advertisement Monolithic DC/DC Converters Break 1MHz to Shrink Board Space – Design Note 125 Mitchell Lee In the never-ending quest for board space, operating frequency remains the most important variable in a DC/DC converter design. Higher frequency equates with smaller coils and capacitors. A new family of fast monolithic converters that allows circuit designers to reduce the size of their finished products is now available. Other improvements include quiescent currents well below those of slower converters and a new switchdrive technique that reduces dynamic losses by at least four fold over previous methods, virtually eliminating these losses as a concern in efficiency calculations. Table 1 shows the salient features of each member of the family. Each is configured as a grounded-switch step-up converter, but is equally useful in positive and negative high efficiency buck, SEPIC, inverting and flyback circuits. The converters are all based on the LT ®1372 design, which operates at 500kHz, draws 4mA quiescent current and contains a 1.5A, 0.5Ω switch. The LT1371 is designed for higher power applications, with a 3A, 0.25Ω switch. Supply current and operating frequency remain unchanged. Reduced quiescent current (1mA) makes the LT1373 useful in low power designs or in applications where the load current has a wide dynamic range. For the ultimate in miniaturization, the LT1377 features 1MHz operating frequency—especially helpful where post filtering is employed. Table 1. Family Characteristics DEVICE IQ SWITCH FREE RUNNING SYNCHRONIZATION FREQUENCY LIMIT LT1371 4mA 3A 500kHz 800kHz LT1372 4mA 1.5A 500kHz 800kHz LT1373 1mA 1.5A 250kHz 360kHz LT1377 4mA 1.5A 1MHz 1.6MHz All devices share the same constant-frequency PWM core, up to 90% duty cycle and 2.7V to 30V supply range with a maximum switch rating of 35V. Unique to these devices is a synchronization input that allows the internal oscillator to be overridden by an external clocking signal. The synchronization limit for each part is also shown in Table 1. Another unique feature is a second Feedback pin that allows direct regulation of negative outputs. A simple boost converter using the LT1372 is shown in Figure 1. 350mA output current is available at 12V from a 5V input. Adaptive switch drive and a 0.5Ω collector resistance result in a peak efficiency of 87%, as shown in Figure 2. Figure 3 shows a buck-boost (SEPIC) converter built around the 3-ampere LT1371. Inputs of 2.7V to 20V L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN 2.7V TO 11V 95 C1 47μF L1* 10μH VOUT = 12V OFF VIN S/S VSW D1 MBRS130T3 85 VOUT 12V LT1372 N/C 3 NFB VC FB GND 1 C2 47nF 4.7nF 8 6, 7 2 R2 10.7k R3 1.24k + C3 33μF w2 R1 2k 80 VIN = 5V 75 70 VIN = 3V 65 60 *SUMIDA CD73-100KC DN125 F01 Figure 1. Schematic Diagram: LT1372 Boost Converter 03/96/125_conv EFFICIENCY (%) + 5 ON 4 VIN = 9V 90 10 100 OUTPUT CURRENT (mA) 1000 DN125 F02 Figure 2. Efficiency of Boost Converter Shown in Figure 1 are converted to a 5V regulated output at up to 1.8A (see Figure 4). In spite of handling 9W output power, the 500kHz operating frequency of the LT1371 allows a 0.37-inch toroidal core to be used for the coupled inductor, with excellent efficiency. In shutdown, the output is completely disconnected from the input source. 250mA. This converter topology exhibits inherently low output ripple and noise and uses a single-core coupled inductor. Operating at 250kHz allows the use of relatively small filter components. The speed-to-power ratio of the LT1373 is quite high; with only 1mA quiescent current, it maintains higher efficiency at light loads. The latest generation of disk drives has adopted magneto-resistive (MR) read-write heads. These operate with a low noise bias supply of – 3V. Figure 5 shows a Cuk-configured LT1373 capable of generating –3V at At the other end of the spectrum is the 1MHz LT1377. It has the same high speed-to-power ratio as the LT1373. In Figure 6 the LT1377 is used as a 50mA charger for 4 to 6 NiCd cells, operating from a 5V input. The charger is clamped against excessive output voltage at 11V, and maintains constant output current from 0V to 11V. MBRS340T3 OUTPUT 5V • L1* 10μH + INPUT 2.7V TO 20V 100μF 20V OS-CON 33μF 20V OS-CON • VIN VSW S/S LT1371 NFB FB VC GND OFF ON + + NC 150μF 6.3V OS-CON 3.6k 1.2k 20k 47nF *HURRICANE ELECTRONICS LAB HL-8798 COILTRONICS CTX10-4 4.7nF 1MHz operation is also useful in radio applications where a 455kHz IF is present, as it gives one octave separation from that critical frequency. Figure 7 shows the LT1377 configured as a high efficiency buck converter, with a 5V, 1A output. A 20μH inductor is used in this application to maintain a low ripple current (10%), thus easing output filtering requirements. L1 4.7μH* DN125 F03 Figure 3. 5V, 9W Converter Operates Over Wide Input Range with Good Efficiency C3 100μF 16V 4 VIN = 5V C1 22μF 10V OUTPUT (mA) R2 2k + C2 47nF 1000 GND GND 6 7 C4 1nF Q2 Si9400DY R3 1k 50mA (11V MAX) *COILCRAFT DO-1608-472 DN125 F06 500 Figure 6. Battery Charger 0 0 10 5 8V TO 30V INPUT 20 15 INPUT (V) 3 • 4 C2 47μF 16V L1* VIN 5V •1 2 5 + 4 7 6 VSW VIN S/S LT1373 GND *SUMIDA CLS62-100L **MOTOROLA MBR0520LT3 NFB VC GND 4 VOUT –3V 250mA R1 1k 1% C6 0.1μF 8 3 1 + D1** + R4 5.1k C4 10nF R2 4.99k 1% NC + C1 2.2μF + D1 1N5818 DN125 F04 Figure 4. Maximum Available Output Current of LT1371 9W Converter (Figure 3) C1 22μF 10V VC Q1 2N3906 D2 10V 400mW VSW 2 FB LT1377 1 R1 12Ω + 8 5 VIN S/S 2000 1500 D1 MBR0520L 3 S/S 100μF 5 8 VIN VSW LT1377 NFB SG FB VC 6 PG 1 4.7nF 3.57k 1.24k 1N4148 10Ω 100nF 7 2k 47nF C3 47μF 16V 2 20μH* + *COILTRONICS CTX20-2P **SANYO VIDEO COMPONENTS MBRS130 5V 1A 150μF 6.3V OS-CON** DN125 F07 † PATENTS MAY APPLY DN125 F05 Figure 7. 1MHz LT1377-Based Buck Converter Figure 5. Low Ripple 5V to –3V “Cuk”† Converter Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn125f_conv LT/GP 0396 155K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1996