MB3785A_03 Datasheet, PDF (7/30 Pages) Fujitsu Component Limited. – Switching Regulator Controller (4 Channels plus High-Precision, High-Frequency Capabilities)

MB3785A
2. Channel Control Function
The MB3785A allows the four channels of power supply circuits to be controlled independently. Set the voltage
levels on the CTL1 (pin 20), CTL2 (pin 21), and CTL3 (pin 22) terminals to turn the circuit of each channel “ON”
or “OFF”, as listed below.
Table 1 Channel by Channel On/Off Setting Conditions.
CTL pin voltage level
On/Off state of channel
CTL1
CTL2
CTL3
Power supply
circuit
CH1
CH2
CH3 and CH4
H
H
L
H
ON
H
L
L
ON
OFF
ON
OFF
ON
OFF
L
X
Standby state*
*: The power supply current value during standby is 10 µA or less.
3. Protective Functions
(1) Timer/latch-actuated short-circuiting protection circuit
The SCP comparator checks the output voltage of each comparator which is used to detect the short-circuiting
of output. When any of these comparators have an output voltage greater than or equal to 2.1 V, the timer circuit
is activated and a protection enable capacitor externally fitted to the SCP terminal (pin 23) begins to charge.
If the comparator’s output voltage is not restored to normal voltage level by the time the capacitor voltage has
risen to the base-emitter junction voltage of the transistor, i.e., VBE (=: 0.65 V), the latch circuit is activated to turn
off the output transistor while at the same time setting the duty (OFF) = 100 %.
When actuated, this protection circuit can be reset by turning on the power supply again.
(2) Under voltage lockout protection circuit
A transient state at power-on or a momentary drop of the power supply voltage causes the control IC to mal-
function, resulting in system breakdown or deterioration. By detecting the internal reference voltage with respect
to the power supply voltage, this protection circuit resets the latch circuit to turn off the output transistor and set
the duty (OFF) = 100 %, while at the same time holding the SCP terminal (pin 23) at the “L”. The reset is cleared
when the power supply voltage becomes greater than or equal to the threshold voltage level of this protection
circuit.
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