DESIGN IDEAS Reduce EMI with Ultralow Noise 48V to 5V, 10W DC/DC Converter by Rick Brewster Introduction Increasingly, designers are using ultralow noise controllers to avoid EMI problems. Lower operating voltages and more sensitive measurements have created the need for quieter supplies. Extra filtering components and shielding are usually required, as is a careful board layout. Ultralow noise switching regulator controllers such as the LT1683 reduce or eliminate the need for extra filtering. The LT1683 controller uses external MOSFETs to create ultralow noise DC/DC converters. Control of the switch voltage and switch current slew rates reduces switcher noise. The LT1683’s use of external switches allows for greater flexibility in the selection of voltage and current ratings of the supply. + 39µF 63V 2µs/DIV Figure 2. Voltage on one of the MOSFET drains and on the sense resistor Circuit Details T1 FZT649 10µF 20V 2N3904 1N4148 CS 50mV/DIV Figure 1 shows the schematic of an ultralow noise 48V to 5V converter using a push-pull forward converter topology. The output broadband noise is a very low 200µV (bandwidth = 48V 510Ω 0.5W 51k DRAIN 20V/DIV + OPTIONAL D3 D1 8.3 + 11V 68µF 20V 2.7M 14 5 130k 6 1.3nF 7 16.9k 8 25k 3.6k 16 25k 3.6k 15 1.5k 12 SHDN GATE A SYNC CT CAP B LT1683 RT GATE B RVSL CS D2 1 10pF 200V 5pF 19 30pF M1 M2 4 PGND VC SS GND 11 FB NFB 20 7.5k 9 30pF 10 10nF MIDCOM 31244 SILICONIX Si9422 ON SEMICONDUCTOR MBRS340 ON SEMICONDUCTOR MBR0530 (605) 886-4385 (800) 554-5565 (602) 244-6600 Figure 1. Ultralow noise 48V to 5V DC/DC converter 36 A 5V/2A 22µF 2×100µF POSCAP 10pF 200V 5pF 2 18 + 0.1Ω RCSL 13 T1: M1, M2: D1, D2: D3: CAP A V5 0.22µF 22nF 150µF OS-CON 17 3 VIN GCL B 22µF 2.49k + 100MHz) at 2A output (10W). The LT1683 contains all the control circuitry for the converter: oscillator, error amp, gate drivers and protection circuitry. The low noise is achieved by controlling the voltage slew rate of the MOSFET drain and the current slew rate of the MOSFET current. The capacitor divider network from the drain to Cap A or Cap B yields an effective 0.33pF capacitor that provides the voltage slew rate feedback information. The current slew feedback occurs internal to the LT1683 by means of the 100mΩ sense resistor. The resistors on the RVSL and RCSL pins allow the user to optimize the slew rates. The trade-off is between noise and converter efficiency. During design, monitor the output supply noise while slowing down the slew rates via the slew control resistors. Adjust the slew rate until the noise requirement is satisfied. In general, the efficiency loss is only a few percent. Figure 2 shows the voltage on the drain of one of the MOSFETs and the voltage across the sense resistor. Linear Technology Magazine • February 2001 DESIGN IDEAS A 200µV/DIV voltage. The CS pin provides the feedback for pulse-by-pulse current control and slew control. A large signal on CS, indicative of a fault, also shuts the MOSFETs off. Converter efficiency is improved by use of a bootstrap winding that powers the part when the converter is up and running. Efficiency at the low noise setting is approximately 77%. 200µVP-P B 20mV/DIV Conclusion The LT1683 provides a unique way to produce an efficient, ultralow noise supply. Novel control circuitry quiets the switcher, allowing a new supply solution for sensitive electronic systems. The use of external MOSFET switches allows the voltage and current ratings of the supply to be tailored to the application. 5µs/DIV Figure 3. 5V output noise (bandwidth = 100MHz) with undervoltage lockout, ensuring that the input is up and running before the converter is allowed to start. In addition, the GCL pin prevents excessive gate voltage on the MOSFET and protects against the MOSFETs turning on without sufficient gate Because of the voltage slew control, clamps or snubbers on the MOSFET drains are not required and switch ringing is greatly reduced. Figure 3 shows the noise at the outputs. The output noise is a very low 200µVP-P. The SHDN pin provides the supply LTC3200, continued from page 29 1µF 1 2 3V TO 4.4V Li-Ion BATTERY 1µF C+ 3 C– VOUT VIN 1µF LTC3200 FB ON OFF 6 (APPLY PWM WAVEFORM FOR ADJUSTABLE BRIGHTNESS CONTROL) SGND SHDN VSHDN UP TO 6 LEDS 8 PGND 7 5 82Ω 82Ω 82Ω 82Ω 82Ω 82Ω 4 t Figure 3. White or blue LED driver with LED current control Conclusion In either constant voltage or current controlled applications of the LTC3200, the LED brightness can be controlled by applying a PWM signal (approximately 100Hz) to the SHDN pin. Varying the pulse width from 4% to 100% gives the LEDs a linear appearance of brightness control from full-on to full-off. Linear Technology Magazine • February 2001 In the tiny 6-pin SOT or 8-pin MSOP packages, the LTC3200 family of charge pumps provides a simple solution for powering white LEDs. Its small size, low external parts count and low noise, constant frequency operation is ideally suited for both communications and other portable products. http://www.linear-tech.com/ezone/zone.html Articles, Design Ideas, Tips from the Lab… 37