DESIGN FEATURES Synchronous Switching Regulator Controller Allows Inputs up to 100V by Greg Dittmer Introduction Industrial, automotive, and telecom systems create harsh, unforgiving environments that demand robust electronic systems. In telecom systems the input rails can vary from 36V to 72V, with transients as high as 100V. In automotive systems the DC battery voltage may be 12V, 24V, or 42V with load dump conditions causing transients up to 60V. Until now, no synchronous buck (or boost) control IC has been capable of operating at 100V, so solutions have been limited to low-side drive topologies that utilize expensive and bulky transformers. The LTC3703 is a 100V synchronous switching regulator controller that can directly step-down high input voltages using a single inductor, thus providing a compact high performance power supply for harsh environments. Key Features for High Voltage Applications The LTC3703 drives external Nchannel MOSFETs using a constant frequency, voltage mode architecture. A high bandwidth error amplifier and patented line feed forward compensation provide very fast line and load transient response. Strong 1Ω gate drivers allow the LTC3703 to drive multiple MOSFETs for higher current applications. A precise internal 0.8V reference provides 1% DC accuracy. The operating frequency is user programmable from 100kHz to 600kHz and can also be synchronized to an external clock for noise-sensitive applications. Selectable Pulse Skip Mode operation improves light load efficiency. Current limit is user programmable and utilizes the voltage drop across the synchronous MOSFET to eliminate the need for a current sense resistor. A low minimum on-time allows high input-to-output step-down ratios such as 72V-to-3.3V at 200kHz. Shutdown mode reduces supply current to 50µA. An internal UVLO circuit guarantees that the driver supply voltage is high enough to sufficiently enhance the MOSFETs before enabling the controller (UV+ = 8.7V, UV– = 6.2V). The LTC3703 is available in a 16-pin narrow SSOP package or, if high voltage 100Ω L1: CIN1: CIN2: COUT: SGND PGND VISHAY IHLP5050EZ SANYO 100MV68AX TDK C4532X7R2A105M OSCON 16SP270M + 1 RC1 10k RSET 30.1k 2 CC1 470pF CC2 1000pF RMAX 15k R2 8.06k 1% RC2 100Ω CC3 2200pF R1 113k 1% 22µF 25V 4 5 7 8 FB IMAX INV RUN/SS GND SW VCC DRVCC BG BGRTN VIN 36V TO 72V 12V DB BAS21 CIN2 1µF 100V X7R ×2 + 15 FSET BOOST LTC3703 3 14 COMP TG 6 CSS 0.1µF MODE/SYNC VIN 16 20k Q1 FZT600 CB 0.1µF 13 10 9 RF 10Ω L1 8µH M2 Si7852DP CDRVCC 10µF CVCC 1µF Figure 1. 36V–72V to 12V/5A synchronous step-down converter Linear Technology Magazine • August 2004 CMDSH-3 M1 Si7852DP 12 11 CIN1 68µF 100V COUT 270µF 16V + D1 MBR1100 VOUT 12V 5A pin spacing is required, in a 28-pin SSOP package. Strong Gate Drivers and Synchronous Drive for High Efficiency Because switching losses are proportional to the square of input voltage, these losses can dominate in high voltage applications with inadequate gate drive. The LTC3703 has strong 1Ω gate drivers that minimize transition times and thus minimize switching losses, even when multiple MOSFETs are used for high current applications. Dual N-channel synchronous drives combined with the strong drivers results in power conversion efficiencies as high as 96%. The LTC3703 provides a separate return pin for the bottom MOSFET driver (see Figure 1), allowing the use of a negative gate drive voltage in the off state. In high voltage switching converters, the switch node dv/dt can be many volts/ns, which pulls up on the gate of the bottom MOSFET through its Miller capacitance, especially in applications with multiple MOSFETs. If this Miller current, times the combined internal gate resistance of the MOSFET plus the driver resistance, exceeds the threshold of the MOSFET, shoot-through will occur, degrading efficiency. By using a negative supply on this pin, the gate can be pulled below ground when turning the bottom MOSFET off. This provides a few extra volts of margin before the gate reaches the turn-on threshold of the MOSFET. Fast Load Transient Response The LTC3703 uses a fast 25MHz op amp as an error amplifier. This allows the compensation network to be optimized for better load transient response. The high bandwidth of the amplifier, along with high switching frequencies and low value inductors, 13 DESIGN FEATURES VOUT 5V/DIV VOUT 50mV/DIV VOUT 50mV/DIV RUN/SS 5V/DIV VIN 20V/DIV IOUT 2A/DIV 50µs/DIV VIN = 50V VOUT = 12V 1A TO 5A LOAD STEP IL 5A/DIV IL 2A/DIV 1ms/DIV VOUT = 12V 20µs/DIV ILOAD = 1A 25V TO 60V VIN STEP Figure 2. Load transient performance Figure 3. Line transient performance Figure 4. Short circuit performance allow very high loop crossover frequencies. Figure 2 illustrates the transient response of a 50V input, 12V output power supply (1A to 5A load step). equals the current limit. During the transient period while the capacitor is being discharged to the proper duty ratio, a cycle-by-cycle comparator guarantees that the peak inductor current remains in control by keeping the top MOSFET off when the VDS of the bottom MOSFET exceeds the programmed limit by more than 50mV. The top MOSFET stays off until the inductor current decays below the limit (VDS < VIMAX). Figure 4 shows the inductor current waveforms during a short-circuit condition. Figure 5 shows a peak efficiency of almost 95% at 50V input and 93% at 75V input. The loop is compensated for a 50kHz crossover frequency which provides ~10µs response time to load transients. The IC and driver bias supply is derived from the 12V output when the output is in regulation, improving the efficiency. During startup or in a short circuit condition when the 12V output is not available, Q1 provides this IC bias voltage from the input supply. For input voltages >30V, the practical choices for input capacitors are limited to ceramics and aluminum electrolytics. Ceramics have very low ESR but bulk capacitance is limited, while aluminum electrolytics have higher bulk capacitance but with much higher ESR. To meet RMS ripple and bulk capacitance requirements, using a combination of the two types is usually the best approach and also prevents excessive LC ringing at the input (by lowering the high Q of the ceramics) when the supply is connected. Another consideration in high voltage converters such as this one is the boost diode. Low leakage and fast reverse recovery is essential. In order to limit power dissipation when this diode is reverse biased at high voltage, ultra-fast reverse recovery silicon diodes such as the BAS21 are recommended. Overcurrent Protection Current limiting is very important in a high voltage supply. Because of the high voltage across the inductor when the output is shorted, the inductor can saturate quickly causing excessive currents to flow. The LTC3703 has current limit protection that uses VDS-sensing of the bottom-side MOSFET to eliminate the need for a current sense resistor. The current limit is user programmable with an external resistor on the IMAX pin to set the maximum VDS at which the current limit kicks in. Current limit works by discharging the RUN/SS capacitor when the VDS exceeds the programmed maximum. The voltage on RUN/SS controls the LTC3703’s maximum duty cycle, so discharging this capacitor reduces the duty ratio until the output current 14 Application Examples 36V–72V to 12V/5A Synchronous Step-Down Regulator The circuit shown in Figure 1 provides direct step-down conversion of a typical 36V-to-72V telecom input rail to 12V at 5A. With the 100V maximum rating of the LTC3703 and the MOSFETs, the circuit can handle input transients of up to 100V without requiring protection devices. The frequency is set to 250kHz to optimize efficiency and output ripple. 100 VIN = 25V VIN = 50V 95 EFFICIENCY (%) Outstanding Line Transient Rejection The LTC3703 achieves outstanding line transient response using a patented feedforward correction scheme. With this circuit the duty cycle is adjusted instantaneously to changes in input voltage without having to slew the COMP pin, thereby avoiding unacceptable overshoot or undershoot. It has the added advantage of making the DC loop gain independent of input voltage. Figure 3 shows how large transient steps at the input have little effect on the output voltage. VIN = 75V 90 85 80 0 1 3 2 LOAD (A) 4 5 Figure 5. Efficiency of the circuit in Figure 1 48V-to-12V 360W Isolated Power Supply The circuit shown in Figure 6 can be used to generate a loosely regulated 12V, 30A isolated power supply for a Linear Technology Magazine • August 2004 DESIGN FEATURES VIN 24V MMBZ5252B 100Ω FZT600 10k .56µH D01813P-561HC 0.33µF X7R 100V ×2 VCC 4.7µF X5R 16V 13V MMBZ5243B VIN 36V–53V 0.33µF X7R 100V VIN BAS19 MODE/SYNC 30.1k 1% 100k 1% 1000pF 20k FSET BOOST LTC3703 COMP TG FB 100k 1% 0.22µF 16V Si7852DP PA0486 SW 10Ω IMAX 0.1µF 16V 1Ω INV VCC N=2 DRVCC RUN/SS GND • • 22µH CDEP105-0R2NC-50 N=1 • Si7852DP N=1 BG BGRTN 0.1µF X5R 16V 20k N=2 1µF X5R 16V 0.22µF X5R 16V 1Ω • 1Ω VBUS 12V 22µF 30A X5R 16V x2 B180 B180 Si7884DP Si7884DP 1µF X7R 100V ×2 10k 200k 0.1µF X5R 25V + VCC 2Ω LT1797 – 0.1µF 16V 1µF X7R 100V ×2 6.8V MMBZ5235B 13V MMBZ5243B • 1k 1µF X5R 16V B140 1µF X5R 16V 13V MMBZ5243B N=1 • N=1 210k 10.0k 1% Figure 6. 48-to-12V 360W isolated power supply Linear Technology Magazine • August 2004 step-down ratio thus generates an output voltage equal to 0.25 • VIN. Running open loop in this fashion eliminates the need for complex feedback circuitry 100 95 EFFICIENCY (%) 360W intermediate bus that can then be stepped down with additional buck regulators to generate multiple low voltage high current outputs. Using this LTC3703-based DC/DC pushpull converter allows one to replace a conventional power module at a lower cost, smaller size and with superior efficiency. The push-pull topology has the advantage over forward/flyback topology of less voltage stress on the MOSFETs, allowing the use of a lower voltage, lower RDS(ON) device to improve efficiency. The LTC3703 runs open loop using the LT1797 amplifier to force 50% duty cycle by driving the FB input of the LTC3703. The 2-to-1 transformer 90 85 80 0 5 10 15 20 25 LOAD CURRENT (A) 30 Figure 7. Efficiency of Figure 6 35 over the isolation barrier. The second stage step-down regulators can then convert this intermediate bus voltage to more tightly regulated outputs. Figure 7 shows that an efficiency of almost 94% can be achieved at 30A. High Efficiency 12V-to-24V 5A Synchronous Step-Up Regulator Synchronous boost converters have a significant advantage over non-synchronous boost converters in higher current applications due to the low power dissipation of the synchronous MOSFET compared to that of the diode in a non-synchronous converter. The high power dissipation in the diode requires a much larger package, 15 DESIGN FEATURES 100 R1 113k 1% RSET 30.1k RC1 10k CC2 0.1µF R2 3.92k 1% CC1 100pF 15 FSET BOOST LTC3703 3 14 COMP TG 5 6 CSS 0.1µF + 2 4 RMAX 15k MODE/SYNC VIN DB CMDSH-3 16 7 8 FB SW IMAX VCC INV RUN/SS GND DRVCC BG BGRTN SGND PGND CB 0.1µF X7R 13 M1 B240A RF 10Ω 10 CDRVCC 10µF X7R 9 L1: COUT1: COUT2: CIN: M1, M2: CVCC 1µF X7R M2 COUT2 10µF 50V X5R ×2 L1 3.3µH 12 11 VOUT 24V 5A COUT1 220µF 35V ×3 CIN 180µF 20V ×2 + VIN 10V TO 15V VISHAY IHLP5050EZ SANYO 35MV220AX UNITED CHEMICON NTS60X5RIH106MT OSCON 20SP180M Si7892DP Figure 8. 12-to-24V, 5A synchronous boost converter e.g. D2PAK, than the small SO8-size package required for the synchronous MOSFET to carry the same current. Figure 8 shows the LTC3703 implemented as a synchronous 12Vto-24V/5A step-up converter that achieves a peak efficiency over 96%. The LTC3703 is set to operate as a synchronous boost converter by simply connecting the INV pin to greater than 2V. In boost mode, the BG pin becomes the main switch and TG, the synchronous switch; and aside from this phase inversion, its operation is similar to the buck mode operation. In boost mode, the LTC3703 can produce output voltages as high as 80V. LT1941, continued from page 12 operate. The LT1941 can supply all of them. Figure 9 shows a typical SLIC. The two step-down switching regulators provide the 3.3V and 1.8V logic supplies. The inverting switching regulator generates both the –21.6V and –65V outputs using a charge pump configuration. The PGOOD3 pin indicates if the –21.6V output is useful because a switching regulator without soft-start can trip a current limited input supply during startup. SLIC Power Supply with Soft-Start SLICs (Subscriber Line Interface Circuits) require many voltages to Conclusion The LTC3703 provides a set of features that make it an ideal foundation for a high input voltage, high performance, high efficiency power supplies. Those features include: 100V capability, synchronous N-channel drive, strong gate drivers, outstanding line and load VOUT1 (1.8V) 2V/DIV VOUT2 (3.3V) 5V/DIV VOUT3 (–21.6V) 50V/DIV VOUT4 (–65V) 100V/DIV IVIN(AVG) (–65V) 2A/DIV 5ms/DIV Figure 10. SLIC start-up waveforms with soft-start 16 95 EFFICIENCY (%) 1 90 85 80 0 1 2 3 LOAD CURRENT (A) 4 5 Figure 9. Efficiency of Figure 8’s circuit regulation, overcurrent protection, and 50µA shutdown current. It is particularly well suited to the harsh environments presented by automotive, telecom, avionics and industrial applications. Its ability to directly step-down input voltages from up to 100V without requiring bulky transformers, or external protection, makes for low cost and compact solutions. The LTC3703 is also versatile—easily applied to a wide variety of output voltages and power levels—mainly due its low minimum on-time (which allows low duty ratios), programmable frequency, programmable current limit, step-up or step-down capability, and package options. in regulation. Figure 10 shows the output voltages and input current during startup. Soft-start helps limit the peak input current. Conclusion The LT1941 is a monolithic triple output switching regulator that has the features and size to fit in a wide variety of applications. The high switching frequency allows the use of small external components, minimizing the total solution size. An internal op amp allows the part to directly regulate negative voltages. The wide input range of 3.5V to 25V and soft-start feature allow the LT1941 to regulate a broad array of power sources. Power good indicators and 2-phase switching help the LT1941 to work with almost any system. Linear Technology Magazine • August 2004