LT1158CS View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1158CS' PDF : 20 Pages View PDF
LT1158
APPLICATIONS INFORMATION
100
90
FIGURE 12 CIRCUIT
VIN = 12V
80
70
60
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
LT1158 F04
Figure 4. Typical Efficiency Curve for Step-Down
Regulator with Synchronous Switch
One fundamental difference in the operation of a step-
down regulator with synchronous switching is that it never
becomes discontinuous at light loads. The inductor cur-
rent 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 negative
inductor current is returned to VIN when the switch turns
back on.
The LT1158 performs the synchronous MOSFET drive and
current sense functions in a step-down switching regula-
tor. 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
Schottky diode across the bottom MOSFET helps to re-
duce reverse-recovery switching losses.
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
Figure 11, an LT1431 controls the DC level of a triangle
wave 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 current for the Figure 12 regulator
with VIN = 12V.
Current Limit in Switching Regulator Applications
Current is sensed by the LT1158 by measuring the voltage
across a current shunt (low valued resistor). 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.
When the voltage across RSENSE exceeds 110mV, the
LT1158 fault pin begins to conduct. By feeding the fault
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:
IOUT
=
110mV
RSENSE
in current limit
In LT3525 based circuits, connecting the fault pin to the
LT3525 soft-start pin accomplishes this function. In cir-
cuits where the LT1158 input is being driven with a ramp
or sawtooth, the fault pin is used to pull down the DC level
of the input.
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 resistor RGS shown in Figure 3 is necessary to prevent
output voltage overshoot due to charge coupled into the
gate of the top MOSFET by a large start-up dv/dt on VIN. If
DC operation of the top MOSFET is required, RGS must be
330k or greater to prevent loading the charge pump.
11
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