AS3843N13 datasheet(17/20 Pages) ETC

17 / 20 page

ASTEC Semiconductor

The current sense comparatorÕs inverting input is

internally clamped to a level of 1.0 V to provide a

current limit (or power limit for multiple output

supplies) function. The value of R

S is selected to

produce 1.0 V at the maximum allowed current.

For example, if 1.5 A is the maximum allowed

peak inductor current, then R

S is selected to

equal 1 V/1.5 A = 0.66 ½. In high power applica-

tions, power dissipation in the current sense

resistor may become intolerable. In such a case,

a current transformer can be used to step down

the current seen by the sense resistor. See

Figure 21.

1.6 Output (OUT)

The output stage of the AS3842 is a high current

totem-pole configuration that is well suited for

directly driving power MOSFETs. It is capable of

sourcing and sinking up to 1 A of peak current.

Cross conduction losses in the output stage have

been minimized resulting in lower power dissipa-

tion in the device. This is particularly important

for high frequency operation. During under-

voltage shutdown conditions, the output is active

low. This eliminates the need for an external pull-

down resistor.

1.7 Over-temperature shutdown

The AS3842 has a built-in over-temperature

shutdown which will limit the die temperature to

130¡C typically. When the over-temperature con-

dition is reached, the oscillator is disabled. All

other circuit blocks remain operational. There-

fore, when the oscillator stops running, output

pulses terminate without losing control of the

supply or losing any peripheral functions that

may be running off the 5 V regulator. The output

may go high during the final cycle, but the PWM

AS384x

Current Mode Controller

5 V REG

COMP

AS3842/3/4/5

ERROR AMP

2.5 V

VFB

1 V

RT/CT

Leading Edge Filter

CLOCK

PWM

COMPARATOR

VREG

VCC

OUTPUT

GND

CURRENT

SENSE

CLOCK

OUTPUT

PRI

SEC

VIN

PWM LOGIC

Figure 20. Current Sense/PWM Latch Circuit and Waveforms

VS =

N:1

Figure 21. Optional Current Transformer

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