MCR03EZPFX1002 datasheet(17/33 Pages) IRF

17 / 33 page

Rev 16.0

PD-97509

IR3843AMPbF

300

400

500

600

700

800

900

1000

1100

1200

Switching Frequency (kHz)

Minimum on time Considerations

The minimum ON time is the shortest amount of

time for which the Control FET may be reliably

turned on, and this depends on the internal

timing delays. For the IR3843A, the typical

minimum on-time is specified as 100 ns.

Any design or application using the IR3843A

must ensure operation with a pulse width that is

higher than this minimum on-time and preferably

higher than 150 ns. This is necessary for the

circuit to operate without jitter and pulse-

skipping, which can cause high inductor current

ripple and high output voltage ripple.

In any application that uses the IR3843A, the

following condition must be satisfied:

The minimum output voltage is limited by the

reference voltage and hence V

out(min) = 0.7 V.

Therefore, for V

out(min) = 0.7 V,

Therefore, even at the minimum switching

frequency 250 kHz, no input voltage higher than

18.5V may be reliably converted down to 0.7V

without excessive jitter or pulse skipping over the

complete load range.

In general, practical considerations dictate that

any application that demands a pulse width

smaller than 175ns may not exhibit jitter free

operation over the entire load range.

V/s

150

0.7

(min)

≤

∴

≤

∴

out

Maximum Duty Ratio Considerations

A fixed off-time of 200 ns maximum is specified

for the IR3843A. This provides an upper limit on

the operating duty ratio at any given switching

frequency. It is clear that, higher the switching

frequency, the lower is the maximum duty ratio at

which the IR3843A can operate. To allow a

margin of 50ns, the maximum operating duty

ratio in any application using the IR3843A should

still accommodate about 250 ns off-time. Fig 10.

shows a plot of the maximum duty ratio v/s the

switching frequency, with 250 ns off-time.

out

Vin ×

(min)

out

≤

∴

≤

∴

≤

Fig. 10. Maximum duty cycle v/s switching

frequency.

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