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









×

−









×

on, making the diode losses more significant than the

switch. The worst case for the diode is during a short

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.

MOSFETs eliminates the need for heatsinks.

T GATE DR

T GATE FB

T SOURCE

B GATE DR

B GATE FB

FAULT

INPUT

LT1158

+

REF

PWM

+

1158 F03