MB3769A
4. Triangular Wave Oscillator
The oscillation frequency is expressed by the following formula:
1
0.8 x C
T
x R
T
+ 0.0002 ms
[kHz] C
T
:µF
R
T
:kΩ
f
OSC
~
For master/slave synchronized operation of several MB3769As, the C
T
and R
T
terminals of the master MB3769A are connected
in the usual way and the C
T
terminals of the master and slave device (s) are connected together. The slave MB3769A’s R
T
terminal is connected to it’s V
REF
terminal to disable the slave’s oscillator. In this case, set 50/n kΩ (n is the number of master
and slave ICs) to the upper limit of R
T
so that internal bias currents do not stop the master oscillation.
Fig. 7 - MB3769A Synchronized Operation
master
R
T
C
T
V
REF
slave
R
T
C
T
5. Overvoltage Detector
The overvoltage detection circuit shuts the system power down if the switching regulator’s output voltage is abnormal or if
abnormal voltage is appeared. The reference voltage is 2.5 V (V
REF
/2). The system power is shut down if the voltage at pin 13
rises above 2.5 V. The output is kept shut down by the latching circuit until the power supply is turned off (see Figure 3).
6. Stand-by Mode and Under-Voltage Lockout (UVLO)
Generally, V
GS
> 6 to 8 V is required to use power MOSFET for switching. UVLO is set so that output is on at V
CC
≥
10 V
(standard) when the power is turned on and is off at V
CC
≤
8 V (standard) when the power is turned off.
In the stand-by mode, the power supply current is limited to 2 mA or less when the output is inhibited by the UVLO circuit. When
the MB3769A is operated from the 100 VAC line, the power supply current is supplied through resistor R (Figure 8). That is, the
IC power supply current is supplied by the AC line through resistor R until operation starts. Current is then supplied from the
transformer tertiary winding, eliminating the need for a second power supply.
Two volts (typical) of hysteresis are provided for return from operation mode to stand-by mode not to return to stand-by mode
until output power is turned on or to avoid malfunction due to noise.
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FUJITSU [ FUJITSU COMPONENT LIMITED. ]
