LT1158CS datasheet(11/20 Pages) LINER

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LT1158

OUTPUT CURRENT (A)

4.0

LT1158 F04

1.0

2.0

3.0

100

0.5

1.5

2.5

3.5

FIGURE 12 CIRCUIT

VIN = 12V

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

in current limit

SENSE

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.

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

Figure 4. Typical Efficiency Curve for Step-Down

Regulator with Synchronous Switch

APPLICATIONS INFORMATION

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