Choose a higher value for R1 than the one calculated in
the previous equation if a longer startup time can be
tolerated to minimize power loss on this resistor.
The above startup method is applicable to a circuit simi-
lar to the one shown in Figure 1. In this circuit, the ter-
tiary winding has the same phase as the output
windings. Thus, the voltage on the tertiary winding at any
given time is proportional to the output voltage and goes
through the same soft-start period as the output voltage.
The minimum discharge time of C1 from 21.6V to 9.74V
must be greater than the soft-start time of 5.6ms.
Another method for bootstrapping the power supply is
to use a bias winding that is in-phase with the MOSFET
on-time (see Figure 5). In this case, the amount of
capacitance required at IN (C1) is much smaller.
However, the input voltage cannot have a range
greater than approximately 2:1 (primary-winding volt-
age to bias-winding voltage ratio).
For hiccup-mode fault protection, make the bias wind-
ing in-phase with the output, then the power-supply hic-
cups and soft-starts under output short-circuit
conditions. The power supply does not hiccup if the
bias winding is in-phase with the MOSFET on-time.
MAX17499/MAX17500
Current-Mode PWM Controllers with
Programmable Switching Frequency
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