IR3831MPBF datasheet(15/31 Pages) IRF

15 / 31 page

11/05/10

IR3831MPbF

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 IR3831, the typical

minimum on-time is specified as 50 ns.

Any design or application using the IR3831 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 IR3831, the

following condition must be satisfied:

The minimum output voltage for the IR3831 is

limited to V

out(min) = 0.6 V.

Furthermore, for the IR3831, especially for active

bus termination applications, it is strongly

recommended to use a switching frequency of

400 kHz to obtain clean and jitter free operation

in sourcing as well as sinking modes. Therefore,

the maximum input voltage that may be stepped

down to 0.6V at 400 kHz without jitter or pulse

skipping is 15 V.

V/s

100

0.6

(min)

≤

∴

≤

∴

out

Maximum Duty Ratio Considerations

A fixed off-time of 200 ns maximum is specified

for the IR3831. 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 IR3831 can operate. To allow some

margin, the maximum operating duty ratio in any

application

using

the

IR3831

should

still

accommodate about 250 ns off-time. Fig 9.

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

switching frequency, with 250 ns off-time.

out

Vin ×

(min)

out

≤

∴

≤

∴

≤

Fig. 9. Maximum duty cycle v/s switching

frequency.

M ax Duty Cycle

250

450

650

850

1050

1250

1450

1650

S w itching Frequency (kH z )

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