MB3769A Datasheet, PDF (9/26 Pages) Fujitsu Component Limited. – SWITCHING REGULATOR CONTROLLER

MB3769A
s FUNCTIONS
1. Error Amplifier
The error amplifier detects the output voltage of the switching regulator.
The error amplifier uses a high-speed operational amplifier with an 8 MHz bandwidth (typical) and 6 V/ms slew rate (typical).
For ease of use, the common mode input voltage ranges from -0.2 V to VCC-3 V. Figure 4 shows the equivalent circuit.
Fig. 4 - MB3769A Equivalent Circuit Differential Amp.
VCC
VREF
-IN (OP)
+IN (OP)
GND
150 Ω
700 µA
To PWM
Comp.
Protection element
2. Overcurrent Detection Comparator
There are two methods for protection of the output transistor of this device from overcurrents; one restricts the transistor’s on-
time if an overcurrent that flows through the output transistor is detected from an average output current, and the other detects
an overcurrent in the external transistor (FET) and shuts the output down instantaneously. Using average output currents, the
peak current of the external transistor (FET) cannot be detected, so an output transistor with a large safe operation area (SOA)
margin is required.
For the method of detecting overcurrents in the external transistor (FET), the output transistor can be protected against a shorted
filter capacitor or power-on surge current.
The MB3769A uses dynamic current limiting to detect overcurrents in the output transistor (FET). A high-speed comparator
and flip-flop are built-in.
To detect overcurrents, compare the voltage at +IN(C) of current detection resistor connected the source of the output transistor
(FET), with the reference voltage (connected to -IN(C) ) using a comparator. To prevent output oscillation during overcurrent,
flip-flop circuit protects against double pulses occurring within a cycle.
The output of overcurrent detector is ORed with other signals at the PWM comparator. See the example Application Example
for details on use.
Figure 5 shows the equivalent circuit of the over-current detection comparator.
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