MB3881 Datasheet, PDF (27/40 Pages) Fujitsu Component Limited. – 8-ch DC/DC Converter IC with Synchronous Rectifier for Voltage Step-up and Step-down

MB3881
s SETTING FOR EXTERNAL SYNCHRONOUS OSCILLATION
For external synchronous operation, connect the timing capacitor (CT) to the CT terminal (pin 34) and the timing
resistor (RT) to the RT terminal (pin 33).
In this case, select the CT and RT so that the oscillation frequency is 5% to 10% lower than the frequency of the
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
CT
34
CT
VB
−
I
0.8 V
+
−
0.3 V
+
Latch1
SQ
R
SYNC
35
“H” level: ON
1.5 I
Ι: Proportional to VRT/RT
+
100 kΩ 1.4 V
−
External synchronization circuit
Latch2
SQ
R
Free-run oscillation
0.8 V
VCT
External synchronous oscillation
0.8 V
VCT
0.3 V
5.0 V
VSYNC
0.3 V
5.0 V
VSYNC
0V
0V
t
T1
t
T
Note: If the external synchronous pulse is not input, the device is started with 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 kΩ.
If the triangular wave has superimposed noise during external synchronous oscillation, insert a CR filter.
27