MCR03EZPFX4021 datasheet(18/35 Pages) IRF

18 / 35 page

Rev 4.0

PD-97516

IR3853MPbF

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 IR3853, the typical

minimum on-time is specified as 50 ns.

Any design or application using the IR3853 must

ensure operation with a pulse width that is higher

than this minimum on-time and preferably higher

than 100 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 IR3853, 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 21V and minimum output voltage, the

converter should be designed at a switching

frequency that does not exceed 333 kHz.

Conversely, for operation at the maximum

recommended operating frequency 1.65 MHz

and minimum output voltage, any voltage above

4.2 V may not be stepped down without pulse-

skipping.

V/s

100

0.7

≤

∴

≤

∴

(min)

out

Maximum Duty Ratio Considerations

A fixed off-time of 200 ns maximum is specified

for the IR3853. 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 IR3853 can operate. To allow a margin

of 50ns, the maximum operating duty ratio in any

application

using

the

IR3853

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. 11. Maximum duty cycle v/s switching

frequency.

M a x Duty Cycle

250

450

650

850

1050

1250

1450

1650

Sw itching Frequency (kH z )

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