AS2845D-8 datasheet(17/20 Pages) ETC

17 / 20 page

ASTEC Semiconductor

Current Mode Controller

AS2842/3/4/5

Figure 21. Optional Current Transformer

The current sense comparator’s 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 RS 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 RS is selected to

equal 1 V/1.5 A = 0.66

Ω. 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 AS2842 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 undervoltage

shutdown conditions, the output is active low.

This eliminates the need for an external pulldown

resistor.

1.7 Over-temperature shutdown

The AS2842 has a built-in over-temperature

shutdown which will limit the die temperature to

130

° C typically. When the over-temperature

condition 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

Figure 20. Current Sense/PWN Latch Circuit and Waveforms

VS =

( )

N:1

–

5 V REG

COMP

AS2842/3/4/5

ERROR AMP

2.5 V

VFB

1 V

RT/CT

Leadong Edge Filter

CLOCK

PWM

COMPARATOR

VREG

VCC

OUTPUT

GND

CURRENT

SENSE

CLOCK

OUTPUT

PRI

SEC

VIN

PWM LOGIC

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