LT1158CS View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1158CS' PDF : 20 Pages View PDF
LT1158
APPLICATIONS INFORMATION
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
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.
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”
=
VOUT
VIN
×
TOTAL
PERIOD
SWITCH
“OFF”
=
VIN
− VOUT
VIN
×
TOTAL
PERIOD
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.
10
VIN
REF
PWM
T GATE DR
T GATE FB
RGS
T SOURCE
LT1158 SENSE+
FAULT
INPUT
SENSE–
B GATE DR
B GATE FB
RSENSE
VOUT
1158 F03
Figure 3. Adding Synchronous Switching to a Step-Down Switching Regulator
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