LM3101M datasheet(14/20 Pages) NSC

14 / 20 page

Typical Applications (Continued)

The duty cycle will reach the RDL limit for several cycles

letting energy build up in the transformersee the drain cur-

rent waveform in

Figure 9 When the residual energy builds

up enough the duty cycle starts to decrease because it is

now determined by the CLIM circuit A voltage of 038V or

greater at this pin will toggle a pulse-by-pulse comparator

on every cycle (see the LM3001 block diagram) In the ap-

plication circuit a 0167X resistor will generate the current

limit threshold voltage when a 228A (peak) current flows

through it With the CLIM circuit in control of the duty cycle

the duty cycle will decrease with each successive cycle

The duty cycle will continue to shrink until the pulse feed-

back from the LM3101 takes control

As the LM3001 switches the Power MOSFET on and off

the Power Transformer starts delivering power to the sec-

ondary side of the circuit This action will cause the supply

voltage of the LM3101 and the output voltage to gradually

rise When the supply voltage reaches the Undervoltage

Lockout Threshold (of 39V) the LM3101 starts supplying a

pulse train to the differentiator circuit on pin 8 The resulting

PWM signals are fed back to the LM3001 via the pulse

transformer The first pulse signal to the LM3001 will cause

it to disconnect its internal oscillator from its PWM and Out-

put Driver circuits and trigger the Output Driver from the

pulse feedback signals (of the LM3101) At this point con-

trol of the frequency and the duty cycle changes from the

LM3001 to the LM3101

The LM3101 also exercises Soft-Start capability (pin 3) An

RC network connected to this pin allows the LM3101 to

gradually increase the duty cycle to its nominal value (in the

example the secondary Soft-Start time delay is 500 ms ap-

proximately)

The method of Soft-Start used by the LM3101 ensures that

the error amplifier is in its linear region before the output

voltage reaches its nominal value thus yielding a smooth

start-up of the output without any overshoot (see

Figure 10 )

(Representative not to scale)

TLH11436 – 15

FIGURE 9 Start-Up Timing Sequence

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