DESIGN FEATURES High Voltage Step-Down Synchronous Controller Offers Single-Supply Operation, Current Mode Control, and 100µA Burst Mode Operation by Jay Celani Introduction As more features and functions are packed into electronics packages, efficient step-down DC/DC conversion circuits that can handle high input voltages at substantial load currents are increasingly necessary. This is especially true for distributed power systems that have high power point of load requirements. The LT3800 is a feature-packed high voltage synchronous step-down controller that simplifies meeting these high power requirements. The LT3800 is the core of singlesupply DC/DC converter solutions that require few external components and maintain high-efficiencies over wide load ranges. Burst Mode operation and a reverse inductor current inhibit feature maximize efficiencies during light-load and no-load conditions, making the LT3800 ideal for use in applications with supply maintenance requirements. Maintenance requirements are common in automotive applications where a low current standby mode is required in addition to high power operating conditions. Both Burst Mode operation and reverse inductor current inhibit can be disabled if desired. The LT3800 contains an integrated start-up regulator that powers the IC directly from the input supply for true single-supply operation. The IC uses a 200kHz fixed-frequency current mode architecture and operates with input voltages from 4V to 60V. A precision shutdown pin threshold allows for easy integration of input supply under-voltage lockout (UVLO) using a simple resistor divider, and quiescent currents are reduced to less than 10µA while the IC is in shutdown. Converter output voltage is programmed using Linear Technology Magazine • March 2005 a resistor divider, and the IC includes a 1% accurate internal reference. The LT3800 also incorporates a programmable ΔV/Δt soft-start that directly controls the rising rate of the converter output voltage at start-up. The LT3800 employs continuous high-side inductor current sensing using an external sense resistor. Inductor “The LT3800 is a versatile platform on which to build high-voltage DC/DC converter solutions that use few external components and maintain high-efficiencies over wide load ranges.” current is limited to the same value in both positive and negative directions, protecting the converter from both source and sink over-current events, and current limit is unaffected by duty-cycle. A LT3800 DC/DC converter uses standard-level N-channel MOSFETs for main and synchronous switches, employing a bootstrapped supply rail for the main switch MOSFET driver. High current switch-drivers allow the use of low RDS(ON) MOSFETs without the need for gate drive buffers. The LT3800 is available in a smallfootprint thermally-enhanced 16-lead TSSOP package. Onboard Start-Up Regulator The LT3800 eliminates the need for an external regulator or a slow-charge hysteretic start scheme through integration of an 8V linear regulator. This regulator generates VCC, the lo- cal supply that runs the IC, from the converter input supply, VIN. The onboard regulator can operate the IC continuously, provided the input voltage and/or FET gate charge currents are low enough to avoid excessive power dissipation in the part. Forcing the VCC pin above its 8V regulated voltage allows use of externally derived power to minimize power dissipation in the IC, reducing thermal considerations. Using the onboard regulator for start-up then deriving power for VCC from the converter output maximizes conversion efficiencies and is common practice. The LT3800 has a start-up requirement of VIN ≥ 7.5V. This assures that the onboard regulator has ample headroom to bring the VCC pin above its UVLO threshold of 6.25V. If VCC is maintained using an external source, such as the converter output, the LT3800 can continue to operate with VIN as low as 4V. Burst Mode Operation The LT3800 supports low current Burst Mode operation to maximize efficiency during low-load and no-load conditions. Burst Mode operation is enabled by shorting the BURST_EN pin to SGND, and can be disabled by shorting BURST_EN to either VFB or VCC. When the peak switch current is below 15% of the programmed current limit, Burst Mode function is engaged. During the Burst interval, switching ceases and all internal IC functions are disabled, which reduces VIN pin current to 20µA and reduces VCC current to 80µA. If no external drive is provided for VCC, all VCC bias currents originate from the VIN pin, giving a 25 DESIGN FEATURES 100 + 56µF ×2 VIN LT3800 1nF R1 20k 1% SHDN 200k R2 174k 1% 100pF 82.5k 680pF SW CSS Si7850DP D1 BAS19 15µH BURST_EN VCC VFB VC 1µF TG D2 1N4148 0.015 + 26 1 10 1 0 VOUT 12V AT 75W 270µF When reverse-current inhibit is enabled, the LT3800 sense amplifier detects inductor currents approaching zero and disables the synchronous switch for the remainder of that switch cycle, simulating the light-load switching characteristics of a non-synchronous converter. Reverse-current inhibit reduces losses associated with inductor ripple currents, improving conversion efficiencies with loads that are less than half of the peak inductor ripple current. VIN supply. The resistor divider is set such that the divider output puts 1.35V onto the SHDN pin when VIN is at the desired UVLO rising threshold voltage. The SHDN pin has 120mV of input hysteresis, which allows the IC to resist almost 10% of input supply droop before disabling the converter. The S H D N pin has a secondary threshold of 0.5V, below which the IC operates in an ultralow-current shutdown mode with IVIN < 10µA. The shutdown function can be disabled by connecting the SHDN pin to VIN through a large value pull-up resistor. Continuous High-Side Inductor Current Sensing Precision Shutdown Threshold The LT3800 uses a wide commonThe LT3800 has a precision-threshold shutdown feature, which allows use of the SHDN pin for analog monitoring applications, as well as logic-level controlled applications. Input supply UVLO for sequencing or start-up over-current protection is easily achieved by driving the SHDN pin with a resistor divider from the 10V/DIV Figure 3. Output soft-start waveform for the DC/DC converter in Figure 1 2 LOSS (48V) Figure 2. Converter efficiency and power loss for the DC/DC converter in Figure 1 Figure 1. This 20V–55V to 12V 75W DC/DC converter features reverse current inhibit and input under-voltage lockout. 1ms/DIV 80 3 ILOAD (A) 10µF 2V/DIV 4 VIN = 48V 85 70 0.1 Si7370DP SENSE– SENSE+ SGND The LT3800 contains a reverse-current inhibit feature, which maximizes efficiency during light load conditions. This mode of operation prevents negative inductor current, and is sometimes called “pulse-skipping” mode. This feature is always enabled with Burst Mode operation when the BURST_EN pin is connected to ground. The reverse-current inhibit feature can also be enabled without Burst Mode by connecting the BURST_EN pin to the VFB pin, which is the configuration used for the DC/DC converter shown in Figure 1. VIN = 55V 90 B160 PGND Reverse Current Inhibit 5 75 1µF BG total VIN current of 100µA. An internal negative-excursion clamp on the VC pin is set 100mV below the switch disable threshold, which limits the negative excursion of the pin voltage during the Burst interval. This clamp minimizes converter output ripple during Burst Mode operation. VIN = 36V 95 POWER LOSS (W) RA RB 1M 82.5k BOOST 6 VIN = 24V 1µF ×3 EFFICIENCY (%) VIN 20V TO 55V mode input range current sense amplifier that operates from 0V to 36V. This current sense amplifier provides continuous inductor current sensing via an external sense resistor. A continuous inductor current sensing scheme does not require blanking intervals or a minimum on-time to monitor current, common to schemes 10V/DIV ILOAD = 2A 2µs/DIV Figure 4. Switching waveform for the DC/DC converter in Figure 1 ILOAD = 0.5A 2µs/DIV Figure 5. Light load switching waveform for the converter in Figure 1 Linear Technology Magazine • March 2005 DESIGN FEATURES VIN 6.5V TO 55V C2 1µF 100V X7R ×3 C8 56µF 63V ×2 + RA 1M C7 1.5nF R1 100k 1% R2 309k 1% R3 62k C9 470pF C10 100pF VIN BOOST NC TG LT3800 SHDN C1 1µF 16V X7R D1 BAS19 M1 Si7850DP ×2 SW R4 75k L1 5.6µH CSS BURST_EN VCC VFB BG VC R5 47k C3 1µF 16V X7R M2 Si7370DP ×2 DS3 B160 ×2 PGND SENSE– SENSE+ SGND RS 0.01 D2 1N4148 DS2 MBRO520L DS1 MBRO520L C4 1µF C6 10µF 6.3V X7R M3 1/2 Si1555DL M4 1/2 Si1555DL + C5 220µF ×2 VOUT 5V AT 10A C5: SANYO POSCAP 6TP220M L1: IHLP-5050FD-01 Figure 6. This 6.5V–55V to 5V 10A DC/DC converter features charge pump doubler VCC refresh and current limit foldback. that sense switch current. The sense amplifier monitors inductor current independent of switch state, so the main switch is not enabled unless the inductor current is below what corresponds to the VC pin voltage. This turn-on decision is performed at the start of each cycle, and individual switch cycles will be skipped should an over-current condition occur. This eliminates many of the potential overcurrent dangers caused by minimum on-time requirements, such as those that can occur during startup, shortcircuit, or abrupt input transients. Soft Start The LT3800 employs an adaptive softstart scheme that directly controls 100 12 10 VIN = 24V VIN = 13.8V VIN = 48V 90 8 VIN = 55V 85 POWER LOSS VIN = 48V 80 75 70 0 2 4 6 IOUT (A) 6 4 POWER LOSS VIN = 13.8V 8 2 10 0 Figure 7. Efficiency and power loss for the DC/DC converter in Figure 6 Linear Technology Magazine • March 2005 POWER LOSS (W) EFFICIENCY (%) 95 the DC/DC converter output voltage during start-up. The rising rate of this voltage is programmed with a capacitor connected to the converter output. The capacitor value is chosen such that the desired ΔV/Δt of the output results in a 2µA charge current through the capacitor. The soft start function maintains this desired output rising rate up to 95% of the regulated output voltage, when the soft-start circuitry is disabled. The soft-start function is automatically re-enabled if the converter output droops below 70% regulation, so converter recovery is graceful from a short duration shutdown or an output short-circuit condition. 20V–55V to 12V, 75W DC/DC Converter Figure 1 shows a 20V–55V to 12V 75W converter, configured for reverse current inhibit operation and input UVLO. Power for the IC is obtained directly from VIN through the LT3800’s internal VCC regulator at start-up. The main switch bootstrapped supply is refreshed via D1 from the 8V generated on the VCC pin. When the converter output comes up, D2 pulls VCC above regulation, disabling the internal regulator and providing a current path from the converter output to the VCC pin. With the VCC pin driven from the converter output, VIN current is reduced to 20µA. Using outputgenerated power in high input voltage converters results in significant reduction of IC power dissipation, which increases overall conversion efficiency, but is critical to reduce IC thermal considerations. Figure 2 shows the conversion efficiency and power loss for this DC/DC converter. Output voltage is programmed using R1 and R2, and the output is in regulation when the voltage at the VFB pin is 1.231V. VIN UVLO is programmed via RA and RB, enabling the LT3800 at 90% of the specified low end of the VIN range, or 18V, which corresponds to 1.35V on the SHDN pin. The SHDN input has 120mV of hysteresis, so the converter will be disabled if VIN drops below 16V. The LT3800 soft-start function controls the rising slope of the output at startup such that the ΔV/Δt current through C8 is 2µA, so the converter output will rise at a controlled rate of 2µA/1nF, or 2V/mS. Figure 3 shows the soft-start ramp. The BURST_EN pin is tied to the VFB pin to disable Burst Mode operation while keeping reverse current inhibit operation enabled. Figure 4 shows continuous current operation when 27 DESIGN FEATURES VIN 9V TO 38V + C8 100µF 50V ×2 RA RB 1M 187k R3 82k C2 330pF C10 100pF BOOST NC TG LT3800 SHDN C1 1nF R1 100k 1% VIN R2 169k 1% C3 100pF R4 39k C5 1µF 16V X7R C9 4.7µF 50V X7R ×3 M1 Si7884DP SW D1 MBR520 CSS BURST_EN VFB VCC BG VC C4 1µF 16V X7R L1 3.3µH M2 Si7884DP DS1 SS14 ×2 PGND SENSE – SENSE+ SGND RS 0.01 C7 10µF 6.3V X7R C6: SANYO POSCAP 4TPD470M L1: IHLP-5050FD-01 + C6 470µF ×2 VOUT 3.3V AT 10A Figure 8. A 9V–38V to 3.3V 10A DC/DC converter with VIN UVLO the load is greater than half of the peak ripple current. With lighter loads, during the switch off interval, as the inductor current approaches zero, the synchronous switch is disabled. The resulting discontinuous switching waveform is shown in Figure 5. 6.5V–55V to 5V, 10A DC/DC Converter In LT3800 converter applications with output voltages in the 9V to 20V range, back-feeding VCC from the converter output is trivial, accomplished by connecting a diode from the output to the VCC pins. Outputs lower than 9V require step-up techniques to generate back-feed voltages greater than the 7 VIN = 13.8V 90 6 88 5 86 4 84 3 82 2 80 1 78 0.1 1 ILOAD (A) 0 10 Figure 9. Efficiency and power loss for the DC/DC converter in Figure 8 POWER LOSS (W) EFFICIENCY (%) 92 VCC regulated output. The 6.5V–55V to 5V 10A DC/DC converter shown in Figure 6 uses an external Si1555DL MOSFET pair (M3, M4) to create a charge pump doubler that steps up the output voltage. This simple doubler uses the synchronous gate drive (BG pin) as a control signal. This converter also uses an external current limit foldback scheme. The foldback circuit consists of a single 1N4148 diode (D2) and a 47k resistor (R5). The current limit foldback circuit provides additional control during the first few switch cycles of start-up, and provides reduced short-circuit output current. When the output is at ground, the diode and resistor clamp the VC pin to a value that corresponds to 25% of the programmed maximum current. This circuit is only active with VOUT close to ground, and becomes completely disabled once the output voltage rises above 10% regulation. Figure 7 shows the conversion efficiency and power loss for this converter. 9V–38V to 3.3V, 10A DC/DC Converter In some DC/DC converter applications, the typical input voltage is moderate, but the converter must withstand or operate through intermittent high-voltage excursions. This is typical of automotive battery-voltage applications, where high voltage line transients, such as during a load-dump condition, must be accommodated. The 9V–38V to 3.3V 10A DC/DC converter with VIN UVLO shown in Figure 8 is an automotive application that typically operates with VIN = 13.8V, but can operate through VIN excursions from 9V up to 38V. Because the typical line voltage is moderate, the LT3800 can operate directly from the internal VCC regulator without excessive power dissipation, eliminating the need for a step-up scheme to regenerate VCC from the converter output. Figure 9 shows the conversion efficiency and power loss for this circuit. Conclusion The LT3800 is a versatile platform on which to build high voltage DC/ DC converter solutions that use few external components and maintain high efficiencies over wide load ranges. The integrated start-up regulator facilitates true single-supply operation and Burst Mode function enables efficient solutions to power-supply maintenance requirements. For more information on parts featured in this issue, see http://www.linear.com/designtools 28 Linear Technology Magazine • March 2005