Direct Efficient DC/DC Conversion of 100V Inputs for Telecom/Automotive Supplies – Design Note 398 Greg Dittmer Introduction Automotive, telecom and industrial systems have harsh, unforgiving environments that demand robust electronic systems. In telecom systems the input rail 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 or more. The LTC® 3810 is a current mode synchronous switching regulator controller that can directly step down input voltages up to 100V, making it ideal for these harsh environments. The ability to step down the high input voltage directly allows a simple single inductor topology, resulting in a compact high performance power supply—in contrast to the low side drive topologies that require bulky, expensive transformers. Feature-Rich Controller The LTC3810 drives two external N-channel MOSFETs using a synchronizable constant on-time, valley current mode architecture. A high bandwidth error amplifier provides fast line and load transient response. Strong 1Ω gate drivers minimize switching losses—often the dominant loss component in high voltage supplies—even when multiple MOSFETs are used for high current applications. The LTC3810 includes an internal linear regulator controller to generate a 10V IC/driver supply from the high voltage input supply with a single external SOT23 MOSFET. When the output voltage is above 6.7V, the 10V supply can be generated from the output, instead of the , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. RNDRV 249k RON 57.6k 1 BOOST ION 4 VON 5 VRNG 6 PGOOD 7 MODE_SYNC 8 ITH 9 V FB 10 PLL_LPF CSS 1000pF 27 TG 26 SW 25 + SENSE 11 BG SS_TRACK 12 SGND 13 SHDN 14 UVIN SHDN RUV2 16.2k CC2 10pF RFB2 2k RC 210k CC1 10pF PGND CB 0.1μF M1 Si7852* L1 3.3μH 21 SENSE– BGRTN DRVCC CIN2 1μF 100V 28 LTC3810 PGOOD CIN1 68μF 100V DB BAS19 CON 100pF RUV1 475k M3 ZVN4210G VIN 36V TO 72V (100V TRANSIENT*) 20 19 18 17 INTVCC 16 EXTVCC 15 NDRV + CDRVCC 0.1μF M2 Si7852* 10Ω CVCC 1μF D1 B1100 C5 1μF RFB1 4.32k DN398 F01 *USE Si7456 FOR 100V OPERATION Figure 1. Compact 36V–72V to 2.5V/6A Synchronous Step-Down Converter 10/06/398 COUT1 470μF 4V POSCAP VOUT 2.5V 5A COUT2 10μF 6.3V input, for higher efficiency. Other features include: • Programmable cycle-by-cycle current limit, with tight tolerances, provides control of the inductor current during a short-circuit condition. No RSENSETM current sensing utilizes the voltage drop across the synchronous MOSFET to eliminate the need for a current sense resistor. • Low minimum on-time (<100ns) for low duty cycle applications. The on-time is programmable with an external resistor and is compensated for changes in input voltage to keep switching frequency relatively constant over a wide input supply range. • Precise 0.8V, ±0.5% reference over the operating temperature range of 0°C to 85°C. • Phase-locked loop for external clock synchronization, selectable pulse-skip mode operation, tracking, programmable undervoltage lockout and power good output voltage monitor. • 28-pin SSOP package with high voltage pin spacing. High Efficiency 36V–72V to 2.5V/6A Power Supply The circuit shown in Figure 1 provides direct step-down conversion of a typical 48V telecom input rail to 2.5V at 5A. With the 100V maximum DC rating of the LTC3810 and 80V for the MOSFETs, the circuit can handle input voltages of up to 80V without requiring protection devices (up to 100V if appropriate MOSFETs are used). This circuit demonstrates how the low minimum on-time of the LTC3810 enables high step-down ratio applications: 2.5V output from a 72V input at 250kHz is a 140ns on-time. 100 The VRNG pin is set to 0V to set the current limit to about 8A (3A after foldback) during a short-circuit condition (see Figure 4). The resistor divider (RUV1, RUV2) sets the input supply undervoltage lockout to 24V, keeping the LTC3810 shut-down until the VIN > 24V. The LTC3810’s internal linear regulator controller generates the 10V IC/driver supply (INTVCC, DRVCC pins) from the input supply with a single external MOSFET, M3. For continuous operation the power rating of M3 must be at least (72V – 10V) • (0.02A) = 1.2W. If another low voltage supply (between 6.2V and 14V) capable of supplying the ~20mA IC/driver current is available, this supply could be connected to INTVCC /DRVCC pins to increase efficiency by up to 10% at loads above 1A. VOUT 100mV/DIV ILOAD 5A/DIV IL 5A/DIV 20μs/DIV DN398 F03 Figure 3. Load Transient Performance of Figure 1 Circuit Shows 20μs Response Time to a 5A Load Step VIN = 24V 90 VIN = 36V EFFICIENCY (%) The frequency is set to 250kHz with the RON resistor to optimize efficiency while minimizing output ripple. Figure 2 shows mid-range efficiencies of 80% to 84% at 36V input and 65% to 70% at 72V input. Type II compensation is used to set the loop bandwidth to about 75kHz, which provides a 20μs response time to load transients (see Figure 3). VOUT 1V/DIV 80 VIN = 48V 70 VIN = 72V IL 5A/DIV 60 50 VOUT = 2.5V 0 1 2 3 LOAD (A) 4 5 6 DN398 F02 Figure 2. Efficiency of the Circuit in Figure 1 100μs/DIV DN398 F04 Figure 4. Short-Circuit Condition in Figure 1 Circuit Shows Tight Control of Inductor Current and Foldback Data Sheet Download For applications help, call (408) 432-1900, Ext. 2134 www.linear.com Linear Technology Corporation dn398f LT/TP 1006 409K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●