DESIGN FEATURES Two New Switching Regulators Deliver Both Power and Ultralow Noise by Rick Brewster Introduction T raditionally, when designers required a very low noise power supply, they would avoid DC/DC switchers because of their notorious reputation for generating high frequency harmonics. If they did use a switcher, careful attention would be required for board layout, shielding and the judicious use of filtering elements. The LT1683 and LT1738 are new switching regulator controllers that create a DC/DC switching supply with significantly lower conducted and radiated electromagnetic interference (EMI). These controllers reduce EMI at its source by controlling the current and voltage slew rates of the main switches. A robust DC/ DC converter for use in noise-sensitive applications can be built with confidence. The LT1683 and LT1738 make use of a patented dual control loop method that allows for control of both the output switch voltage slew rate and 48V 510Ω 0.5W 51k 39µF 63V T1 Q1 10µF 20V 1N4148 D2 17 VIN 2.7M 14 5 130k 6 1.3nF 7 16.9k 8 25k 25k 3.6k 3.6k 1.5k 16 15 12 SHDN CAP A V5 GATE A SYNC CT CAP B LT1683 RT GATE B RVSL 150µF OS-CON 3 GCL CS PGND VC 22nF SS 0.22µF 5V/2A 10pF 200V 5pF 18 30pF 19 M1 M2 4 7.5k 1% 2.49k 1% GND 13 11 FB NFB 20 9 30pF 10 10nF T1: MIDCOM 31244 Q1: ZETEX FZT649 L1: COILCRAFT DS5022P M1, M2: SILICONIX Si9422 D1: ON SEMI MBR0530 D2, D3: ON SEMI MBRS340 (800) 643-2661 (631) 534-7100 (847) 639-6400 (800) 554-5565 (602) 244-6600 Figure 2. LT1683 push-pull converter CV + CONTROL IVSL GATE DRIVER IVSL M1 – + – CC + CS RSEN SENSE_AMP – 2×100µF POSCAP D3 1 CAP + A 22µH 0.1Ω RCSL IVSL VT B 10pF 200V 5pF 2 LO ICSL L1 22µH 8.2 68µF 20V 11V OPTIONAL D1 Q2 2N3904 – GND GC CURRENT-SLEW AMP the output switch current slew rate. Bringing these slew rates under control eliminates the source of much of the high frequency harmonic energy in both the output voltage and in the input current; thus, less filtering is needed. The slew rates are user adjustable so that an optimal tradeoff between noise and converter efficiency can be made. Typically, the optimal setting requires a loss of efficiency of only a few percent. The LT1683, with two external MOSFET switches, is designed for use in a push-pull topology, whereas the LT1738 has a single external MOSFET for single-switch topologies. Because the switches are external, the user has complete control over voltage and current ratings of the supply. Figure 1. LT1683/LT1738 slew control (one channel) 18 Linear Technology Magazine • September 2001 DESIGN FEATURES A 200µV/DIV based on a user-defined external resistor, RCSL. The magnitude of the current in CC is translated to a current proportional to IVSL and is applied to the integrating node (Cap pin) to control the switch. In this manner the slew rates are both regulated and linked to provide quick and efficient on and off transitions. 200µVP-P B 20mV/DIV Ultralow Noise 48V to 5V Converter 5µs/DIV Figure 3. 5V output noise for Figure 2’s circuit The Magic of Slew Control Analyzing the Fourier components of a square wave signal shows that the harmonic peaks roll off at 20dB/dec starting at the fundamental frequency. If the edges of that square wave are slewed so that it is trapezoidal, a second 20dB/dec roll-off at 1/tSLEW results, where tSLEW is the time it takes for the waveform transition. The LT1683 and LT1738 are new switching regulator controllers that create a DC/DC switching supply with significantly lower conducted and radiated electromagnetic interference (EMI). These controllers reduce EMI at its source by controlling the current and voltage slew rates of the main switches. gracefully switch over to slewing the voltage. When the switch turns off, the reverse should happen. The LT1683 and LT1738 employ dual feedback loops to handle the slew control. Figure 1 shows symbolically how a single channel looks. The voltage slew control is a simple integrator using the external cap, CV, and a charging or discharging current, IVSL, based on a user-defined external resistor, RVSL. The current slew control is derived in the following manner. The voltage developed across the sense resistor is amplified and differentiated via an internal capacitor, CC. Thus, the current in CC is proportional to dI/dt. That current is limited to a value, ICSL, L1 22µH 5V VIN Figure 2 shows the schematic of a converter using the LT1683 in a pushpull forward converter topology. The LT1683 contains all the control circuitry for the converter: oscillator, error amp, gate drivers and protection circuitry. The capacitor divider networks from the drains to Cap A and Cap B provides for an effective 0.33pF capacitor that is used for the voltage slew rate feedback. The current slew feedback occurs internal to the LT1683 and uses the CS voltage as its feedback (switch current). The push-pull topology is desirable for low noise switchers because it draws current more continuously from the input, thus reducing conducted emissions. The output noise of the converter is shown in Figure 3. The bottom trace is the output voltage of the regulator. The noise is dominated by the funda- D1 OSCON + In a switch-mode power supply, most of the noise is generated because of the switching action of the power switch. For efficiency these transitions are generally made as abrupt as possible, but slewing these edges allows for a great reduction of noise at a modest loss of efficiency. So the challenge is how to apply this to both the switch voltage and switch current. In a normal switching cycle with an inductive load, the switch will turn on, current will rise until the switch diverts current from the inductor and then the voltage will fall. Therefore, we wish to slew the current and then Linear Technology Magazine • September 2001 B + 4 × 150µF 100µF 17 VIN 14 5 6 1.3nF 7 16.9k 25k 3.6k 25k 3.6k 22nF 0.22µF 8 16 15 12 CAP 150µF + A 12V 1A OSCON OPTIONAL 3 GCL SHDN 10µH 5pF 2 V5 SYNC CT GATE 1 M1 L1: COILCRAFT DS5022P-223 (847) 639-6400 D1: ON SEMI MBRD620 (602) 244-6600 M1: SILICONIX Si9426 (800) 544-5565 LT1738 RT RVSL CS 4 25mΩ RCSL PGND VC SS 1.5k 13 GND 11 FB NFB 10 20 9 21.5k 1% 2.49k 1% 10nF Figure 4. 5V to 12V/1A boost converter using the LT1738 19 DESIGN FEATURES A 500µV/DIV slew control, clamps or snubbers on the MOSFET drains are eliminated and switch ringing amplitude and duration are reduced by more than 80%. 400µVP-P Ultralow Noise 5V to 12V Boost Converter B 50mV/DIV Figure 4 shows a 5V to 12V/1A boost converter using the LT1738. The single switch provides a simpler solution at the expense of slightly higher noise. Figure 5 shows noise at the output voltage at 1A. It is a very low 400µVP-P. 5µs/DIV Figure 5. 12V output noise for Figure 4’s circuit (bandwidth = 100MHz) mental frequency of the switcher. When this component is further filtered out by the added filter, it is apparent how quiet the switcher is— the residual noise is a miniscule 200µVP-P at a 2A output (bandwidth = 100MHz). The LT1683 has many protection features. First, the SHDN pin provides the supply with undervoltage Figure 6 shows the flexibility of having external switches. The circuit is an AC line (90VAC to 264VAV) to 12V/ 2.5A converter using the LT1738 and a single high voltage MOSFET. The basic topology is a flyback converter, which, in conjunction with the optoisolator, provides for complete continued on page 30 DANGER: HIGH VOLTAGE L1 X1 + 1M 90VAC TO 264VAC Ultralow Noise 30W Offline Supply 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 prevents the MOSFETs from turning on without sufficient gate voltage. Excessive switch current will be detected at the CS pin and the MOSFETs will be turned off. Because of the voltage 0.1µF 250VAC “X2” 1M BR1 100µF 400V D2 510Ω 1 T1 11 2W MUR160 100k 2W 7.5V 1N755A 510k + VIN 14 5 510k 6 1.5nF 7 51k 165k 8 19.6k 2N2222 3.9k 3.9k 2N2222 51k 10Ω 17 56µF 35V 16 15 9 7 12 3 6 19 NC D4 BA521 10 NFB 470pF 5 8 10Ω D1 A1 220pF X3 200pF 200V + A2 C2 330µF 25V + C3 330µF 25V + +VOUT 12V 2.5A C4 330µF 25V – VOUT 15pF 600V SHDN V5 15pF SYNC CT CAP U3 LT1738 GATE RT CS RVSL NC RCSL PGND VC FB GND SS 13 11 10nF BR1: GENERAL INSTRUMENTS W06G (215) 323-1000 C2–C4: SANYO MV-GX (619) 661-6835 ISO1: FAIRCHILD CNY17-3 (888) 522-5373 UNLESS OTHERWISE NOTED: ALL RESISTORS 1206, 5% D1: D2: L1: T1: 2 1 MTP2N60E 0.1µF 4 18 12 ISO1 3 6 1 5 2 0.1µF ON SEMI MBR20200CT ON SEMI P6KE200A BI TECH HM18-10001 PREMIER MAGNETICS POL-15033 2 COMP V U2 REF 8 LT1431 4 COLL RTOP 7 RMIO G-F G-S 6 5 3 + GCL 3 +VOUT 1k 0.068Ω 1/2W 20 1k 4 38.3k 1% 0.22µF 1k 10k 1% (602) 244-6600 (602) 244-6600 (714) 447-2345 (949) 362-4211 INPUT FILTER IS REQUIRED TO ATTENUATE SWITCHING FREQUENCY HARMONICS AND PASS FCC CLASS B (LT1738 DOES NOT ATTENUATE THESE LOW FREQUENCY HARMONICS) MAIN ADVANTAGE WITH LT1738 IS IT MAKES SUPPRESSING THE HIGH FREQUENCY NOISE AND EMI EASY. Figure 6. Ultralow noise 30W offline power supply 20 Linear Technology Magazine • September 2001 DESIGN FEATURES The –24V output of this supply uses one secondary winding in a capacitively coupled flyback configuration, whereas the –72V output uses the other two windings in a conventional flyback mode. The –24V output uses an LT1783 op amp to level-shift the feedback voltage, and the –72V output is obtained by stacking additional windings on the –24V output. A size 5, 6-winding Versapak transformer (VP5-0155) was used for convenience, with three windings connected in parallel on the primary to satisfy the primary current demand. Unlike the previous, low voltage boost and SEPIC designs, which take advantage of the No RSENSE technology, this flyback converter places significant stress on the drain of the power MOSFET. As a result, a 100V BVDSS device is used (International Rectifier’s IRL2910), along with a conventional 12mΩ sense resistor in the MOSFET source (the absolute maximum voltage rating for the SENSE pin on the LTC1871 is 36V). The increase in losses due to this sense resistor are relatively small in this system (approximately 1%), due to the high input voltage. For systems where control of the maximum output current is more important than overall efficiency, the use of a sense resistor can improve performance. The initial tolerance of a discrete sense resistor is commonly better than ±5%, whereas the initial tolerance of the RDS(ON) of a power MOSFET is typically ±20%–30%. In addition, the temperature coefficient of the discrete resistor can easily be an order of magnitude lower than for a power MOSFET (whose R DS(ON) increases approximately 50% from 25°C to 125°C). The resistor divider formed by R1 and R2 is used to detect an undervoltage condition on the input supply in order to shut down the converter when the battery pack has discharged below 5.0V. For a falling input voltage (a discharging battery), the RUN pin on the LTC1871 is compared to an internal micropower 1.248V reference. If the RUN pin falls below this LT1683/LT1783, continued from page 20 components, eliminating the need for the grounded caps on the input filter. electrical isolation of the output. An overwinding coil on the transformer provides the supply voltage to the part. An input filter is used to attenuate the low frequency harmonics so the supply can pass FCC Class B regulations. The LT1738 reduces the high frequency harmonics and EMI noise Authors can be contacted at (408) 432-1900 30 Conclusion Two new controllers have been created that allow designers to confidently tackle ultralow noise applications. The LT1683 and LT1738 not only provide normal controller functions but also regulate the main compo- threshold, the chip shuts down and the quiescent current drops to 10µA in order to reduce the load on the battery. The hysteresis on the RUN pin comparator was chosen to be 100mV in order to compensate for the rise in the unloaded battery voltage (or other input supply) and to provide good noise immunity in general. In this particular design, the rising input start-up threshold is approximately 5.4V. The optional capacitor CR could be used to give the converter some ride-through capability for brief input transients. Conclusion The LTC1871 is a versatile control IC optimized for a wide variety of singleended DC/DC converter topologies. Flexible, high performance operation is provided in a small, convenient MS10 package in order to improve efficiency, reduce the size and weight of the power supply, and reduce the total component and manufacturing cost. nents of EMI in a switcher, thus easily reducing output noise. DC/DC converters can now be used in application areas such as precision instrumentation and noise-sensitive wideband communication equipment without the uncertain noise implications associated with normal converter design solutions. for the latest information on LTC products, visit www.linear-tech.com Linear Technology Magazine • September 2001