LT3685EMSE-PBF

LT3685 LINER | Alldatasheet

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FEATURES

APPLICATIONS

DESCRIPTION

1.2A, Step-Down Switching Regulator in 3mm × 3mm DFN The LT® 3505 is a current mode PWM step-down DC/DC converter with an internal 1.4A power switch. The wide operating input range of 3.6V to 36V (40V maximum) makes the LT3505 ideal for regulating power from a wide variety of sources, including unregulated wall transform- ers, 24V industrial supplies and automotive batteries. The oscillator can be programmed for high frequency operation allowing the use of tiny, low cost external components or it can be programmed for lower frequency operation to maximize effi ciency. Cycle-by-cycle current limit provides protection against shorted outputs and soft-start eliminates input current surge during start-up. The low current (<2µA) shutdown mode provides output disconnect, enabling easy power management in battery-powered systems. 750kHz, 3.3V Step-Down Converter , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. ! Wide Input Range: 3.6V to 36V Operating, 40V Maximum ! Up to 1.2A Output Current ! Resistor-Programmable Fixed-Frequency Operation from 200kHz to 3MHz ! Output Adjustable Down to 780mV ! Short-Circuit Robust ! Uses Tiny Capacitors and Inductors ! Soft-Start ! Low Shutdown Current: <2µA ! Low VCESAT Switch: 350mV at 1A ! Thermally Enhanced, Low Profi le 3mm x 3mm DFN-8 and MSOP-8 Packages ! Automotive Battery Regulation ! Industrial Control Supplies ! Wall Transformer Regulation ! Distributed Supply Regulation ! Battery-Powered Equipment VIN 4.2V TO 36V ON OFF 0.1µF 10µH 36.5k 10µF

3505 TA01

68pF1µF 75.0k 11.3k VOUT 3.3V 1.1A, VIN > 5V 1.2A, VIN > 8V 69.8k GND BOOST FB VC SW LT3505 VIN SHDN RT Effi ciency LOAD CURRENT (A) EFFICIENCY (%) VIN = 12V VOUT = 3.3V fSW = 750kHz L = 10 H

V ABSOLUTE MAXIMUM RATINGS (Note 1) TOP VIEW DD PACKAGE 8-LEAD (3mm × 3mm) PLASTIC DFN

1 BOOST

V IN SHDN VC FB RT GND TJMAX = 125°C, θJA = 43°C/W, θJC = 5°C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB BOOST 8 V C FB R T GND TOP VIEW MS8E PACKAGE 8-LEAD PLASTIC MSOP SW VIN SHDN TJMAX = 125°C, θJA = 40°C/W, θJC = 5°C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB PIN CONFIGURATION Operating Temperature Range (Note 2) ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT3505EDD#PBF LT3505EDD#TRPBF LCHB 8-Lead (3mm x 3mm) Plastic DFN –40°C to 85°C LT3505IDD#PBF LT3505IDD#TRPBF LCHC 8-Lead (3mm x 3mm) Plastic DFN –40°C to 125°C LT3505EMS8E#PBF LT3505EMS8E#TRPBF LTCNX 8-Lead Plastic MSOP –40°C to 85°C LT3505IMS8E#PBF LT3505IMS8E#TRPBF LTCNY 8-Lead Plastic MSOP –40°C to 125°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT3505EDD LT3505EDD#TR LCHB 8-Lead (3mm x 3mm) Plastic DFN –40°C to 85°C LT3505IDD LT3505IDD#TR LCHC 8-Lead (3mm x 3mm) Plastic DFN –40°C to 125°C LT3505EMS8E LT3505EMS8E#TR LTCNX 8-Lead Plastic MSOP –40°C to 85°C LT3505IMS8E LT3505IMS8E#TR LTCNY 8-Lead Plastic MSOP –40°C to 125°C Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/

Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3505E is guaranteed to meet performance specifi cations from 0°C to 85°C. Specifi cations over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3505I specifi cations are guaranteed over the –40°C to 125°C temperature range. PARAMETER CONDITIONS MIN TYP MAX UNITS VIN Operating Range 3.6 36 V Undervoltage Lockout 3.1 3.35 3.6 V Feedback Voltage " 765 780 795 mV FB Pin Bias Current VFB = Measured VREF (Note 4) " 55 150 nA Quiescent Current Not Switching, RT = 75.0k 2.0 2.7 mA Quiescent Current in Shutdown VSHDN = 0V 0.01 2 µA Reference Line Regulation VIN = 5V to 36V 0.007 %/V Switching Frequency VFB = 0.7V, RT = 13.7k VFB = 0.7V, RT = 75.0k VFB = 0.7V, RT = 357k 2.70 675 180 3.01 750 200 3.30 825 220 MHz kHz kHz Maximum Duty Cycle RT = 75.0k " 90 94 % Error Amp Transconductance VFB = 0.78V 200 µA/V Error Amp Voltage Gain VFB = 0.78V 400 V/V VC Source Current VFB = 0V, VC = 1.5V 10 µA VC Sink Current VFB = 1V, VC = 1.5V 14 µA VC Switching Threshold Voltage IOUT = 0mA 0.9 V VC Clamp Voltage VFB = 0V 1.7 V RT Bias Voltage VFB = 0.6V VFB = 0V, RT = 75.0k 0.5 V mV Switch Current Limit (Note 3) 1.4 1.75 2.2 A Switch VCESAT ISW = 1A 350 mV Switch Leakage Current 0.1 2 µA Minimum Boost Voltage Above Switch ISW = 1A 1.6 2.2 V BOOST Pin Current ISW = 1A 24 50 mA SHDN Input Voltage High 2.3 V SHDN Input Voltage Low 0.3 V SHDN Bias Current VSHDN = 2.3V (Note 5) VSHDN = 0V 0.01 0.1 µA µA ELECTRICAL CHARACTERISTICS The " denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C, VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2). Note 3: Current limit guaranteed by design and/or correlation to static test. Slope compensation reduces current limit at higher duty cycle. Note 4: Current fl ows out of pin. Note 5: Current fl ows into pin.

TYPICAL PERFORMANCE CHARACTERISTICS Effi ciency (VOUT = 5V, L = 10µH, fSW = 750kHz) Effi ciency (VOUT = 3.3V, L = 10µH, fSW = 750kHz) Effi ciency (VOUT = 3.3V, L = 4.7µH, fSW = 2.2MHz) LOAD CURRENT (A) EFFICIENCY (%) 0.8 1.0 0.2 0.4 0.6 1.2 VIN = 8V VIN = 12V VIN = 24V TA = 25° C LOAD CURRENT (A) EFFICIENCY (%) 0.8 1.0 0.2 0.4 0.6 1.2 VIN = 8V VIN = 12V VIN = 24V TA = 25° C LOAD CURRENT (A) EFFICIENCY (%) 0.8 1.0 0.2 0.4 0.6 1.2 VIN = 8V VIN = 12V TA = 25° C Effi ciency (VOUT = 5V, L = 4.7µH, fSW = 2.2MHz) LOAD CURRENT (A) EFFICIENCY (%) 0.8 1.0 0.2 0.4 0.6 1.2 VIN = 8V VIN = 12V TA = 25° C Max Load Current (VOUT = 3.3V, L = 6.8µH, fSW = 750kHz) INPUT VOLTAGE (V) OUTPUT CURRENT (A) 1.2 1.3 1.4

3505 G05

1.1 1.0 0.8 9 137 11 1715 0.9 1.7 1.6 1.5 TYPICAL MINIMUM TA = 25° C *10% DROPOUT Max Load Current (VOUT = 5V, fSW = 750kHz) INPUT VOLTAGE (V) 0.8 OUTPUT CURRENT (A) 0.9 1.1 1.2 1.3 1.7

3505 G06

1.0 1410 26188 241612 2820 30 1.4 1.5

1.6 TYPICAL, L = 22µH

TYPICAL, L = 10µH MINIMUM, L = 10µH TA = 25° C *10% DROPOUT Max Load Current (VOUT = 3.3V, L = 2.2µH, fSW = 2.2MHz) INPUT VOLTAGE (V) 0.8 OUTPUT CURRENT (A) 0.9 1.1 1.2 1.3 1.8 1.5 7 9 10

3505 G07

1.0 1.6 1.7 1.4 6 8 11 12 TYPICAL MINIMUM TA = 25° C *10% DROPOUT Max Load Current (VOUT = 5V, L = 3.3µH, fSW = 2.2MHz) INPUT VOLTAGE (V) 0.80 OUTPUT CURRENT (A) 0.90 1.10 1.20 1.30 1.80 1.50 9 11 12

3505 G08

1.00 1.60 1.70 1.40 8 10 14 16 18 TYPICAL MINIMUM TA = 25° C *10% DROPOUT Switch Voltage Drop SWITCH CURRENT (mA) VCE(SWITCH) (mV)300 400 500 1200

3505 G09

TA = –45 ° C TA = 85° C TA = 25° C

TYPICAL PERFORMANCE CHARACTERISTICS Undervoltage Lockout Switching Frequency Frequency Foldback, RT = 75.0k TEMPERATURE (° C) UVLO (V) 3.60 3.80 4.00

3505 G10

3.40 3.20 3.50 3.70 3.90 3.30 3.10 3.00 –25–50 250 75 100 12550 FB VOLTAGE (V) RT PIN BIAS VOLTAGE (V) 0.1 0.2 0.3 0.4 0.2 0.4 0.6 0.8

3505 G12

0.5 0.6 0.1 0.3 0.5 0.7 TA = 25° C Soft-Start SHDN Pin Current Typical Minimum Input Voltage, (VOUT = 5V, fSW = 750kHz) SHDN PIN VOLTAGE (V) SWITCH CURRENT LIMIT (A) 0.2 0.6 0.8 1.0 2.0 1.4 0.50 1 1.25 0.4 1.6 1.8 1.2 0.25 0.75 1.50 1.75 2 TA = 25° C VSHDN (V) ISHDN (µA) 30

3505 G14

TA = 25° C LOAD CURRENT (mA) 6.0 INPUT VOLTAGE (V) 6.2 6.4 6.6 6.8 10 100 1000

3505 G15

5.8 5.6 5.4 5.2 7.0 7.2 TO START TO RUN TA = 25° C Typical Minimum Input Voltage, (VOUT = 3.3V, fSW = 750kHz) LOAD CURRENT (mA) 4.3 INPUT VOLTAGE (V) 4.5 4.7 4.9 5.1 10 100 1000 4.1 3.9 3.7 3.5 5.3 5.5 TO START TA = 25° C TO RUN Typical Minimum Input Voltage, (VOUT = 3.3V, fSW = 2.2MHz) LOAD CURRENT (mA) 3.5 INPUT VOLTAGE (V) 5.0 5.5 10 100 1000 4.5 4.0 TO START TO RUN TA = 25° C Typical Minimum Input Voltage, (VOUT = 5V, fSW = 2.2MHz) LOAD CURRENT (mA) 5.0 INPUT VOLTAGE (V) 6.2 6.6 7.0 10 100 1000 5.8 5.4 6.0 6.4 6.8 5.6 5.2 TO START TO RUN TA = 25° C TEMPERATURE (° C)

0.60 SWITCHING FREQUENCY (MHz)

0.80 1.20 1.40 1.60 100 2.40 1.00 5025–50 –25 75 125 1.80 2.00

2.20 RT = 21k

RT = 30.1k RT = 75.0k

TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit Typical Minimum On Time RT Pin Bias Voltage Operating Waveforms Operating Waveforms, Discontinuous Mode TEMPERATURE (° C) –50 1.0 SWITCH CURRENT LIMIT (A) 1.1 1.3 1.4 1.5 2.0 1.7 0 25 100 125 1.2 1.8 1.9 1.6 –25 50 75 Switch Current Limit, RT = 75.0k TEMPERATURE (° C) –50 MINIMUM ON TIME (ns) 120 125 0 50 100–25 25 75 160 100 140 Switching Frequency Switching Frequency TEMPERATURE(° C) –50 BIAS VOLTAGE (mV) 495 500 505 25 75 125 490 485 480 –25 0 50 100 VFB = 0.78V RT PIN BIAS CURRENT (µA) SWITCHING FREQUENCY (MHz)0.5 1.5 2.0 2.5 20 35 3.0 1.0 10 305 2515 TA = 25° C RT PIN RESISTANCE (kΩ) 0.1 SWITCHING FREQUENCY (MHz) 100

3505 G24

TA = 25° C VSW 5V/DIV IL 0.5A/DIV VOUT 20mV/DIV VIN = 12V VOUT = 3.3V IOUT = 0.5A L = 10µH COUT = 10µF RT = 75.0k 1µs/DIV

3505 G18

0.5A/DIV VOUT 20mV/DIV 1µs/DIV

3505 F26

VIN = 12V VOUT = 3.3V IOUT = 50mA L = 10µH COUT = 10µF RT = 75.0k DUTY CYCLE (%) 1.2 SWITCH CURRENT LIMIT (A) 1.4 1.6 1.8 1.3 1.5 1.7 20 40 60 80

3505 G20

TA = 25° C

BOOST (Pin 1): The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pin 2): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. VIN (Pin 3): The VIN pin supplies current to the LT3505’s internal regulator and to the internal power switch. This pin must be locally bypassed. SHDN (Pin 4): The SHDN pin is used to put the LT3505 in shutdown mode. Tie to ground to shut down the LT3505. Tie to 2.3V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. SHDN also provides a soft-start function; see the Applications Infor- mation section. GND (Pin 5): Tie the GND pin to a local ground plane below the LT3505 and the circuit components. Return the feedback divider to this pin. RT (Pin 6): The RT pin is used to program the switching frequency of the LT3505 by connecting a resistor from this pin to ground. The Applications Information section of the data sheet includes a table to determine the resistance value based on the desired switching frequency. Minimize capacitance at this pin. FB (Pin 7): The LT3505 regulates its feedback pin to 780mV. Connect the feedback resistor divider tap to this pin. Set the output voltage by selecting R1 according to: R R V V OUT1 2 0 78 1=  . – A good value for R2 is 10.0k. VC (Pin 8): The VC pin is used to compensate the LT3505 control loop by tying an external RC network from this pin to ground. Exposed Pad (Pin 9): The Exposed Pad must be soldered to the PCB and electrically connected to ground. Use a large ground plane and thermal vias to optimize thermal performance.

Σ R DRIVER Q1S OSC SLOPE COMP FREQUENCY FOLDBACK INT REG AND UVLO VC gm 780mV 3505 BD Q Q BOOST SW FB VOUT C1D1 VIN VIN ON OFF GND RT VC SHDN OPERATION (Refer to Block Diagram) The LT3505 is a constant frequency, current mode step- down regulator. A resistor-programmed oscillator enables an RS fl ip-fl op, turning on the internal 1.4A power switch Q1. An amplifi er and comparator monitor the current fl owing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at the VC pin. An error amplifi er measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifi er’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp (not shown) on the VC node provides current limit. The VC node is also clamped to the voltage on the SHDN pin; soft-start is implemented by generating a voltage ramp at the SHDN pin using an external resistor and capacitor. An internal regulator provides power to the control circuitry. This regulator includes an undervoltage lockout to prevent switching when VIN is less than ~3.4V. The SHDN pin is used to place the LT3505 in shutdown, disconnecting the output and reducing the input current to less than 2µA. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for effi cient opera- tion. When the FB pin is low, the voltage at the RT pin decreases to reduce the oscillator frequency. This frequency foldback helps to control the output current during start-up and overload.

The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis- tors according to: R R V V OUT1 2 0 78 1=    . – R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. Input Voltage Range The input voltage range for LT3505 applications depends on the output voltage, on the absolute maximum ratings of the VIN and BOOST pins, and on the programmed switching frequency. The minimum input voltage is determined by either the LT3505’s minimum operating voltage of 3.6V, or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC V V V V V OUT D IN SW D = + where VD is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.4V at maximum load). This leads to a minimum input voltage of: V V V DC V VIN MIN OUT D MAX D SW( ) –= + + with DCMAX = 1 – fSW/8.33, where fSW is in MHz. The maximum input voltage is determined by the abso- lute maximum ratings of the VIN and BOOST pins. For constant-frequency operation, the maximum input voltage is determined by the minimum duty cycle requirement. As the input voltage increases, the required duty cycle to regulate the output voltage decreases. The minimum duty-cycle is: DCMIN = fSW • tON(MIN) where fSW is the switching frequency in hertz and tON(MIN) is the worst-case minimum on-time in seconds. The minimum on-time of the LT3505 is a strong function of temperature. The typical performance characteristics section of the datasheet contains a graph of minimum on-time versus temperature to help determine the worst-case minimum on-time for the intended application. If the input voltage is high enough that the duty-cycle requirement is lower than DCMIN, the part enters pulse- skipping mode. Specifi cally, the onset of pulse-skipping occurs at: VIN(PS) = (VOUT + VD) / DCMIN – VD + VSW Above VIN(PS) the part turns on for brief periods of time to control the inductor current and regulate the output voltage, possibly producing a spectrum of frequencies below the programmed switching frequency. To remain in constant-frequency operation the input voltage should remain below VIN(PS). See the “Minimum On Time” sec- tion of the data sheet for more information on operating above VIN(PS). Note that this is a restriction on the operating input voltage to remain in constant-frequency operation; the circuit will tolerate brief transient inputs up to the absolute maximum ratings of the VIN and BOOST pins when the output is in regulation. The input voltage should be limited to VIN(PS) during overload conditions (short-circuit or start-up). Minimum On Time For switching frequencies less than 750kHz, the part will still regulate the output at input voltages that exceed VIN(PS) (up to 40V), however, the output voltage ripple increases as the input voltage is increased. Figure 1 il- lustrates switching waveforms in continuous mode for a 3V output application near VIN(PS) = 33V. As the input voltage is increased, the part is required to switch for shorter periods of time. Delays associated with turning off the power switch determine the minimum on time of the part. The worst-case typical minimum on-time is 130ns. Figure 2 illustrates the switching waveforms when the input voltage is increased to VIN = 35V.

Now the required on time has decreased below the minimum on time of 130ns. Instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the switch pulse width remains fi xed at 130ns. In Figure 2 the inductor current ramps up to a value exceeding the load current and the output ripple increases to ~200mV. The part then remains off until the output voltage dips below 100% of the programmed value before it begins switching again. For switching frequencies above 750kHz, the input voltage must not exceed VIN(PS). See the “Input Voltage Frequency Foldback” section of the datasheet for a circuit solution that provides safe operation above VIN(PS) at switching frequencies exceeding 750kHz. For switching frequencies below 750kHz, operation above VIN(PS) is safe and will not damage the part as long as the output voltage stays in regulation and the inductor does not saturate. Figure 3 shows the switching waveforms of a 750kHz applica- tion when the input voltage is increased to its absolute maximum rating of 40V. As the input voltage increases, the inductor current ramp rate increases, the number of skipped pulses increases and the output voltage ripple increases. The part is robust enough to survive prolonged operation under these condi- tions as long as the programmed switching frequency is less than 750kHz and the peak inductor current does not exceed 2.2A. Inductor current saturation may further limit performance in this operating regime. Frequency Selection The maximum frequency that the LT3505 can be pro- grammed to is 3MHz. The minimum frequency that the LT3505 can be programmed to is 200kHz. The switching frequency is programmed by tying a 1% resistor from the RT pin to ground. Table 1 can be used to select the value of RT. Minimum on-time and edge loss must be taken into consid- eration when selecting the intended frequency of operation. Higher switching frequency increases power dissipation and lowers effi ciency. APPLICATIONS INFORMATION VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10 F VOUT = 3V VIN = 35V ILOAD = 0.75A L = 10 H RT = 75.0k 2 s/DIV

3505 F02

0.5A/DIV Figure 2 Figure 3 VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10µF VOUT = 3V VIN = 40V ILOAD = 0.75A L = 10µH RT = 75.0k 2µs/DIV

3505 F03

0.5A/DIV VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10 F VOUT = 3V VIN = 30V ILOAD = 0.75A L = 10 H RT = 75.0k 2 s/DIV

3505 F01

0.5A/DIV Figure 1

voltage to remain in continuous mode operation, VIN(PS). LT3505 circuit as shown in Figure 4. 25µA, which programs a 2.2MHz switching frequency. encountered in pulse-skipping mode. Figure 4. 2.2MHz, 5V Application with Input Voltage Frequency Foldback Circuit

3505 F04

application follow the procedure outlined in this section. First select the value of R4 from Table 1. Table 1. RT Pin Resistance in the “Input Voltage Range” section of the data sheet. a switching frequency equal to fSW(MIN). start into large output current loads. the input voltage is less than the zener voltage. maximum load that the LT3505 can regulate will be lower. section of this data sheet for more information.

Table 2. Inductor Vendors several vendors and types that are suitable. current and a maximum reverse voltage of 40V. the input with a 2.2µF or higher value ceramic capacitor. with a low performance electrolytic capacitor. switching current into a tight local loop, minimizing EMI.

Table 3. Capacitor Vendors the Hot Plugging Safely section. sistance (ESR) and provide the best ripple performance. adjusted to maintain the loop bandwidth. lead capacitor from FB to VOUT. voltage. The upper photo shows the recommended value.

3505 F06a

3505 F06b

Figure 6. Two Circuits for Generating the Boost Voltage

3505 F05a

3505 F05b

3505 F05c

Figure 5. Transient Load Response of the LT3505 with Different Output Capacitors as the

(7a) Typical Minimum Input Voltage, VOUT = 5V, fSW = 750kHz (7b) Typical Minimum Input Voltage, VOUT = 3.3V, fSW = 750kHz capacitor and a small Schottky diode (such as the BAT-54). For lower output voltages tie a Schottky diode to the input (Figure 6b). The circuit in Figure 6a is more effi cient because the BOOST pin current comes from a lower voltage source. You must also be sure that the maximum voltage rating of the BOOST pin is not exceeded. The minimum operating voltage of an LT3505 applica- tion is limited by the undervoltage lockout (3.6V) and by the maximum duty cycle as outlined above. For proper start-up, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3505 is turned on with its SHDN pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on the input and output voltages and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 7 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher which will allow it to start. The plots show the worst-case situation where VIN is ramping verly slowly. For lower start-up voltage, the boost diode can be tied to VIN; however this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. At light loads, the inductor current becomes discontinu- ous and the effective duty cycle can be very high. This reduces the minimum input voltage to approximately 400mV above VOUT. At higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT3505, requiring a higher input voltage to maintain regulation. Soft-Start The SHDN pin can be used to soft-start the LT3505, reducing the maximum input current during start-up. The SHDN pin is driven through an external RC fi lter to create a voltage ramp at this pin. Figure 8 shows the start-up waveforms with and without the soft-start circuit. By choosing a large RC time constant, the peak start up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 20µA when the SHDN pin reaches 2.3V. Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate exces- sively, an LT3505 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3505 is absent. This may occur in battery charging ap- LOAD CURRENT (mA) 6.0 INPUT VOLTAGE (V) 6.2 6.4 6.6 6.8 10 100 1000 5.8 5.6 5.4 5.2 7.0 7.2 TO START TO RUN TA = 25° C LOAD CURRENT (mA) 4.3 INPUT VOLTAGE (V) 4.5 4.7 4.9 5.1 10 100 1000 4.1 3.9 3.7 3.5 5.3 5.5 TO START TA = 25° C TO RUN

a shorted or reversed input. Figure 8. To Soft-Start the LT3505, Add a Resistor and Capacitor Figure 9. Diode D4 Prevents a Shorted Input from Discharging

3505 F09

3505 F08a

3505 F08b

3505 F10

Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and smaller and less expensive than the electrolytic capacitor. achieve low output ripple and small circuit size. rings as high as 35V and the input current peaks at 20A.

is used or if the output capacitor has high ESR.

3505 F12

Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation

3505 F11

Figure 11. Model for Loop Response

system ground in only one place. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board and their connections should be made on that layer. Place a local, unbroken ground plane below these components and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor C1. The SW and BOOST nodes should be as small as possible. Finally, keep the FB node small so that the ground pin and ground traces will shield it from the SW and BOOST nodes. Include vias near the exposed GND pad of the LT3505 to help remove heat from the LT3505 to the ground plane. High Temperature Considerations The die temperature of the LT3505 must be lower than the maximum rating of 125°C. This is generally not a concern unless the ambient temperature is above 85°C. For higher temperatures, care should be taken in the layout of the circuit to ensure good heat sinking of the LT3505. The maximum load current should be derated as the ambient temperature approaches 125°C. The die temperature is calculated by multiplying the LT3505 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3505 can be estimated by calculat- ing the total power loss from an effi ciency measurement and subtracting the catch diode loss. Thermal resistance depends on the layout of the circuit board, but 43°C/W is typical for the (3mm × 3mm) DFN (DD) package. Outputs Greater Than 6V For outputs greater than 6V, add a 1k to 2.5k resistor across the inductor to damp the discontinuous ringing of the SW node, preventing unintended SW current. The 12V Step-Down Converter circuit in the Typical Applica- tions section shows the location of this resistor. Also note that for outputs above 10V, the input voltage range will be limited by the maximum rating of the BOOST pin. The 12V circuit shows how to overcome this limitation using an additional zener diode. Other Linear Technology Publications detailed descriptions and design information for Buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note DN100 shows how to generate a bipolar output supply using a Buck regulator.

0.1µF 3.3µH 1N4148 36.5k 10µF

3505 TA02

1µF 20.0k 698k 11.3k VOUT 3.3V MBRM140 10V CMPZ5240B GND 22pF 100k BOOST FB VC SW LT3505 VIN SHDN RT 2.2MHz, 3.3V Step-Down Converter TYPICAL APPLICATIONS 1.2MHz, 1.8V Step-Down Converter VIN 3.6V TO 25V ON OFF 0.1µF 4.7µH BAT54 26.1k 22µF

3505 TA03

120pF2.2µF 44.2k 1.5M 20.0k VOUT 1.8V 1.2A 60.4k MBRM140 12V CMPZ5242B BOOST FB VC SW GND LT3505 VIN SHDN RT Input Voltage [V] 0.00 Frequency [MHz] / Load Current [A]0.25 0.75 1.0 1.25 2.50 1.75 15 25 30 LTC3505 • TA02b 0.50 2.00 2.25 1.50 10 20 35 40 Switching Frequency Maximum Load Current INPUT VOLTAGE (V) Frequency [MHz] / Load Current [A] 0.60 0.80 1.00 15 25 LT3505 • TA03b 0.40 0.20 0.00 5 10 20 1.20 1.40 1.60 Switching Frequency Maximum Load Current

750kHz, 3.3V Step-Down Converter VIN 4.2V TO 36V ON OFF 0.1µF 10µH 36.5k 10µF

3505 TA04

1µF 11.3k VOUT 3.3V 1.1A, VIN > 5V 1.2A, VIN > 8V 1N4148 MBRM140 70pF 69.8k BOOST FB VC SW 75.0k VIN SHDN RT GND LT3505 1MHz, 12V Step-Down Converter VIN 13.5V TO 36V ON OFF 0.1µF 15µH 1k* 0.25W CMDZ5235B 6V 1N4148 71.5k 10µF

3505 TA05

22pF3.3µF 54.9k 4.99k VOUT 12V 1A, VIN > 16.5V 1.1A, VIN > 20.5V 100k MBRM140 *FOR CONTINUOUS OPERATION ABOVE 30V, USE TWO 2k, 0.25W RESISTORS IN PARALLEL BOOST GND FB VC SW LT3505 VIN SHDN RT TYPICAL APPLICATIONS

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. PACKAGE DESCRIPTION 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698) 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1662) NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON TOP AND BOTTOM OF PACKAGE 3.00 ±0.10 (4 SIDES) 0.75 ±0.05 PIN 1 TOP MARK (NOTE 6)

0.200 REF

0.00 – 0.05 0.25 ± 0.05 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 1.65 ±0.05 (2 SIDES)2.15 ±0.05 0.50 BSC 0.675 ±0.05 3.5 ±0.05 PACKAGE OUTLINE 0.38 ± 0.10 BOTTOM VIEW—EXPOSED PAD 1.65 ± 0.10 (2 SIDES) R = 0.115 TYP 2.38 ±0.10 (2 SIDES) (DD) DFN 1203 0.25 ± 0.05

0.50 BSC

NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 0.53 ± 0.152 (.021 ± .006) 0.254 DETAIL “A” GAUGE PLANE 0.18 (.007) DETAIL “A” MSOP (MS8E) 0603 SEATING PLANE 1.10 (.043) MAX 0.22 – 0.38 (.009 – .015) TYP 0.127 ± 0.076 (.005 ± .003) 0.86 (.034) REF 0.65 (.0256) BSC 1 2 3 4 4.90 ± 0.152 (.193 ± .006) 8 7 6 5 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 0.52 (.0205) REF BOTTOM VIEW OF EXPOSED PAD OPTION 1.83 ± 0.102 (.072 ± .004) 2.06 ± 0.102 (.081 ± .004) 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 2.083 ± 0.102 (.082 ± .004) 2.794 ± 0.102 (.110 ± .004) 0.889 ± 0.127 (.035 ± .005) RECOMMENDED SOLDER PAD LAYOUT 0.42 ± 0.038 (.0165 ± .0015) TYP 0.65 (.0256) BSC

Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 " FAX: (408) 434-0507 #"# www.linear.com  LINEAR TECHNOLOGY CORPORATION 2006 LT 0807 REV C • PRINTED IN USA RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (IOUT), 200kHz, High Effi ciency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD < 25µA, TSSOP16/TSSOP16E Packages LT1767 25V, 1.2A (IOUT), 1.25MHz, High Effi ciency Step-Down DC/DC Converter VIN: 3V to 25V, VOUT(MIN) = 1.20V, IQ = 1mA, ISD < 6µA, LT1933 500mA (IOUT), 500kHz, Step-Down Switching Regulator in SOT-23 VIN: 3.6V to 36V, VOUT(MIN) = 1.25V, IQ = 1.6mA, ISD < 1µA, TSSOP16/TSSOP16E Packages LT1936 36V, 1.4A (IOUT), 500kHz, High Effi ciency Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 1.20V, IQ = 1.9mA, ISD < 1µA, LT1940 Dual 25V, 1.4A (IOUT), 1.1MHz, High Effi ciency Step-Down DC/DC Converter VIN: 3.6V to 25V, VOUT(MIN) = 1.25V, IQ = 3.8mA, ISD < 30µA, LT1976/LT1977 60V, 1.2A (IOUT), 200kHz/500kHz, High Effi ciency Step- Down DC/DC Converters with Burst Mode Operation VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA, LT3434/LT3435 60V, 2.4A (IOUT), 200kHz/500kHz, High Effi ciency Step- Down DC/DC Converters with Burst Mode Operation VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA, LT3437 60V, 400mA (IOUT), Micropower Step-Down DC/DC Converter with Burst Mode Operation VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD = <1µA, LT3493 36V, 1.2A (IOUT), 750kHz, High Effi ciency Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 0.78V, IQ = 1.9mA, ISD < 2µA, Burst Mode is a registered trademark of Linear Technology Corporation. TYPICAL APPLICATIONS 300kHz, 3.3V Step-Down Converter VIN 4V TO 36V ON OFF 0.47µF 22µH 36.5k 68µF KEMET A700D686M010ATE015

3505 TA06

150pF2.2µF 226k 11.3k VOUT 3.3V 1A, VIN > 5V 1.2A, VIN > 8.5V 100k VIN BOOST GND FB VC SHDN SW LT3505 RT 1N4148 MBRM140