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LT1158IN Datasheet(PDF) 10 Page - Linear Technology |
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LT1158IN Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LT1158 10 Figure 3. Adding Synchronous Switching to a Step-Down Switching Regulator Ugly Transient Issues In PWM applications the drain current of the top MOSFET is a square wave at the input frequency and duty cycle. To prevent large voltage transients at the top drain, a low ESR electrolytic capacitor must be used and returned to the power ground. The capacitor is generally in the range of 250 µF to 5000µF and must be physically sized for the RMS current flowing in the drain to prevent heating and premature failure. In addition, the LT1158 requires a separate 10 µF capacitor connected closely between pins 2 and 7. The LT1158 top source and sense pins are internally protected against transients below ground and above supply. However, the gate drive pins cannot be forced below ground. In most applications, negative transients coupled from the source to the gate of the top MOSFET do not cause any problems. However, in some high current (10A and above) motor control applications, negative transients on the top gate drive may cause early tripping of the current limit. A small Schottky diode (BAT85) from pin 15 to ground avoids this problem. Switching Regulator Applications The LT1158 is ideal as a synchronous switch driver to improve the efficiency of step-down (buck) switching regulators. Most step-down regulators use a high current Schottky diode to conduct the inductor current when the switch is off. The fractions of the oscillator period that the switch is on (switch conducting) and off (diode conduct- ing) are given by: SWITCH “ON” = V V TOTAL PERIOD SWITCH “OFF” = VV V TOTAL PERIOD OUT IN IN OUT IN × − × Note that for VIN > 2VOUT, the switch is off longer than it is on, making the diode losses more significant than the switch. The worst case for the diode is during a short circuit, when VOUT approaches zero and the diode con- ducts the short-circuit current almost continuosly. Figure 3 shows the LT1158 used to synchronously drive a pair of power MOSFETs in a step-down regulator applica- tion, where the top MOSFET is the switch and the bottom MOSFET replaces the Schottky diode. Since both conduc- tion paths have low losses, this approach can result in very high efficiency –– from 90% to 95% in most applications. And for regulators under 5A, using low RDS(ON) N-channel MOSFETs eliminates the need for heatsinks. VOUT T GATE DR T GATE FB T SOURCE SENSE+ SENSE– B GATE DR B GATE FB FAULT INPUT LT1158 + REF PWM RSENSE RGS + VIN 1158 F03 APPLICATIONS INFORMATION |
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