LM3478

LM3478/LM3478Q February 26, 2009 High Efficiency Low-Side N-Channel Controller for Switching Regulator General Description The LM3478 is a versatile Low-Side N-Channel MOSFET controller for switching regulators. It is suitable for use in topologies requiring a low side MOSFET, such as boost, fly- back, SEPIC, etc. Moreover, the LM3478 can be operated at extremely high switching frequency in order to reduce the overall solution size. The switching frequency of the LM3478 can be adjusted to any value between 100kHz and 1MHz by using a single external resistor. Current mode control pro- vides superior bandwidth and transient response, besides cycle-by-cycle current limiting. Output current can be pro- grammed with a single external resistor. The LM3478 has built in features such as thermal shutdown, short-circuit protection, over voltage protection, etc. Power saving shutdown mode reduces the total supply current to 5µA and allows power supply sequencing. Internal soft-start limits the inrush current at start-up. Key Specifications ■ Wide supply voltage range of 2.97V to 40V ■ 100kHz to 1MHz Adjustable clock frequency ■ ±2.5% (over temperature) internal reference ■ 10µA shutdown current (over temperature) Features ■ LM3478Q is AEC-Q100 qualified and manufactured on an Automotive Grade Flow ■ 8-lead Mini-SO8 (MSOP-8) package ■ Internal push-pull driver with 1A peak current capability ■ Current limit and thermal shutdown ■ Frequency compensation optimized with a capacitor and a resistor ■ Internal softstart ■ Current Mode Operation ■ Undervoltage Lockout with hysteresis Applications ■ Distributed Power Systems ■ Battery Chargers ■ Offline Power Supplies ■ Telecom Power Supplies ■ Automotive Power Systems Typical Application Circuit 10135501 Typical High Efficiency Step-Up (Boost) Converter © 2009 National Semiconductor Corporation 101355 www.national.com LM 3478/LM 3478Q High Efficiency Low-Side N-Channel Controller for Switching Regulator Connection Diagram 10135502 8 Lead Mini SO8 Package (MSOP-8 Package) Package Marking and Ordering Information Order Number Package Type Package Marking Supplied As: Feature LM3478MM MSOP-8 S14B 1000 units on Tape and Reel LM3478MMX 3500 units on Tape and Reel LM3478QMM MSOP-8 S14B 1000 units on Tape and Reel AEC-Q100 (Grade 1) qualified. Automotive Grade Production Flow*LM3478QMMX 3500 units on Tape and Reel * Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including defect detection methodologies. Reliability qualification is compliant with the requirements and temperature grades defined in the AEC-Q100 standard. Automotive grade products are identified with the letter Q. For more information go to http://www.national.com/automotive. Pin Descriptions Pin Name Pin Number Description ISEN 1 Current sense input pin. Voltage generated across an external sense resistor is fed into this pin. COMP 2 Compensation pin. A resistor, capacitor combination connected to this pin provides compensation for the control loop. FB 3 Feedback pin. The output voltage should be adjusted using a resistor divider to provide 1.26V at this pin. AGND 4 Analog ground pin. PGND 5 Power ground pin. DR 6 Drive pin. The gate of the external MOSFET should be connected to this pin. FA/SD 7 Frequency adjust and Shutdown pin. A resistor connected to this pin sets the oscillator frequency. A high level on this pin for longer than 30 µs will turn the device off. The device will then draw less than 10µA from the supply. VIN 8 Power Supply Input pin. www.national.com 2 LM 34 78 /L M 34 78 Q Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Input Voltage 45V FB Pin Voltage -0.4V < VFB < 7V FA/SD Pin Voltage -0.4V < VFA/SD < 7V Peak Driver Output Current (<10µs) 1.0A Power Dissipation Internally Limited Storage Temperature Range −65°C to +150°C Junction Temperature +150°C ESD Susceptibilty Human Body Model (Note 2) 2kV Lead Temperature MM Package Vapor Phase (60 sec.) Infared (15 sec.) 215°C 220°C DR Pin Voltage −0.4V ≤ VDR ≤ 8V ISEN Pin Voltage 500mV Operating Ratings (Note 1) Supply Voltage 2.97V ≤ VIN ≤ 40V Junction Temperature Range −40°C ≤ TJ ≤ +125°C Switching Frequency 100kHz ≤ FSW ≤ 1MHz Electrical Characteristics Specifications in Standard type face are for TJ = 25°C, and in bold type face apply over the full Operating Temperature Range. Unless otherwise specified, VIN = 12V, RFA = 40kΩ Symbol Parameter Conditions Typical Limit Units VFB Feedback Voltage VCOMP = 1.4V, 2.97 ≤ VIN ≤ 40V 1.26 1.2416/1.228 1.2843/1.292 V V(min) V(max) ΔVLINE Feedback Voltage Line Regulation 2.97 ≤ VIN ≤ 40V 0.001 %/V ΔVLOAD Output Voltage Load Regulation IEAO Source/Sink ±0.5 %/V (max) VUVLO Input Undervoltage Lock-out 2.85 2.97 V V(max) VUV(HYS) Input Undervoltage Lock-out Hysteresis 170 130 210 mV mV (min) mV (max) Fnom Nominal Switching Frequency RFA = 40KΩ 400 350 440 kHz kHz(min) kHz(max) RDS1 (ON) Driver Switch On Resistance (top) IDR = 0.2A, VIN= 5V 16 Ω RDS2 (ON) Driver Switch On Resistance (bottom) IDR = 0.2A 4.5 Ω VDR (max) Maximum Drive Voltage Swing(Note 6) VIN < 7.2V VIN V VIN ≥ 7.2V 7.2 Dmax Maximum Duty Cycle(Note 7) 100 % Tmin (on) Minimum On Time 325 210 600 nsec nsec(min) nsec(max) ISUPPLY Supply Current (non- switching) (Note 9) 2.7 3.3 mA mA (max) IQ Quiescent Current in Shutdown Mode VFA/SD = 5V (Note 10), VIN = 5V 5 10 µA µA (max) VSENSE Current Sense Threshold Voltage VIN = 5V 156 135/ 125 180/ 190 mV mV (min) mV (max) VSC Short-Circuit Current Limit Sense Voltage VIN = 5V 343 250 415 mV mV (min) mV (max) 3 www.national.com LM 3478/LM 3478Q Symbol Parameter Conditions Typical Limit Units VSL Internal Compensation Ramp Voltage VIN = 5V 92 52 132 mV mV(min) mV(max) VOVP Output Over-voltage Protection (with respect to feedback voltage) (Note 8) VCOMP = 1.4V 50 32/ 25 78/ 85 mV mV(min) mV(max) VOVP(HYS) Output Over-Voltage Protection Hysteresis(Note 8) VCOMP = 1.4V 60 20 110 mV mV(min) mV(max) Gm Error Ampifier Transconductance VCOMP = 1.4V IEAO = 100µA (Source/Sink) 800 600/ 365 1000/ 1265 µmho µmho (min) µmho (max) AVOL Error Amplifier Voltage Gain VCOMP = 1.4V IEAO = 100µA (Source/Sink) 38 26 44 V/V V/V (min) V/V (max) IEAO Error Amplifier Output Current (Source/ Sink) Source, VCOMP = 1.4V, VFB = 0V 110 80/ 50 140/ 180 µA µA (min) µA (max) Sink, VCOMP = 1.4V, VFB = 1.4V −140 −100/ −85 −180/ −185 µA µA (min) µA (max) VEAO Error Amplifier Output Voltage Swing Upper Limit VFB = 0V COMP Pin = Floating 2.2 1.8 2.4 V V(min) V(max) Lower Limit VFB = 1.4V 0.56 0.2 1.0 V V(min) V(max) TSS Internal Soft-Start Delay VFB = 1.2V, VCOMP = Floating 4 msec Tr Drive Pin Rise Time Cgs = 3000pf, VDR = 0 to 3V 25 ns Tf Drive Pin Fall Time Cgs = 3000pf, VDR = 0 to 3V 25 ns VSD Shutdown threshold (Note 5) Output = High 1.27 1.4 V V (max) Output = Low 0.65 0.3 V V (min) ISD Shutdown Pin Current VSD = 5V −1 µA VSD = 0V +1 IFB Feedback Pin Current 15 nA TSD Thermal Shutdown 165 °C Tsh Thermal Shutdown Hysteresis 10 °C θJA Thermal Resistance MM Package 200 °C/W www.national.com 4 LM 34 78 /L M 34 78 Q Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 3: All limits are guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100% tested. 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 4: Typical numbers are at 25°C and represent the most likely norm. Note 5: The FA/SD pin should be pulled to VIN through a resistor to turn the regulator off. The voltage on the FA/SD pin must be above the maximum limit for Output = High to keep the regulator off and must be below the limit for Output = Low to keep the regulator on. Note 6: The voltage on the drive pin, VDR is equal to the input voltage when input voltage is less than 7.2V. VDR is equal to 7.2V when the input voltage is greater than or equal to 7.2V. Note 7: The limits for the maximum duty cycle can not be specified since the part does not permit less than 100% maximum duty cycle operation. Note 8: The over-voltage protection is specified with respect to the feedback voltage. This is because the over-voltage protection tracks the feedback voltage. The overvoltage protection threshold is given by adding the feedback voltage, VFB to the over-voltage protection specification. Note 9: For this test, the FA/SD pin is pulled to ground using a 40K resistor. Note 10: For this test, the FA/SD pin is pulled to 5V using a 40K resistor. 5 www.national.com LM 3478/LM 3478Q Typical Performance Characteristics Unless otherwise specified, VIN = 12V, TJ = 25°C. IQ vs Input Voltage (Shutdown) 10135503 ISupply vs Input Voltage (Non-Switching) 10135534 ISupply vs VIN (Switching) 10135535 Switching Frequency vs RFA 10135504 Frequency vs Temperature 10135554 Drive Voltage vs Input Voltage 10135505 www.national.com 6 LM 34 78 /L M 34 78 Q Current Sense Threshold vs Input Voltage 10135545 COMP Pin Voltage vs Load Current 10135562 Efficiency vs Load Current (3.3V In and 12V Out) 10135559 Efficiency vs Load Current (5V In and 12V Out) 10135558 Efficiency vs Load Current (9V In and 12V Out) 10135560 Efficiency vs Load Current (3.3V In and 5V Out) 10135553 7 www.national.com LM 3478/LM 3478Q Error Amplifier Gain 10135555 Error Amplifier Phase 10135556 COMP Pin Source Current vs Temperature 10135536 Short Circuit Sense Voltage vs Input Voltage 10135557 Compensation Ramp vs Compensation Resistor 10135551 Shutdown Threshold Hysteresis vs Temperature 10135546 www.national.com 8 LM 34 78 /L M 34 78 Q Duty Cycle vs Current Sense Voltage 10135594 9 www.national.com LM 3478/LM 3478Q Functional Block Diagram 10135506 Functional Description The LM3478 uses a fixed frequency, Pulse Width Modulated (PWM) current mode control architecture. The block diagram above shows the basic functionality. In a typical application circuit, the peak current through the external MOSFET is sensed through an external sense resistor. The voltage across this resistor is fed into the Isen pin. This voltage is fed into the positive input of the PWM comparator. The output voltage is also sensed through an external feedback resistor divider network and fed into the error amplifier negative input (feedback pin, FB). The output of the error amplifier (COMP pin) is added to the slope compensation ramp and fed into the negative input of the PWM comparator. At the start of any switching cycle, the oscillator sets the RS latch using the switch logic block. This forces a high signal on the DR pin (gate of the external MOSFET) and the external MOSFET turns on. When the voltage on the positive input of the PWM comparator exceeds the negative input, the RS latch is reset and the external MOSFET turns off. The voltage sensed across the sense resistor generally con- tains spurious noise spikes, as shown in Figure 2. These spikes can force the PWM comparator to reset the RS latch prematurely. To prevent these spikes from resetting the latch, a blank-out circuit inside the IC prevents the PWM comparator from resetting the latch for a short duration after the latch is set. This duration is about 325ns and is called the blanking interval and is specified as minimum on-time in the Electrical Characteristics section. Under extremely light-load or no-load conditions, the energy delivered to the output capacitor when the external MOSFET in on during the blanking interval is more than what is delivered to the load. An over-voltage com- parator inside the LM3478 prevents the output voltage from rising under these conditions. The over-voltage comparator senses the feedback (FB pin) voltage and resets the RS latch. The latch remains in reset state until the output decays to the nominal value. OVER VOLTAGE PROTECTION The LM3478 has over voltage protection (OVP) for the output voltage. OVP is sensed at the feedback pin (pin 3). If at any- time the voltage at the feedback pin rises to VFB+ VOVP, OVP is triggered. See ELECTRICAL CHARACTERISTICS section for limits on VFB and VOVP. OVP will cause the drive pin to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage will drop. The LM3478 will begin switching again when the feed- back voltage reaches VFB + (VOVP - VOVP(HYS)). See ELECTRICAL CHARACTERISTICS for limits on VOVP(HYS). OVP can be triggered if the unregulated input voltage crosses 7.2V, the output voltage will react as shown in Figure 1. The internal bias of the LM3478 comes from either the internal LDO as shown in the block diagram or the voltage at the Vin pin is used directly. At Vin voltages lower than 7.2V the inter- nal IC bias is the Vin voltage and at voltages above 7.2V the internal LDO of the LM3478 provides the bias. At the switchover threshold at 7.2V a sudden small change in bias voltage is seen by all the internal blocks of the LM3478. The control voltage shifts because of the bias change, the PWM comparator tries to keep regulation. To the PWM comparator, www.national.com 10 LM 34 78 /L M 34 78 Q the scenario is identical to a step change in the load current, so the response at the output voltage is the same as would be observed in a step load change. Hence, the output voltage overshoot here can also trigger OVP. The LM3478 will regu- late in hysteretic mode for several cycles, or may not recover and simply stay in hysteretic mode until the load current drops or Vin is not crossing the 7.2V threshold anymore. Note that the output is still regulated in hysteretic mode. Depending on the requirements of the application there is some influence one has over this effect. The threshold of 7.2V can be shifted to higher voltages by adding a resistor in series with Vin. In case Vin is right at the threshold of 7.2V it can happen that the threshold is crossed over and over due to some slight ripple on Vin. To minimize the effect on the output voltage one can filter the Vin pin with an RC filter. 10135511 FIGURE 1. The Feedback Voltage Experiences an Oscillation if the Input Voltage crosses the 7.2V Internal Bias Threshold 10135507 FIGURE 2. Basic Operation of the PWM Comparator SLOPE COMPENSATION RAMP The LM3478 uses a current mode control scheme. The main advantages of current mode control are inherent cycle-by-cy- cle current limit for the switch and simpler control loop char- acteristics. It is also easy to parallel power stages using current mode control since current sharing is automatic. How- ever, current mode control has an inherent instability for duty cycles greater than 50%, as shown in Figure 3. A small increase in the load current causes the switch current to increase by ΔI0. The effect of this load change is ΔI1. The two solid waveforms shown are the waveforms compared at the internal pulse width modulator, used to generate the MOSFET drive signal. The top waveform with the slope Se is the internally generated control waveform VC. The bottom waveform with slopes Sn and Sf is the sensed inductor current waveform VSEN. 10135512 FIGURE 3. Sub-Harmonic Oscillation for D>0.5 and Compensation Ramp to Avoid Sub-Harmonic Oscillation 11 www.national.com LM 3478/LM 3478Q Sub-harmonic Oscillation can be easily understood as a ge- ometric problem. If the control signal does not have slope, the slope representing the inductor current ramps up until the control signal is reached and then slopes down again. If the duty cycle is above 50%, any perturbation will not converge but diverge from cycle to cycle and causes sub-harmonic os- cillation. It is apparent that the difference in the inductor current from one cycle to the next is a function of Sn, Sf and Se as follows: Hence, if the quantity (Sf - Se)/(Sn + Se) is greater than 1, the inductor current diverges and subharmonic oscillation results. This counts for all current mode topologies. The LM3478 has some internal slope compensation VSL which is enough for many applications above 50% duty cycle to avoid subhar- monic oscillation . For boost applications, the slopes Se, Sf and Sn can be cal- culated with the formulas below: Se = VSL x fs Sf = (VOUT - VIN)/L Sn = VIN/L When Se increases then the factor which determines if sub- harmonic oscillation will occur decreases. When the duty cycle is greater than 50%, and the inductance becomes less, the factor increases. For more flexibility slope compensation can be increased by adding one external resistor, RSL, in the Isens path. Figure 4 shows the setup. The externally generated slope compensa- tion is then added to the internal slope compensation of the LM3478. When using external slope compensation, the for- mula for Se becomes: Se = (VSL + (K x RSL)) x fs A typical value for factor K is 40 µA. The factor changes with switching frequency. Figure 5 is used to determine the factor K for individual applications and the formula below gives the factor K. K = ΔVSL / RSL It is a good design practice to only add as much slope com- pensation as needed to avoid subharmonic oscillation. Addi- tional slope compensation minimizes the influence of the sensed current in the control loop. With very large slope com- pensation the control loop characteristics are similar to a voltage mode regulator which compares the error voltage to a saw tooth waveform rather than the inductor current. 10135513 FIGURE 4. Adding External Slope Compensation 10135595 FIGURE 5. External Slope Compensation ΔVSL vs RSL FREQUENCY ADJUST/SHUTDOWN The switching frequency of the LM3478 can be adjusted be- tween 100kHz and 1MHz using a single external resistor. This resistor must be connected between FA/SD pin and ground, as shown in Figure 6. To determine the value of the resistor required for a desired switching frequency refer to the typical performance characteristics. www.national.com 12 LM 34 78 /L M 34 78 Q 10135514 FIGURE 6. Frequency Adjust The FA/SD pin also functions as a shutdown pin. If a high signal (>1.35V) appears on the FA/SD pin, the LM3478 stops switching and goes into a low current mode. The total supply current of the IC reduces to less than 10 uA under these con- ditions. Figure 7 shows implementation of the shutdown func- tion when operating in frequency adjust mode. In this mode a high signal for more than 30us shuts down the IC. However, the voltage on the FA/SD pin should be always less than the absolute maximum of 7V to avoid any damage to the device. 10135516 FIGURE 7. Shutdown Operation in Frequency Adjust Mode SHORT-CIRCUIT PROTECTION When the voltage across the sense resistor measured on the Isen pin exceeds 343 mV, short circuit current limit protection gets activated. A comparator inside the LM3478 reduces the switching frequency by a factor of 5 and maintains this con- dition until the short is removed. In normal operation the sensed current will trigger the power MOSFET to turn off. During the blanking interval the PWM comparator will not re- act to an over current so that this additional 343 mV current limit threshold is implemented to protect the device in a short circuit or severe overload condition. Typical Applications The LM3478 may be operated in either the continuous (CCM) or the discontinuous current conducti