LM2717 NSC | Alldatasheet
Document overview
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Technical content
Features
n Fixed 3.3V output buck converter with a 2.2A, 0.16 Ω, internal switch n Adjustable buck converter with a 3.2A, 0.16 Ω, internal switch n Operating input voltage range of 4V to 20V n Input undervoltage protection n 300kHz to 600kHz pin adjustable operating frequency n Over temperature protection n Small 24-Lead TSSOP package
Applications
Typical Application Circuit 20078501 PRELIMINARY May 2005 LM2717 Dual Step-down DC/DC Converter © 2005 National Semiconductor Corporation DS200785 www.national.com
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 LM2717 www.national.com 2
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. C1 Fixed buck compensation network connection. Connected to the output of the voltage error amplifier. 6V BG Bandgap connection. 7V C2 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 V IN pins and SW2 pin. 14 V IN Analog power input. V IN pins should be connected together directly at the part. 15 V IN Analog power input. V IN 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 V IN Analog power input. V IN pins should be connected together directly at the part. 24 SW1 Fixed buck power switch input. Switch connected between V IN pins and SW1 pin. LM2717 www.national.com3
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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. V IN −0.3V to 22V SW1 Voltage −0.3V to 22V SW2 Voltage −0.3V to 22V FB1 Voltage −0.3V to 7V FB2 Voltage −0.3V to 7V V C1 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 T J = 25˚C and those with boldface type apply over the full Operating Tempera- ture Range (TJ = −40˚C to +125˚C). VIN = 5V, IL = 0A, and F SW = 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 =0 V 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) V IN = 20V 65 µA IB2 Adjustable Buck FB Pin Bias Current (Note 8) V IN = 20V 65 nA VIN Input Voltage Range 42 0 V gm1 Fixed Buck Error Amp Transconductance ∆I = 20µA 1340 µmho gm2 Adjustable Buck Error Amp Transconductance ∆I = 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 R F = 46.4k 200 300 400 kHz RF = 22.6k 475 600 775 kHz LM2717 www.national.com5
Electrical Characteristics (Continued) Specifications in standard type face are for T J = 25˚C and those with boldface type apply over the full Operating Tempera- ture Range (TJ = −40˚C to +125˚C). VIN = 5V, IL = 0A, and F SW = 300kHz unless otherwise specified. Symbol Parameter Conditions Min (Note 4) Typ (Note 5) Max (Note 4) Units 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 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 R DSON (Note 9) 160 m Ω RDSON2 Adjustable Buck Switch RDSON (Note 9) 160 m Ω 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 Ch aracteristics. Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, T J(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, T A. 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) −T A)/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 1.5k Ω 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% tes ted or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical QualityControl (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 PERFORMANCEsection for Switch Current Limit vs. V IN Note 8: Bias current flows into FB pin. Note 9: Includes the bond wires, R DSON 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. LM2717 www.national.com 6
Typical Performance Characteristics Shutdown IQ vs. Input Voltage Switching IQ vs. Input Voltage (FSW = 300kHz) 20078560 20078561 Switching Frequency vs. Input Voltage (FSW = 300kHz) Fixed Buck R DS(ON) vs. Input Voltage 20078562 20078563 Adjustable Buck RDS(ON) vs. Input Voltage Fixed Buck Efficiency vs. Load Current 20078564 20078565 LM2717 www.national.com7
Typical Performance Characteristics (Continued) Adjustable Buck Efficiency vs. Load Current (VOUT = 15V) Adjustable Buck Efficiency vs. Load Current (VOUT = 5V) 20078566 20078567 Fixed Buck Switch Current Limt vs. Input Voltage Adjustable Buck Switch Current Limt vs. Input Voltage (VOUT = 5V) 20078568 20078569 LM2717 www.national.com 8
PROTECTION (BOTH REGULATORS) The LM2717 has dedicated protection circuitry running dur- ing normal operation to protect the IC. The Thermal Shut- down 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 shutdown mode for each converter decreas- ing the supply current to approximately 10µA (both in shut- down mode). CONTINUOUS CONDUCTION MODE The LM2717 contains current-mode, PWM buck regulators. A buck regulator steps the input voltage down to a lower output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady state), the buck regulator operates in two cycles. The power switch is connected between V IN 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 C OUT 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 cannot 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 required 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 inFigure 1. The feedback pin voltage is 1.26V, so the ratio of the feed- back resistors sets the output voltage according to the fol- lowing equation: INPUT CAPACITOR A low ESR aluminum, tantalum, or ceramic capacitor is needed betwen the input pin and power ground. This capaci- tor prevents large voltage transients from appearing at the input. The capacitor is selected based on the RMS current and voltage requirements. The RMS current is given by: The RMS current reaches its maximum (I OUT/2) when VIN equals 2VOUT. 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 ca- pacitor 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 con- ditions. For higher output current applications or dynamic load conditions a 68µF to 100µF low ESR capacitor is rec- ommended. 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 increases 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 requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple current increases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resis- tance 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, Nichi- con 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 tem- perature. Ceramic or tantalum capacitors have much better ESR specifications at cold temperature and is preferred for low temperature applications. LM2717 www.national.com9
Buck Operation (Continued) 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 recommended, generally 0.1µF to 1µF to ensure plenty of gate drive for the internal switches and a consistently low R DS(ON). 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 application. A current (I SS) charges the external soft-start capacitor, CSS. The soft-start time can be estimated as: TSS =C SS*0.6V/ISS When programming the softstart time simply use the equa- tion given in the Soft-Start Capacitorsection above. SCHOTTKY DIODE The breakdown voltage rating of D1 and D2 is preferred to be 25% higher than the maximum input voltage. The current rating 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, approximately (1-D)*I OUT 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 feedback and compensation networks should be connected directly 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 inject 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 resistance which effects input voltage ripple of the IC. For additional input voltage filtering, a 0.1µF to 4.7µF bypass capacitors can be placed in parallel with C IN, close to the VIN pins to shunt any high frequency noise to ground. The output capacitors, C OUT1 and COUT2, should also be placed close to the IC. Any copper trace connections for the C OUTX capaci- tors can increase the series resistance, which directly effects output voltage ripple. The feedback network, resistors R FB1 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 inductors and schottky diodes should be minimized to re- duce power dissipation and increase overall efficiency. For more detail on switching power supply layout considerations see Application Note AN-1149:Layout Guidelines for Switch- ing 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 LM2717 www.national.com 10
Physical Dimensions inches (millimeters) unless otherwise noted TSSOP-24 Pin Package (MTC) For Ordering, Refer to Ordering Information Table National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. BANNED SUBSTANCE COMPLIANCE National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. National Semiconductor Americas Customer Support Center Email: new.feedback@nsc.com Tel: 1-800-272-9959 National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: ap.support@nsc.com National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: jpn.feedback@nsc.com Tel: 81-3-5639-7560 www.national.com LM2717 Dual Step-down DC/DC Converter