MCR03EZPFX1002 datasheet(17/34 Pages) IRF

17 / 34 page

Rev 13.0

PD-97507

IR3843WMPbF

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 IR3843W, the typical

minimum on-time is specified as 100 ns.

Any design or application using the IR3843W

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 IR3843W, 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, at the maximum recommended input

voltage 16V and minimum output voltage, the

converter should be designed at a switching

frequency that does not exceed 290 kHz.

However, 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. This means

that for 16 V input voltage, the operation

frequency should be limited to 250 kHz.

V/s

150

0.7

(min)

≤

∴

≤

∴

out

Maximum Duty Ratio Considerations

A fixed off-time of 200 ns maximum is specified

for the IR3843W. 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 IR3843W can operate. To allow a

margin of 50ns, the maximum operating duty

ratio in any application using the IR3843W

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.

M ax Duty Cycle

250

450

650

850

1050

1250

1450

1650

Sw itching Frequency (kH z )

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