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LT1737IGN データシート(PDF) 24 Page - Linear Technology |
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LT1737IGN データシート(HTML) 24 Page - Linear Technology |
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24 / 28 page  24 LT1737 1737f TYPICAL APPLICATIO S R3 and D2. (Diode D2 prevents reverse emitter/base breakdown in Q1 when MOSFET M1 is in the “ON” state.) The raw flyback voltage at the drain of MOSFET M1 minus the VBE of Q1 is converted to a current by R3 and then back to a voltage at R4. Or, stated mathematically: VV V R R FB FLBK BE = () – 4 3 Resistor R13 provides an initial pre-load to the supply output to improve light load regulation. Resistor divider R8/R9 sets the undervoltage lockout threshold at nomi- nally 10.4V for turn-on, with turn-off about 600mV lower. Overall power supply efficiency and output regulation versus input voltage and load current may be seen in Figures 13 and 14. 5VIN APPLICATION The LT1737 is a bipolar technology IC specified to operate down to a minimum input supply voltage of 4.5V. Although its GATE pin drives “low” nearly to ground, its “high” capability is limited by a headroom requirement of roughly 2VBEs. Thus when operating at a worst case 4.5V supply, the GATE output will only drive up to a nominal 3V or so. Fortunately, MOSFETs are now available with specified performance at this level of gate voltage. The circuit shown in Figure 15 provides an isolated 5V output from an input between 4.5V and 5.5V. Two Si9804 low gate voltage MOSFETs are paralleled to handle the primary-side current—up to 12A peak. This circuit pro- vides more output power than the previous examples. It PGND ISENSE GATE VC FB R1 0.02 Ω M1 SI9804 10 4 9 T1 COILTRONICS VP5-0083 VIN 3 VCC UVLO 3VOUT SGND 1737 F15 RCMPC ROCMP MINENAB LT1737 R6 51k 5% R5 51k 5% C5 47pF 50V NPO 6 8 7 3 910 15 16 2 14 13 12 4 5 11 1 ENDLY SFST OSCAP tON C3 1nF 25V X7R R7 2.2k 5% R8 10 Ω 5% D4 1N5240 D3 MBR0520 R4 75k 5% C1A-B, C2A-B: SANYO POSCAP (10TPB220M) D1: MOTOROLA 35V, 8A SCHOTTKY DIODE D2: SIGNAL DIODE D3: MOTOROLA 20V, 0.5A SCHOTTKY DIODE D4: 10V, 500mW ZENER DIODE M1, M2: SILICONIX/VISHAY 25V, 0.023 Ω N-CH MOSFET R1: 5 × 0.10Ω, 1W (IRC LR2512) T1: COILTRONICS TRANSFORMER R3 3.01k 1% C8 1 µF 25V Z5U C4 0.1 µF 25V Z5U C9 1nF 25V X7R R11 51 Ω 5% 11 2 12 1 M2 SI9804 C6 4.7nF 50V X7R C7 2.2nF 50V X7R • • • • 5 8 • 6 7 • D1 MBRD835L C2A 220 µF 10V + C2B 220 µF 10V + R10 270 Ω 5% VOUT 5V 3A R9 15 Ω 5% D2 BAS16 Q1 2N3906 R2 11.0k 1% C1A 220 µF 10V + C1B 220 µF 10V + Figure 15. 4.5V-5.5V to Isolated 5V Converter |
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