mb3881pff Fujitsu Microelectronics, Inc., mb3881pff Datasheet - Page 27

• 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.

Note: If the external synchronous pulse is not input, the device is started with free-run oscillation.
SETTING FOR EXTERNAL SYNCHRONOUS OSCILLATION
For external synchronous operation, connect the timing capacitor (C
resistor (R
In this case, select the C
external synchronous signal excluding the setting error of the oscillation frequency.
The duty cycle (T1/T) of the external synchronous signal must be set within a range from 10% to 90%.
Triangular wave oscillator (OSC) equivalent circuit
Free-run oscillation
For free-run oscillation, set the SYNC terminal (pin 35) to “Lo” or “HiZ” level.
The external synchronization circuit starts operation after a VREF rise.
The CT pin oscillation frequency at startup is 500 kHz when the voltage at the VB terminal (pin 36) is 2 V
with CT = 100 pF and RT = 11 k .
If the triangular wave has superimposed noise during external synchronous oscillation, insert a CR filter.
0.8 V
V
0.3 V
5.0 V
V
0 V
CT
SYNC
T
) to the RT terminal (pin 33).
CT
T
and R
SYNC
CT
34
35
T
so that the oscillation frequency is 5% to 10% lower than the frequency of the
External synchronization circuit
VB
100 k
“H” level: ON
I
1.5 I
: Proportional to V
0.8 V
0.3 V
1.4 V
t
External synchronous oscillation
0.8 V
V
0.3 V
5.0 V
V
0 V
CT
SYNC
RT
/R
T1
T
T
) to the CT terminal (pin 34) and the timing
T
S
R
S
R
Latch1
Latch2
Q
Q
MB3881
t
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