LT1158
APPLICATIONS INFORMATION
Ugly Transient Issues
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:
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 250μF 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 10μF capacitor connected closely between pins
2 and 7.
⎛
⎞
VOUT
SWITCH“ON”=
• TOTALPERIOD
⎜
⎟
V
⎝
⎠
IN
⎛
⎞
V − VOUT
IN
SWITCH“OFF”=
• TOTALPERIOD
⎜
⎟
V
⎝
⎠
IN
Note that for V > 2V , the switch is off longer than it
IN
OUT
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 cur-
rent (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.
is on, making the diode losses more significant than the
switch. The worst case for the diode is during a short cir-
cuit, when V
approaches zero and the diode conducts
OUT
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-
tionpathshavelowlosses,thisapproachcanresultinvery
high efficiency—from 90% to 95% in most applications.
Switching Regulator Applications
And for regulators under 5A, using low R
N-channel
DS(ON)
MOSFETs eliminates the need for heatsinks.
The LT1158 is ideal as a synchronous switch driver to
improve the efficiency of step-down (buck) switching
V
IN
+
T GATE DR
T GATE FB
R
GS
R
SENSE
V
OUT
T SOURCE
+
LT1158
+
SENSE
SENSE
–
FAULT
REF
PWM
B GATE DR
B GATE FB
INPUT
1158 F03
Figure 3. Adding Synchronous Switching to a Step-Down Switching Regulator
1158fb
11
LINEAR_DIMENSIONS [ Linear Dimensions ]
