LM2717_08 NSC | Alldatasheet
Document overview
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Technical content
Features
■ Fixed 3.3V output buck converter with a 2.2A, 0.16Ω, internal switch ■ Adjustable buck converter with a 3.2A, 0.16Ω, internal switch ■ Operating input voltage range of 4V to 20V ■ Input undervoltage protection ■ 300kHz to 600kHz pin adjustable operating frequency ■ Over temperature protection ■ Small 24-Lead TSSOP package
Applications
■ TFT-LCD Displays ■ Handheld Devices ■ Portable Applications ■ Laptop Computers Typical Application Circuit 20078501 © 2008 National Semiconductor Corporation 200785 www.national.com LM2717 Dual Step-Down DC/DC Converter
Ordering Information
Order Number Spec Package Type NSC Package Drawing Supplied As LM2717MT TSSOP-24 MTC24 61 Units, Rail LM2717MTX TSSOP-24 MTC24 2500 Units, Tape and Reel LM2717MT NOPB TSSOP-24 MTC24 61 Units, Rail LM2717MTX NOPB TSSOP-24 MTC24 2500 Units, Tape and Reel www.national.com 2 LM2717
1 PGND Power ground. PGND and AGND pins must be connected together directly at the part. 2 PGND Power ground. PGND and AGND pins must be connected together directly at the part. 3 AGND Analog ground. PGND and AGND pins must be connected together directly at the part. 4 FB1 Fixed buck output voltage feedback input. 5 VC1 Fixed buck compensation network connection. Connected to the output of the voltage error amplifier. 6 VBG Bandgap connection. 7 VC2 Adjustable buck compensation network connection. Connected to the output of the voltage error amplifier. 8 FB2 Adjustable buck output voltage feedback input. 9 AGND Analog ground. PGND and AGND pins must be connected together directly at the part. 10 AGND Analog ground. PGND and AGND pins must be connected together directly at the part. 11 PGND Power ground. PGND and AGND pins must be connected together directly at the part. 12 PGND Power ground. PGND and AGND pins must be connected together directly at the part. 13 SW2 Adjustable buck power switch input. Switch connected between VIN pins and SW2 pin. 14 VIN Analog power input. VIN pins should be connected together directly at the part. 15 VIN Analog power input. VIN pins should be connected together directly at the part. 16 CB2 Adjustable buck converter bootstrap capacitor connection. 17 SHDN2 Shutdown pin for adjustable buck converter. Active low. 18 SS2 Adjustable buck soft start pin. 19 FSLCT Switching frequency select input. Use a resistor to set the frequency anywhere between 300kHz and 600kHz. 20 SS1 Fixed buck soft start pin. 21 SHDN1 Shutdown pin for fixed buck converter. Active low. 22 CB1 Fixed buck converter bootstrap capacitor connection. 23 VIN Analog power input. VIN pins should be connected together directly at the part. 24 SW1 Fixed buck power switch input. Switch connected between VIN pins and SW1 pin. 3 www.national.com LM2717
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Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN −0.3V to 22V SW1 Voltage −0.3V to 22V SW2 Voltage −0.3V to 22V FB1, FB2 Voltages −0.3V to 7V CB1, CB2 Voltages −0.3V to VIN+7V (VIN=VSW) VC1 Voltage 1.75V ≤ VC1 ≤ 2.25V VC2 Voltage 0.965V ≤ VC2 ≤ 1.565V SHDN1 Voltage −0.3V to 7.5V SHDN2 Voltage −0.3V to 7.5V SS1 Voltage −0.3V to 2.1V SS2 Voltage −0.3V to 2.1V FSLCT Voltage AGND to 5V Maximum Junction Temperature 150°C Power Dissipation(Note 2) Internally Limited Lead Temperature 300°C Vapor Phase (60 sec.) 215°C Infrared (15 sec.) 220°C ESD Susceptibility (Note 3) Human Body Model 2kV Operating Conditions Operating Junction Temperature Range (Note 4) −40°C to +125°C Storage Temperature −65°C to +150°C Supply Voltage 4V to 20V SW1 Voltage 20V SW2 Voltage 20V
Electrical Characteristics
Specifications in standard type face are for TJ = 25°C and those with boldface type apply over the full Operating Temperature Range (TJ = −40°C to +125°C). VIN = 5V, IL = 0A, and FSW = 300kHz unless otherwise specified. Symbol Parameter Conditions Min (Note 4) Typ (Note 5) Max (Note 4) Units IQ Total Quiescent Current (both switchers) Not Switching 2.7 6 mA Switching, switch open 6 12 mA VSHDN = 0V 9 27 µA VFB1 Fixed Buck Feedback Voltage 3.3 V VFB2 Adjustable Buck Feedback Voltage 1.267 V ICL1(Note 6) Fixed Buck Switch Current Limit VIN = 8V (Note 7) 2.2 A ICL2(Note 6) Adjustable Buck Switch Current Limit VIN = 8V (Note 7) 3.2 A IB1 Fixed Buck FB Pin Bias Current (Note 8) VIN = 20V 65 µA IB2 Adjustable Buck FB Pin Bias Current (Note 8) VIN = 20V 65 nA VIN Input Voltage Range 4 20 V gm1 Fixed Buck Error Amp Transconductance ΔI = 20µA 1340 µmho gm2 Adjustable Buck Error Amp Transconductance ΔI = 20µA 1360 µmho AV1 Fixed Buck Error Amp Voltage Gain
134 V/V
AV2 Adjustable Buck Error Amp Voltage Gain
136 V/V
DMAX Maximum Duty Cycle 89 93 % FSW Switching Frequency RF = 46.4k 200 300 400 kHz RF = 22.6k 475 600 775 kHz ISHDN1 Fixed Buck Shutdown Pin Current 0V < VSHDN1 < 7.5V −5 5 µA ISHDN2 Adjustable Buck Shutdown Pin Current 0V < VSHDN2 < 7.5V −5 5 µA 5 www.national.com LM2717
Symbol Parameter Conditions Min (Note 4) Typ (Note 5) Max (Note 4) Units IL1 Fixed Buck Switch Leakage Current VIN = 20V 0.01 5 µA IL2 Adjustable Buck Switch Leakage Current VIN = 20V 0.01 5 µA RDSON1 Fixed Buck Switch RDSON (Note 9) 160 mΩ RDSON2 Adjustable Buck Switch RDSON (Note 9) 160 mΩ ThSHDN1 Fixed Buck SHDN Threshold Output High 1.8 1.36 VOutput Low 1.33 0.7 ThSHDN2 Adjustable Buck SHDN Threshold Output High 1.8 1.36 VOutput Low 1.33 0.7 ISS1 Fixed Buck Soft Start Pin Current 4 9 15 µA ISS2 Adjustable Buck Soft Start Pin Current 4 9 15 µA UVP On Threshold 4 3.8 VOff Threshold 3.6 3.3 θJA Thermal Resistance (Note 10) TSSOP, package only 115 °C/W Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance. The maximum allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 4: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 5: Typical numbers are at 25°C and represent the most likely norm. Note 6: Duty cycle affects current limit due to ramp generator. Note 7: Current limit at 0% duty cycle. See TYPICAL PERFORMANCE section for Switch Current Limit vs. VIN Note 8: Bias current flows into FB pin. Note 9: Includes the bond wires, RDSON from VIN pin(s) to SW pin. Note 10: Refer to National's packaging website for more detailed thermal information and mounting techniques for the TSSOP package. www.national.com 6 LM2717
Typical Performance Characteristics Shutdown IQ vs. Input Voltage 20078560 Switching IQ vs. Input Voltage (FSW = 300kHz) 20078561 Switching Frequency vs. Input Voltage (FSW = 300kHz) 20078562 Fixed Buck RDS(ON) vs. Input Voltage 20078563 Adjustable Buck RDS(ON) vs. Input Voltage 20078564 Fixed Buck Efficiency vs. Load Current 20078565 7 www.national.com LM2717
Adjustable Buck Efficiency vs. Load Current (VOUT = 15V) 20078566 Adjustable Buck Efficiency vs. Load Current (VOUT = 5V) 20078567 Fixed Buck Switch Current Limt vs. Input Voltage 20078568 Adjustable Buck Switch Current Limt vs. Input Voltage (VOUT = 5V) 20078569 www.national.com 8 LM2717
PROTECTION (BOTH REGULATORS) The LM2717 has dedicated protection circuitry running during normal operation to protect the IC. The Thermal Shutdown circuitry turns off the power devices when the die temperature reaches excessive levels. The UVP comparator protects the power devices during supply power startup and shutdown to prevent operation at voltages less than the minimum input voltage. The OVP comparator is used to prevent the output voltage from rising at no loads allowing full PWM operation over all load conditions. The LM2717 also features a shut- down mode for each converter decreasing the supply current to approximately 10µA (both in shutdown mode). CONTINUOUS CONDUCTION MODE The LM2717 contains current-mode, PWM buck regulators. A buck regulator steps the input voltage down to a lower out- put voltage. In continuous conduction mode (when the induc- tor current never reaches zero at steady state), the buck regulator operates in two cycles. The power switch is con- nected between VIN and SW1 and SW2. In the first cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and the load current is supplied by COUT and the rising current through the inductor. During the second cycle the transistor is open and the diode is forward biased due to the fact that the inductor current can- not instantaneously change direction. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined approximately as: where D is the duty cycle of the switch, D and D ′ will be re- quired for design calculations. DESIGN PROCEDURE This section presents guidelines for selecting external com- ponents. SETTING THE OUTPUT VOLTAGE (ADJUSTABLE REGULATOR) The output voltage is set using the feedback pin and a resistor divider connected to the output as shown in Figure 1. The feedback pin voltage is 1.26V, so the ratio of the feedback resistors sets the output voltage according to the following equation: INPUT CAPACITOR A low ESR aluminum, tantalum, or ceramic capacitor is need- ed betwen the input pin and power ground. This capacitor prevents large voltage transients from appearing at the input. The capacitor is selected based on the RMS current and volt- age requirements. The RMS current is given by: The RMS current reaches its maximum (I OUT/2) when VIN equals 2V OUT. This value should be calculated for both regulators and added to give a total RMS current rating. For an aluminum or ceramic capacitor, the voltage rating should be at least 25% higher than the maximum input voltage. If a tantalum capacitor is used, the voltage rating required is about twice the maximum input voltage. The tantalum capac- itor should be surge current tested by the manufacturer to prevent being shorted by the inrush current. The minimum capacitor value should be 47µF for lower output load current applications and less dynamic (quickly changing) load condi- tions. For higher output current applications or dynamic load conditions a 68µF to 100µF low ESR capacitor is recom- mended. It is also recommended to put a small ceramic capacitor (0.1µF to 4.7µF) between the input pins and ground to reduce high frequency spikes. INDUCTOR SELECTION The most critical parameters for the inductor are the induc- tance, peak current and the DC resistance. The inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages (for 300kHz operation): A higher value of ripple current reduces inductance, but in- creases the conductance loss, core loss, and current stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple require- ment. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple current in- creases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss equal 2% of the output power. OUTPUT CAPACITOR The selection of C OUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant fre- quency, PWM mode is approximated by: The ESR term usually plays the dominant role in determining the voltage ripple. Low ESR ceramic, aluminum electrolytic, or tantalum capacitors (such as Taiyo Yuden MLCC, Nichicon PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX TPS, and CDE polymer aluminum) is recommended. An electrolytic capacitor is not recommended for temperatures below −25°C since its ESR rises dramatically at cold temperature. Ceramic or tantalum capacitors have much better ESR specifications at cold temperature and is preferred for low temperature ap- plications. BOOTSTRAP CAPACITOR A 4.7nF ceramic capacitor or larger is recommended for the bootstrap capacitor. For applications where the input voltage is less than twice the output voltage a larger capacitor is rec- ommended, generally 0.1µF to 1µF to ensure plenty of gate drive for the internal switches and a consistently low R DS (ON). 9 www.national.com LM2717
SOFT-START CAPACITOR (BOTH REGULATORS) The LM2717 does not contain internal soft-start which allows for fast startup time but also causes high inrush current. Therefore for applications that need reduced inrush current the LM2717 has circuitry that is used to limit the inrush current on start-up of the DC/DC switching regulators. This inrush current limiting circuitry serves as a soft-start. The external SS pins are used to tailor the soft-start for a specific applica- tion. A current (ISS) charges the external soft-start capacitor, CSS. The soft-start time can be estimated as: TSS = CSS*0.6V/ISS When programming the softstart time simply use the equation given in the Soft-Start Capacitor section above. SHUTDOWN OPERATION (BOTH REGULATORS) The shutdown pins of the LM2717 are designed so that they may be controlled using 1.8V or higher logic signals. If the shutdown function is not to be used the pin may be left open. The maximum voltage to the shutdown pin should not exceed 7.5V. If the use of a higher voltage is desired due to system or other constraints it may be used, however a 100k or larger resistor is recommended between the applied voltage and the shutdown pin to protect the device. SCHOTTKY DIODE The breakdown voltage rating of D1 and D2 is preferred to be 25% higher than the maximum input voltage. The current rat- ing for the diode should be equal to the maximum output current for best reliability in most applications. In cases where the input voltage is much greater than the output voltage the average diode current is lower. In this case it is possible to use a diode with a lower average current rating, approximate- ly (1-D)*IOUT however the peak current rating should be higher than the maximum load current. LAYOUT CONSIDERATIONS The LM2717 uses two separate ground connections, PGND for the drivers and boost NMOS power device and AGND for the sensitive analog control circuitry. The AGND and PGND pins should be tied directly together at the package. The feed- back and compensation networks should be connected di- rectly to a dedicated analog ground plane and this ground plane must connect to the AGND pin. If no analog ground plane is available then the ground connections of the feed- back and compensation networks must tie directly to the AGND pin. Connecting these networks to the PGND can in- ject noise into the system and effect performance. The input bypass capacitor C IN, as shown in Figure 1, must be placed close to the IC. This will reduce copper trace re- sistance which effects input voltage ripple of the IC. For additional input voltage filtering, a 0.1µF to 4.7µF bypass ca- pacitors can be placed in parallel with C IN, close to the V IN pins to shunt any high frequency noise to ground. The output capacitors, COUT1 and COUT2, should also be placed close to the IC. Any copper trace connections for the COUTX capacitors can increase the series resistance, which directly effects out- put voltage ripple. The feedback network, resistors RFB1 and RFB2, should be kept close to the FB pin, and away from the inductor to minimize copper trace connections that can inject noise into the system. Trace connections made to the induc- tors and schottky diodes should be minimized to reduce pow- er dissipation and increase overall efficiency. For more detail on switching power supply layout considerations see Appli- cation Note AN-1149: Layout Guidelines for Switching Power Supplies.
Application Information
Some Recommended Inductors (Others May Be Used) Manufacturer Inductor Contact Information Coilcraft DO3316 and DO5022 series www.coilcraft.com Coiltronics DRQ73 and CD1 series www.cooperet.com Pulse P0751 and P0762 series www.pulseeng.com Sumida CDRH8D28 and CDRH8D43 series www.sumida.com Some Recommended Input And Output Capacitors (Others May Be Used) Manufacturer Capacitor Contact Information Vishay Sprague 293D, 592D, and 595D series tantalum www.vishay.com Taiyo Yuden High capacitance MLCC ceramic www.t-yuden.com Cornell Dubilier ESRD seriec Polymer Aluminum Electrolytic SPV and AFK series V-chip series www.cde.com Panasonic High capacitance MLCC ceramic EEJ-L series tantalum www.panasonic.com www.national.com 10 LM2717
Physical Dimensions inches (millimeters) unless otherwise noted TSSOP-24 Pin Package (MTC) For Ordering, Refer to Ordering Information Table www.national.com 12 LM2717
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