MB3778 Datasheet, PDF (15/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3778
s SETTING THE IDLE PERIOD
When voltage step-up, fly-back step-up or inverted output are set, the voltage at the FB terminal may go higher
than the triangular wave voltage due to load fluctuation, etc. In this case the output transistor will be in full-on
state(ON duty 100%). This can be prevented by setting the maximum duty for the output transistor. This is done
by setting the DTC1 terminal (pin 6) voltage using resistance division of the VREF voltage as illustrated below.
When the DTC1 terminal voltage is higher than the triangular waveform voltage, the output transistor is turned
on. If the triangular waveform amplitude specified by the maximum duty calculation formula is 0.6 V, and the
lower voltage limit of the triangular waveform is 1.3 V, the formula would be as follows (other channels are similar) :
Duty (ON) max (%) =: (Vdt − 1.3 V) / 0.6 V × 100, Vdt (V) = Rb / (Ra + Rb) × VREF
Also, if no output duty setting is required, the voltage should be set greater than the upper limit voltage of the
triangular waveform, which is 1.9 V.
• Setting the idle time at DTC1 (DTC2 is similar)
VREF 16
DTC1 ⁶
Ra
Rb
Vdt
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