MB3785A Datasheet, PDF (21/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller (4 Channels plus High-Precision, High-Frequency Capabilities)

MB3785A
s METHOD OF SETTING THE DEAD TIME AND SOFT START
1. Dead Time
When the device is set for step-up inverted output based on the flyback method, the output transistor is fixed to a
full-on state (ON-duty = 100 %) at power switch-on. To prevent this problem, you may determine the voltages on
the DTC terminals (pins 4, 9, 28, and 33) from the VREF voltage so you can easily set the output transistor’s dead
time (maximum ON-duty) independently for each channel as shown below.
(1) CH1 and CH2 Channels
When the voltage on the DTC terminals (pins 4 and 9) is higher than the triangular-wave output voltage from the
oscillator, the output transistor turns off. The dead time calculation formula assuming that triangular-wave amplitude
≅ 0.6 V and triangular-wave minimum voltage ≅ 1.3 V is given below.
Duty (OFF) = ^{Vdt – 1.3} × 100 [%], Vdt =
R2
× VREF
0.6
R1 + R2
When you do not use these DTC terminals, connect them to GND.
Figure 9. When Using DTC to Set Dead Time Figure 10. When Not Using DTC
19 VREF
R1
DTC1
(DTC2)
Vdt
R2
DTC1
(DTC2)
(2) CH3 and CH4 Channels
When the voltage on the DTC terminals (pins 28 and 33) is lower than the triangular-wave output voltage from the
oscillator, the output transistor turns off. The dead time calculation formula assuming that traingular-wave amplitude
≅ 0.6 V and triangular-wave maximum voltage ≅ 1.9 V is given below.
Duty (OFF) ≅ ^{1.9 –Vdt} × 100 [%], Vdt =
R2
× VREF
0.6
R1 + R2
When you do not use these DTC terminals, connect them to VREF.
21