SL40307A-R19KHF datasheet(28/53 Pages) IRF

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IR3846

28 www.irf.com

August 01, 2013







, with body braking

(3)





, without body braking

(4)

= Inductor current

= Forward voltage drop of the body diode of

the Sync FET.

= output voltage

= Inductor value

The Body Braking mechanism is kept OFF during pre-

bias operation. Also, in the event of an extremely

severe load step-down transient causing OVP, the

Body Brake is overridden by the OVP latch, which

turns on the Sync FET.

MINIMUM ON TIME CONSIDERATIONS

The minimum ON time is the shortest amount of time

for Ctrl FET to be reliably turned on. This is very

critical parameter for low duty cycle, high frequency

applications. Conventional approach limits the pulse

width to prevent noise, jitter and pulse skipping. This

results to lower closed loop bandwidth.

IR has developed a proprietary scheme to improve

and enhance minimum pulse width which utilizes the

benefits of voltage mode control scheme with higher

switching frequency, wider conversion ratio and higher

closed loop bandwidth, the latter results in reduction

of output capacitors. Any design or application using

IR3846 must ensure operation with a pulse width that

is higher than the minimum on-time. 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.

out





(5)

In any application that uses IR3846, the following

condition must be satisfied:



(min)

(6)

out





(min)

(7)

(min)

out







(8)

The minimum output voltage is limited by the

reference voltage and hence Vout(min) = 0.6V.

Therefore, for Vout(min) = 0.6V,

(min)

out







(9)











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 571 kHz. Conversely,

for

operation

the

maximum

recommended

operating frequency (1.5 MHz) and minimum output

voltage (0.6V). The input voltage (PVin) should not

exceed 8V, otherwise pulse skipping may happen.

MAXIMUM DUTY RATIO

A certain off-time is specified for IR3846. This

provides an upper limit on the operating duty ratio at

any given switching frequency. The off-time remains

at a relatively fixed ratio to switching period in low and

mid frequency range, while in high frequency range

this ratio increases, thus the lower the maximum duty

ratio at which IR3846 can operate. Figure 30 shows a

plot of the maximum duty ratio vs. the switching

frequency with built in input voltage feed forward

mechanism.

Figure 30: Maximum duty cycle vs. switching

frequency

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