May 2005 LM2717 Dual Step-down DC/DC Converter General Description Features The LM2717 is composed of two PWM DC/DC buck (stepdown) converters. The first converter is used to generate a fixed output voltage of 3.3V. The second converter is used to generate an adjustable output voltage. Both converters feature low RDSON (0.16Ω) internal switches for maximum efficiency. Operating frequency can be adjusted anywhere between 300kHz and 600kHz allowing the use of small external components. External soft-start pins for each enables the user to tailor the soft-start times to a specific application. Each converter may also be shut down independently with its own shutdown pin. The LM2717 is available in a low profile 24-lead TSSOP package ensuring a low profile overall solution. n Fixed 3.3V output buck converter with a 2.2A, 0.16Ω, internal switch n Adjustable buck converter with a 3.2A, 0.16Ω, 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 n n n n TFT-LCD Displays Handheld Devices Portable Applications Laptop Computers Typical Application Circuit 20078501 © 2005 National Semiconductor Corporation DS200785 www.national.com LM2717 Dual Step-down DC/DC Converter PRELIMINARY LM2717 Connection Diagram Top View 20078504 24-Lead TSSOP Ordering Information Order Number Package Type NSC Package Drawing LM2717MT Spec TSSOP-24 MTC24 61 Units, Rail Supplied As 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 Pin Description Pin Name Function 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 14 VIN Adjustable buck power switch input. Switch connected between VIN pins and SW2 pin. 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 18 SS2 19 FSLCT 20 SS1 21 SHDN1 22 CB1 23 VIN 24 SW1 Shutdown pin for adjustable buck converter. Active low. Adjustable buck soft start pin. Switching frequency select input. Use a resistor to set the frequency anywhere between 300kHz and 600kHz. Fixed buck soft start pin. Shutdown pin for fixed buck converter. Active low. Fixed buck converter bootstrap capacitor connection. Analog power input. VIN pins should be connected together directly at the part. Fixed buck power switch input. Switch connected between VIN pins and SW1 pin. 3 www.national.com LM2717 Block Diagram 20078503 www.national.com 4 Power Dissipation(Note 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Lead Temperature VIN −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 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 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 AGND to 5V Maximum Junction Temperature Internally Limited 4V to 20V SW1 Voltage 20V SW2 Voltage 20V 150˚C 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 IQ Parameter Conditions Min (Note 4) Total Quiescent Current (both Not Switching switchers) Switching, switch open VSHDN = 0V VFB1 Fixed Buck Feedback Voltage VFB2 Adjustable Buck Feedback Voltage ICL1(Note 6) Fixed Buck Switch Current Limit VIN = 8V (Note 7) ICL2(Note 6) Adjustable Buck Switch Current Limit VIN = 8V (Note 7) IB1 IB2 Fixed Buck FB Pin Bias Current (Note 8) VIN = 20V Adjustable Buck FB Pin Bias Current (Note 8) VIN = 20V VIN Input Voltage Range gm1 Fixed Buck Error Amp Transconductance ∆I = 20µA gm2 Adjustable Buck Error Amp Transconductance ∆I = 20µA AV1 Typ (Note 5) Max (Note 4) Units 2.7 6 mA 6 12 mA 9 27 µA 3.3 V 1.267 V 2.2 A 3.2 A 65 µA 65 nA 4 20 V 1340 µmho 1360 µmho Fixed Buck Error Amp Voltage Gain 134 V/V AV2 Adjustable Buck Error Amp Voltage Gain 136 V/V DMAX Maximum Duty Cycle FSW Switching Frequency 89 93 RF = 46.4k 200 300 400 kHz RF = 22.6k 475 600 775 kHz 5 % www.national.com LM2717 Absolute Maximum Ratings (Note 1) LM2717 Electrical Characteristics (Continued) 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 −5 5 µA −5 5 µA 0.01 5 µA 0.01 5 µA ISHDN1 Fixed Buck Shutdown Pin Current 0V < VSHDN1 < 7.5V ISHDN2 Adjustable Buck Shutdown Pin Current 0V < VSHDN2 < 7.5V IL1 Fixed Buck Switch Leakage Current VIN = 20V IL2 Adjustable Buck Switch Leakage Current VIN = 20V 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 Output Low ThSHDN2 Adjustable Buck SHDN Threshold 1.36 1.33 Output High 1.8 Output Low 0.7 1.36 1.33 0.7 V V ISS1 Fixed Buck Soft Start Pin Current 4 9 15 µA ISS2 Adjustable Buck Soft Start Pin Current 4 9 15 µA On Threshold 4 UVP Off Threshold θJA Thermal Resistance (Note 10) 3.8 3.6 TSSOP, package only 3.3 115 V ˚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 Switching IQ vs. Input Voltage (FSW = 300kHz) Shutdown IQ vs. Input Voltage 20078560 20078561 Switching Frequency vs. Input Voltage (FSW = 300kHz) Fixed Buck RDS(ON) vs. Input Voltage 20078562 20078563 Adjustable Buck RDS(ON) vs. Input Voltage Fixed Buck Efficiency vs. Load Current 20078565 20078564 7 www.national.com LM2717 Typical Performance Characteristics (Continued) Adjustable Buck Efficiency vs. Load Current (VOUT = 15V) Adjustable Buck Efficiency vs. Load Current (VOUT = 5V) 20078566 20078567 Adjustable Buck Switch Current Limt vs. Input Voltage (VOUT = 5V) Fixed Buck Switch Current Limt vs. Input Voltage 20078569 20078568 www.national.com 8 LM2717 Buck Operation 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 shutdown mode for each converter decreasing the supply current to approximately 10µA (both in shutdown mode). The RMS current reaches its maximum (IOUT/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 capacitor 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 conditions. For higher output current applications or dynamic load conditions a 68µF to 100µF low ESR capacitor is recommended. 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. 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 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 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: INDUCTOR SELECTION The most critical parameters for the inductor are the inductance, 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 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. 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 components. OUTPUT CAPACITOR The selection of COUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant frequency, PWM mode is approximated by: 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: 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 applications. INPUT CAPACITOR A low ESR aluminum, tantalum, or ceramic capacitor is needed 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 voltage requirements. The RMS current is given by: 9 www.national.com LM2717 Buck Operation LAYOUT CONSIDERATIONS (Continued) 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 feedback 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 CIN, 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 CIN, close to the VIN 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 output 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 inductors and schottky diodes should be minimized to reduce power dissipation and increase overall efficiency. For more detail on switching power supply layout considerations see Application Note AN-1149: Layout Guidelines for Switching Power Supplies. 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 RDS(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 (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. 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)*IOUT however the peak current rating should be higher than the maximum load current. Application Information Some recommended Inductors (others may be used) Manufacturer Inductor Coilcraft DO3316 and DO5022 series Contact Information 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 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 Contact Information LM2717 Application Information (Continued) 20078558 FIGURE 1. 15V, 3.3V Output Application 20078559 FIGURE 2. 5V, 3.3V Output Application 11 www.national.com LM2717 Dual Step-down DC/DC Converter Physical Dimensions inches (millimeters) unless otherwise noted TSSOP-24 Pin Package (MTC) For Ordering, Refer to Ordering Information Table NS Package Number MTC24 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. 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