mb3881pff Fujitsu Microelectronics, Inc., mb3881pff Datasheet - Page 29

• Manufacturer Part Number: mb3881pff
• Description: 8-ch Dc/dc Converter Ic With Synchronous Rectifier For Voltage Step-up And Step-down
• Manufacturer: Fujitsu Microelectronics, Inc.

SETTING THE DEAD TIME
When the device is set for step-up inverted output based on the step-up or step-up/down Zeta method or flyback
method, the FB pin voltage may reach and exceed the rectangular wave voltage due to load fluctuation. If this
is the case, the output transistor is fixed to a full-ON state (ON duty = 100%). To prevent this, set the maximum
duty of the output transistor. To set it, set the voltage at the DTC1 terminal (pin 48) by applying a resistive voltage
divider to the VREF voltage as shown below.
When the voltage at the DTC1 terminal (pin 48) is higher than the triangular wave voltage (CT1), the output
transistor is turned on. The maximum duty calculation formula assuming that triangular wave amplitude : = 0.73
V and triangular wave minimum voltage : = 1.0 V is given below. (Same to other channels.)
When the DTC1 terminal (pin 48) is not used, connect it directly to the VREF terminal (pin 24) as shown below.
(Same to other channels.)
DUTY (ON) max : =
When using DTC to set dead time
When no soft start time is set
Vdt
0.73 V
1.0 V
100[%], Vdt
( Same to other channels.)
VREF
DTC1
(Same to other channels.)
VREF
DTC1
Ra
24
48
Rb
Rb
24
48
Ra
Rb
VREF
Vdt
( CH 1)
( CH1)
MB3881
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