LT1158
APPLICATIONS INFORMATION
100
Current Limit in Switching Regulator Applications
Current is sensed by the LT1158 by measuring the voltage
acrossacurrentshunt(lowvaluedresistor).Normally,this
shunt is placed in the source lead of the top MOSFET (see
Short-Circuit Protection in Bridge Applications). However,
in step-down switching regulator applications, the remote
current sensing capability of the LT1158 allows the actual
inductor current to be sensed. This is done by placing
the shunt in the output lead of the inductor as shown in
Figure 3. Routing of the SENSE and SENSE PC traces
is critical to prevent stray pickup. These traces must be
routed together at minimum spacing and use a Kelvin
connection at the shunt.
90
FIGURE 12 CIRCUIT
IN
V
= 12V
80
70
60
+
–
1.5 2.0 2.5
OUTPUT CURRENT (A)
3.5
4.0
0
0.5 1.0
3.0
LT1158 F04
Figure 4. Typical Efficiency Curve for Step-Down
Regulator with Synchronous Switch
When the voltage across R
exceeds 110mV, the
SENSE
One fundamental difference in the operation of a step-
down regulator with synchronous switching is that it
never becomes discontinuous at light loads. The induc-
tor current doesn’t stop ramping down when it reaches
zero, but actually reverses polarity resulting in a constant
ripple current independent of load. This does not cause
any efficiency loss as might be expected, since the nega-
LT1158FAULTpinbeginstoconduct.ByfeedingtheFAULT
signal back to a control input of the PWM, the LT1158 will
assume control of the duty cycle forming a true current
mode loop to limit the output current:
110mV
RSENSE
IOUT
=
in current limit
tive inductor current is returned to V when the switch
IN
In LT3525 based circuits, connecting the FAULT pin to
the LT3525 soft-start pin accomplishes this function. In
circuitswheretheLT1158inputisbeingdrivenwitharamp
or sawtooth, the FAULT pin is used to pull down the DC
level of the input.
turns back on.
The LT1158 performs the synchronous MOSFET drive
and current sense functions in a step-down switching
regulator. A reference and PWM are required to complete
the regulator. Any voltage-mode PWM controller may be
used, but the LT3525 is particularly well suited to high
power, high efficiency applications such as the 10A circuit
shown in Figure 13. In higher current regulators a small
SchottkydiodeacrossthebottomMOSFEThelpstoreduce
reverse-recovery switching losses.
The constant off-time circuits shown in Figures 10 and 12
are unique in that they also use the current sense during
normal operation. The LT1431 output reduces the normal
LT1158 110mV fault conduction threshold such that the
FAULT pin conducts at the required load current, thus
discharging the input ramp capacitor. In current limit the
LT1431 output turns off, allowing the fault conduction
threshold to reach its normal value.
The LT1158 input pin can also be driven directly with a
ramp or sawtooth. In this case, the DC level of the input
waveform relative to the 1.4V threshold sets the LT1158
duty cycle. In the 5V to 3.3V converter circuit shown in
Figure11,anLT1431controlstheDClevelofatrianglewave
generated by a CMOS 555. The Figure 10 and 12 circuits
use an RC network to ramp the LT1158 input back up to
its 1.4V threshold following each switch cycle, setting a
constant off time. Figure 4 shows the efficiency vs output
TheresistorR showninFigure3isnecessarytoprevent
GS
output voltage overshoot due to charge coupled into the
gate of the top MOSFET by a large start-up dv/dt on V .
IN
If DC operation of the top MOSFET is required, R must
GS
be 330k or greater to prevent loading the charge pump.
current for the Figure 12 regulator with V = 12V.
IN
1158fb
12
LINEAR_DIMENSIONS [ Linear Dimensions ]
