation (%) -15.0 -12.0 -9.0 -6.0 -3.0 0.0 3.06.0 9.0 12.0 15.0 -40 -20 0 20 40 60 80 100 Step-Down Converter Output Voltage Error vs. Temperature (VIN = 3.6V; VO = 1.5V; EN = VIN; ENLDO = GND) Temperature (°°C) Output Error (%) -2.0 -1.0 0.01.02.0 -40 -20 0 20 40 60 80 100 Step-Down Converter Frequency vs. Input Voltage (VOUT = 1.8V; EN = VIN; ENLDO = GND) Input Voltage (V) Frequency Variation (%) -2.0 -1.5 -1.0 -0.5 0.0 0.51.0 Step-Down Converter DC Regulation (VOUT = 1.8V; L = 4.7µH; ENLDO = GND) Output Current (mA) Output Error (%) -1.0 -0.5 0.0 0.5 1.0 0.1 1 10 100 1000 VIN = 3.6V VIN = 4.2V VIN = 2.7V Step-Down Converter Efficiency vs. Load (VOUT = 1.5V; L = 4.7μμH; ENLDO = GND) Output Current (mA) Efficiency (%) 6070 100 0.1 1 10 100 1000 VIN = 3.6V VIN = 4.2V VIN = 2.7V
Dual Channel, Step-Down Converter/Linear Regulator 10 2505.2006.06.1.1 Typical Characteristics Unless otherwise noted, VIN = 5V, TA = 25°C. Step-Down Converter Line Regulation (VOUT = 1.8V) Input Voltage (V) Accuracy (%) -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 IOUT = 400mA IOUT = 1mA IOUT = 10mA Step-Down Converter Line Transient (VOUT = 1.8V @ 400mA) Output Voltage (top) (V) Input Voltage (bottom) (V) Time (25µs/div) 1.74 1.761.78 1.80 1.821.84 2.6 3.64.6 5.6 6.67.6 Step-Down Converter Load Transient Response (1mA to 300mA; VIN = 3.6V; VOUT = 2.5V; C1 = 4.7µF; ENLDO = GND) Output Voltage (top) (V) Load and Inductor Current (200mA/div) (bottom) Time (50µs/div) 0.9 1.1 1.3 1.5 1.7 1.9 2.12.32.5 2.7 2.9 -0.2 0.0 0.2 0.4 0.6 0.8 1.01.21.4 1.6 1.8 Step-Down Converter Load Transient Response (1mA to 300mA; VIN = 3.6V; VOUT = 1.8V; C1 = 4.7µF; ENLDO = GND) Output Voltage (top) (V) Load and Inductor Current (200mA/div) (bottom) Time (50µs/div) 1.0 1.1 1.2 1.31.4 1.5 1.6 1.7 1.8 1.9 2.0 -0.2 0.0 0.2 0.40.6 0.8 1.0 1.2 1.4 1.6 1.8 Step-Down Converter N-Channel RDS(ON) vs. Input Voltage (EN = VIN; ENLDO = GND) Input Voltage (V) RDS(ON) (mΩΩ) 300 350400 450 500550600 650 700750 25°C 120°C 100°C 85°C Step-Down Converter P-Channel RDS(ON) vs. Input Voltage (EN = VIN; ENLDO = GND) Input Voltage (V) RDS(ON) (mΩΩ) 300 350400450 500 550 600 650700750 25°C 120°C 100°C 85°C
Unless otherwise noted, VIN = 5V, TA = 25°C. Step-Down Converter Output Ripple (VIN = 3.6V; VOUT = 1.8V; 400mA; EN = VIN; ENLDO = GND) Output Voltage (AC Coupled) (top) (mV) Inductor Current (bottom) (A) Time (250ns/div) -120 -100 -80 -60 -40 -20 2040 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.9 Step-Down Converter Soft Start (VIN = 3.6V; VOUT = 1.8V; 400mA; EN = VIN; ENLDO = GND) Time (100μμs/div) Enable and Output Voltage (top) (V) Inductor Current (bottom) (A) -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.03.0 4.0 5.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.01.2 1.4 1.6 VEN IL VO AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 11
Dual Channel, Step-Down Converter/Linear Regulator 12 2505.2006.06.1.1 Functional Block Diagram Note: Internal resistor divider included for ≥ 1.2V versions. For low voltage versions, the feedback pin is tied directly to the error amplifier input. EN LX Error Amp. Logic DH DL PGND VP FB GND Voltage Reference Voltage Reference OUT Over-Current Protection Control Logic VLDO Fast Start ControlENLDO SGND VCC See Note Error Amp. 94% POK Functional Description The AAT2505 is a high performance power man- agement IC comprised of a buck converter and a linear regulator. The high efficiency buck convert- er is capable of delivering up to 600mA. Designed to operate at 1.4MHz, the converter requires only three external components (C IN, COUT, and LX) and is stable with a ceramic output capacitor. The lin- ear regulator delivers 300mA and also is stable with a ceramic output capacitor. Linear Regulator The advanced circuit design of the linear regulator has been specifically optimized for very fast start-up and shutdown timing. This proprietary CMOS LDO has also been tailored for superior transient response characteristics. These traits are particularly important for applications that require fast power supply timing. The high-speed turn-on capability is enabled through implementation of a fast-start control cir- cuit which accelerates the power-up behavior of fundamental control and feedback circuits within the LDO regulator. Fast turn-off time response is achieved by an active output pull-down circuit, which is enabled when the LDO regulator is placed in shutdown mode. This active fast shutdown cir- cuit has no adverse effect on normal device opera- tion. The LDO regulator output has been specifi- cally optimized to function with low-cost, low-ESR ceramic capacitors; however, the design will allow for operation over a wide range of capacitor types. Other features include an integrated Power-OK comparator which indicates when the output is out of regulation. The POK open-drain output is low when OUT is 6% below its nominal regulation voltage. The open-drain signal is held low when the linear regula- tor is in shutdown mode. The regulator comes with complete short-circuit and thermal protection. The combination of these two internal protection circuits gives a comprehensive safety system to guard against extreme adverse operating conditions.
The regulator features an enable/disable function. This pin (ENLDO) is active high and is compatible with CMOS logic. To assure the LDO regulator will switch on, the ENLDO turn-on control level must be greater than 1.5V. The LDO regulator will go into the disable shutdown mode when the voltage on the EN pin falls below 0.6V. If the enable function is not needed in a specific application, it may be tied to V IN to keep the LDO regulator in a continuously on state. When the regulator is in shutdown mode, an inter- nal 20kΩ resistor is connected between OUT and GND. This is intended to discharge COUT when the LDO regulator is disabled. The internal 20kΩ resis- tor has no adverse impact on device turn-on time. Step-Down Converter The AAT2505 buck is a constant frequency peak current mode PWM converter with internal compen- sation. It is designed to operate with an input voltage range of 2.7V to 5.5V. The output voltage ranges from 0.6V to the input voltage for the internally fixed version (see Figure 1) , and up to 3.3V for the exter- nally adjustable version (see Figure 2). The 0.6V fixed model is also the adjustable version and is externally programmable with a resistive divider. The converter MOSFET power stage is sized for 600mA load capability with up to 96% efficiency. Light load efficiency exceeds 80% at a 500µA load. Soft Start The AAT2505 soft-start control prevents output voltage overshoot and limits inrush current when either the input power or the enable input is applied. When pulled low, the enable input forces the converter into a low-power, non-switching state with a bias current of less than 1µA. A startup time of 150µs is achieved across the operating range. Low Dropout Operation For conditions where the input voltage drops to the output voltage level, the converter duty cycle increases to 100%. As 100% duty cycle is approached, the minimum off-time initially forces the high side on-time to exceed the 1.4MHz clock cycle and reduce the effective switching frequency. Once the input drops below the level where the out- put can be regulated, the high side P-channel MOSFET is turned on continuously for 100% duty cycle. At 100% duty cycle, the output voltage tracks the input voltage minus the IR drop of the high side P-channel MOSFET R DS(ON). Low Supply The under-voltage lockout (UVLO) guarantees suf- ficient V IN bias and proper operation of all internal circuitry prior to activation. Fault Protection For overload conditions, the peak inductor current is limited. Thermal protection disables switching when the internal dissipation or ambient temperature becomes excessive. The junction over-temperature threshold is 140°C with 15°C of hysteresis. AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 13 Figure 1: AAT2505 Fixed Output. Figure 2: AAT2505 with Adjustable Step-Down Output and Enhanced Transient Response. 4.7µF VIN 59k R24.7µF 4.7µH 10µF VOUTLDO VOUTBUCK 100pFC8PGND LX VP VCC ENLDO EN FB SGND VLDO OUT GND POK AAT2505U1 100kΩ 4.7µH 4.7µF VOUTBUCK VIN 4.7µF 10µFC3 VOUTLDO PGND LX VP VCC ENLDO EN FB SGND VLDO OUT GND POK AAT2505 100kΩ
Dual Channel, Step-Down Converter/Linear Regulator 14 2505.2006.06.1.1 Applications Information Linear Regulator Input and Output Capacitors: An input capacitor is not required for basic operation of the linear reg- ulator. However, if the AAT2505 is physically locat- ed more than three centimeters from an input power source, a C IN capacitor will be needed for stable operation. Typically, a 1µF or larger capaci- tor is recommended for C IN in most applications. CIN should be located as closely to the device V IN pin as practically possible. An input capacitor greater than 1µF will offer supe- rior input line transient response and maximize power supply ripple rejection. Ceramic, tantalum, or aluminum electrolytic capacitors may be select- ed for C IN. There is no specific capacitor ESR requirement for CIN. However, for 300mA LDO reg- ulator output operation, ceramic capacitors are rec- ommended for C IN due to their inherent capability over tantalum capacitors to withstand input current surges from low impedance sources such as bat- teries in portable devices. For proper load voltage regulation and operational stability, a capacitor is required between OUT and GND. The C OUT capacitor connection to the LDO regulator ground pin should be made as directly as practically possible for maximum device perform- ance. Since the regulator has been designed to function with very low ESR capacitors, ceramic capacitors in the 1.0µF to 10µF range are recom- mended for best performance. Applications utilizing the exceptionally low output noise and optimum power supply ripple rejection should use 2.2µF or greater for C OUT. In low output current applications, where output load is less than 10mA, the minimum value for C OUT can be as low as 0.47µF. Equivalent Series Resistance: ESR is a very important characteristic to consider when selecting a capacitor. ESR is the internal series resistance asso- ciated with a capacitor that includes lead resistance, internal connections, size and area, material compo- sition, and ambient temperature. Typically, capacitor ESR is measured in milliohms for ceramic capaci- tors and can range to more than several ohms for tantalum or aluminum electrolytic capacitors. Ceramic Capacitor Materials:Ceramic capacitors less than 0.1µF are typically made from NPO or C0G materials. NPO and C0G materials generally have tight tolerance and are very stable over tem- perature. Larger capacitor values are usually com- posed of X7R, X5R, Z5U, or Y5V dielectric materi- als. Large ceramic capacitors (i.e., greater than 2.2µF) are often available in low-cost Y5V and Z5U dielectrics. These two material types are not rec- ommended for use with the regulator, since the capacitor tolerance can vary more than ±50% over the operating temperature range of the device. A 2.2µF Y5V capacitor could be reduced to 1µF over temperature; this could cause problems for circuit operation. X7R and X5R dielectrics are much more desirable. The temperature tolerance of X7R dielec- tric is better than ±15%. Capacitor area is another contributor to ESR. Capacitors that are physically large in size will have a lower ESR when compared to a smaller sized capacitor of an equivalent material and capaci- tance value. These larger devices can improve cir- cuit transient response when compared to an equal value capacitor in a smaller package size. Consult capacitor vendor datasheets carefully when select- ing capacitors for LDO regulators. Step-Down Converter Inductor Selection: The step-down converter uses peak current mode control with slope com- pensation to maintain stability for duty cycles greater than 50%. The output inductor value must be selected so the inductor current down slope meets the internal slope compensation require- ments. The internal slope compensation for the adjustable and low-voltage fixed versions of the AAT2505 is 0.24A/µsec. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.5V output and 4.7µH inductor. 0.75 ⋅ VO m = = = 0.24 L 0.75 ⋅ 1.5V 4.7μH A μsec
This is the internal slope compensation for the adjustable (0.6V) version or low-voltage fixed ver- sions. When externally programming the 0.6V ver- sion to 2.5V, the calculated inductance is 7.5µH. In this case, a standard 6.8µH value is selected. For high-voltage fixed versions (2.5V and above), m = 0.48A/µsec. Table 1 displays inductor values for the AAT2505 fixed and adjustable options. Manufacturer's specifications list both the inductor DC current rating, which is a thermal limitation, and the peak current rating, which is determined by the saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. Some inductors may meet the peak and average current ratings yet result in excessive loss- es due to a high DCR. Always consider the losses associated with the DCR and its effect on the total converter efficiency when selecting an inductor. The 4.7µH CDRH3D16 series inductor selected from Sumida has a 105mΩ DCR and a 900mA DC current rating. At full load, the inductor DC loss is 17mW which gives a 2.8% loss in efficiency for a 400mA, 1.5V output. Input Capacitor Select a 4.7µF to 10µF X7R or X5R ceramic capac- itor for the input. To estimate the required input capacitor size, determine the acceptable input rip- ple level (V PP) and solve for C. The calculated value varies with input voltage and is a maximum when V IN is double the output voltage. Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the prop- er value. For example, the capacitance of a 10µF, 6.3V, X5R ceramic capacitor with 5.0V DC applied is actually about 6µF. The maximum input capacitor RMS current is: VOBUCK VIN VOBUCK VIN CIN(MIN) = 1 VPP IOBUCK ⎛⎞ · 1 - = for VIN = 2 × VOBUCK⎝⎠ VOBUCK VIN VOBUCK VIN VOBUCK VIN CIN = VOBUCK VIN VPP IOBUCK 0.75 ⋅ VO L = = ≈ 3 ⋅ VO = 3 ⋅ 2.5V = 7.5μH m 0.75 ⋅ VO 0.24A μsec A μsec A A μsec AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 15 Table 1: Inductor Values. Configuration Output Voltage Inductor 0.6V Adjustable With 1V, 1.2V 2.2µH External Feedback 1.5V, 1.8V 4.7µH 2.5V, 3.3V 6.8µH Fixed Output 0.6V to 3.3V 4.7µH
Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 17 The output voltage droop due to a load transient is dominated by the capacitance of the ceramic out- put capacitor. During a step increase in load cur- rent, the ceramic output capacitor alone supplies the load current until the loop responds. Within two or three switching cycles, the loop responds and the inductor current increases to match the load current demand. The relationship of the output volt- age droop during the three switching cycles to the output capacitance can be estimated by: Once the average inductor current increases to the DC load level, the output voltage recovers. The above equation establishes a limit on the minimum value for the output capacitor with respect to load transients. The internal voltage loop compensation limits the minimum output capacitor value to 4.7µF. This is due to its effect on the loop crossover frequency (bandwidth), phase margin, and gain margin. Increased output capacitance will reduce the crossover frequency with greater phase margin. The maximum output capacitor RMS ripple current is given by: Dissipation due to the RMS current in the ceramic output capacitor ESR is typically minimal, resulting in less than a few degrees rise in hot-spot temperature. Adjustable Output Resistor Selection For applications requiring an adjustable output volt- age, the 0.6V version can be externally pro- grammed. Resistors R1 and R2 of Figure 5 program the output to regulate at a voltage higher than 0.6V. To limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum suggested value for R2 is 59k Ω. Although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensitive to external noise and interference. Table 2 summarizes the resistor values for various output voltages with R2 set to either 59k Ω for good noise immunity or 221kΩ for reduced no load input current. The adjustable version of the AAT2505, combined with an external feedforward capacitor (C8 in Figures 2 and 5), delivers enhanced transient response for extreme pulsed load applications. The addition of the feedforward capacitor typically requires a larger output capacitor C1 for stability. Table 2: Adjustable Resistor Values For Use With 0.6V Step-Down Converter. R2 = 59kΩΩ R2 = 221kΩΩ VOUT (V) R1 (k ΩΩ) R1 (k ΩΩ) 0.8 19.6 75 0.9 29.4 113 1.0 39.2 150 1.1 49.9 187 1.2 59.0 221 1.3 68.1 261 1.4 78.7 301 1.5 88.7 332 1.8 118 442 1.85 124 464 2.0 137 523 2.5 187 715 3.3 267 1000 VOUT VREF 1.5V 0.6V VOUT · (VIN(MAX) - VOUT) RMS(MAX)I L · FS · VIN(MAX) COUT = 3 · ΔILOAD VDROOP · FS
Figure 5: AAT2505 Evaluation Board Schematic. Table 3 4.7µF1 10µF GND VIN1 Buck Enable LX1 GND Table 3 59k 100k R2PGND LX VP VCC ENLDO EN FB SGND IN OUT GND POK AAT2505 4.7µF C410µFC3 LDO Enable VOUTLDO VOUTBUCK LDO Input 0.01µFC7 C81 n/a POK AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 18 2505.2006.06.1.1 Thermal Calculations There are three types of losses associated with the AAT2505 step-down converter: switching losses, conduction losses, and quiescent current losses. Conduction losses are associated with the R DS(ON) characteristics of the power output switching devices. Switching losses are dominated by the gate charge of the power output switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of the step-down convert- er and LDO losses is given by: I QBUCK is the step-down converter quiescent cur- rent and I QLDO is the LDO quiescent current. The term tsw is used to estimate the full load step-down converter switching losses. For the condition where the buck converter is in dropout at 100% duty cycle, the total device dissi- pation reduces to: Since R DS(ON), quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input voltage range. PTOTAL = IOBUCK 2 · RDSON(H) + IOLDO · (VIN - VOLDO) + (IQBUCK + IQLDO) · VIN PTOTAL IOBUCK 2 · (RDSON(H) · VOBUCK + RDSON(L) · [VIN - VOBUCK]) VIN + (tsw · FS · IOBUCK + IQBUCK + IQLDO) · VIN + IOLDO · (VIN - VOLDO) 1. For step-down converter, enhanced transient configuration C8 = 100pF and C1 = 10µF.
Given the total losses, the maximum junction tem- perature can be derived from the θJA for the TDFN33-12 package which is 50°C/W. PCB Layout The following guidelines should be used to ensure a proper layout. 1. The input capacitor C2 should connect as closely as possible to VP and PGND, as shown in Figure 4. 2. The output capacitor and inductor should be connected as closely as possible. The connec- tion of the inductor to the LX pin should also be as short as possible. 3. The feedback trace should be separate from any power trace and connect as closely as possible to the load point. Sensing along a high-current load trace will degrade DC load regulation. If external feedback resistors are used, they should be placed as closely as pos- sible to the FB pin. This prevents noise from being coupled into the high impedance feed- back node. 4. The resistance of the trace from the load return to GND should be kept to a minimum. This will help to minimize any error in DC regulation due to differences in the potential of the internal sig- nal ground and the power ground. 5. For good thermal coupling, PCB vias are required from the pad for the TDFN paddle to the ground plane. The via diameter should be 0.3mm to 0.33mm and positioned on a 1.2mm grid. TJ(MAX) = PTOTAL · ΘJA + TAMB AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 19
Dual Channel, Step-Down Converter/Linear Regulator 20 2505.2006.06.1.1 Step-Down Converter Design Example Specifications VOBUCK = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ΔILOAD = 300mA VOLDO = 3.3V @ 300mA VIN = 2.7V to 4.2V (3.6V nominal) FS = 1.4MHz TAMB = 85°C 1.8V Buck Output Inductor (use 4.7µH; see Table 1) For Sumida inductor CDRH3D16, 4.7µH, DCR = 105m Ω. 1.8V Buck Output Capacitor VDROOP = 0.1V 4.7µH · 1.4MHz · 4.2V 23 RMSI L1 · FS · VIN(MAX) = · 3 · ΔILOAD VDROOP · FS 3 · 0.3A 0.1V · 1.4MHz COUT = = = 6.4 µF; use 10µF
- = 45mArms (VO) · (VIN(MAX) - VO) = Pesr = esr · IRMS 2 = 5mΩ · (45mA)2 = 10µW IPKL1 = IO + ΔIL1 = 0.4A + 0.068A = 0.468A2 PL1 = IO 2 ⋅ DCR = 0.4A2 ⋅ 105mΩ = 17mW L1 = 3 ⋅ VO2 = 3 ⋅ 1.8V = 5.4µHµsec A µsec A
1.8V Buck Input Capacitor Input Ripple VPP = 25mV AAT2505 Total Losses TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 395mW = 105°C PTOTAL + (tsw · FS · IOBUCK + IQBUCK + IQLDO) · VIN + (VIN - VOLDO) · IOLDO IOBUCK 2 · (RDSON(H) · VOBUCK + RDSON(L) · [VIN - VOBUCK]) VIN 4.2V IOBUCK RMSI P = esr · IRMS 2= = 0.2Arms CIN = = = 4.75 μF1 VPP IOBUCK 25mV 0.4A AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 21
Dual Channel, Step-Down Converter/Linear Regulator 22 2505.2006.06.1.1 Table 3: Evaluation Board Component Values. Table 4: Typical Surface Mount Inductors. Inductance Max DC DCR Size (mm) Manufacturer Part Number (µH) Current (A) ( ΩΩ) LxWxH Type VOUT (V) R1 (k ΩΩ) R1 (k ΩΩ) L1 (µH) Adjustable Version R2 = 59kΩΩ R2 = 221kΩΩ11 (0.6V device) 0.8 19.6 75.0 4.7 0.9 29.4 113 4.7 1.0 39.2 150 4.7 1.1 49.9 187 4.7 1.2 59.0 221 4.7 1.3 68.1 261 4.7 1.4 78.7 301 4.7 1.5 88.7 332 4.7 1.8 118 442 4.7 1.85 124 464 4.7 2.0 137 523 4.7 or 6.8 2.5 187 715 10 3.3 267 1000 10 VOUT (V) R1 (k ΩΩ) L1 (µH) Fixed Version R2 Not Used 0.6-3.3V 0 4.7 1. For reduced quiescent current R2 = 221k Ω.
Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 23 Table 5: Surface Mount Capacitors. Manufacturer Part Number Value Voltage Temp. Co. Case MuRata GRM219R61A475KE19 4.7µF 10V X5R 0805 MuRata GRM21BR60J106KE19 10µF 6.3V X5R 0805
Dual Channel, Step-Down Converter/Linear Regulator 24 2505.2006.06.1.1
Ordering Information
All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means Voltage Package Buck Converter LDO Marking1 Part Number (Tape and Reel)2 TDFN33-12 Adj. 2.8V POXYY AAT2505IWP-AQ-T1 TDFN33-12 Adj. 2.6V PPXYY AAT2505IWP-AO-T1 1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD. Legend Voltage Code Adjustable A(0.6V) 0.9 B 1.2 E 1.5 G 1.8 I 1.9 Y 2.5 N 2.6 O 2.7 P 2.8 Q 2.85 R 2.9 S 3.0 T 3.3 W 4.2 C
All dimensions in millimeters. Top View Bottom View Detail "B" Detail "A"Side View 3.00 ± 0.05 Index Area (D/2 x E/2) Detail "A" Detail "B" 1.70 ± 0.05 3.00 ± 0.05 0.05 ± 0.05 0.229 ± 0.051 7.5° ± 7.5° 2.40 ± 0.05 0.16 Pin 1 Indicator (optional) 0.075 ± 0.075
0.1 REF
0.8 + 0.05 -0.20 Option A: C0.30 (4x) max Chamfered corner Option B: R0.30 (4x) max Round corner AAT2505 Dual Channel, Step-Down Converter/Linear Regulator 2505.2006.06.1.1 25
Dual Channel, Step-Down Converter/Linear Regulator 26 2505.2006.06.1.1 Advanced Analogic Technologies, Inc. 830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 © Advanced Analogic Technologies, Inc. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold sub- ject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. AnalogicTech warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with AnalogicTech’s standard warranty. Testing and other quality con- trol techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and produ ct names appearing in this document are regis- tered trademarks or trademarks of their respective holders.