MB3886 Fujitsu Microelectronics, Inc., MB3886 Datasheet - Page 13

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MB3886

Manufacturer Part Number
MB3886
Description
2-channel Dc/dc Converter Ic With Overcurrent Protection Symmetrical-phase Type
Manufacturer
Fujitsu Microelectronics, Inc.
Datasheet
2. Channel Control Function
3. Protective Functions
(1) Timer-Latch Short-Circuit Protection Circuit Block
(2) Undervoltage Lockout Circuit Block
(3) Overcurrent Protection Circuit
(4) Overvoltage Protection Circuit (OVP)
Channels are turned on and off depending on the voltage levels at the CTL1 terminal (pin 2) and CTL2 terminal
(pin 29).
The short-circuit detection comparator (SCP Comp.) provided for the two channels detects the output voltage
level and, if either channel output voltage falls below the short-circuit detection voltage, the timer circuit is actuated
to start charging the external capacitor Cscp connected to the CSCP terminal (pin 25) .
When the capacitor voltage reaches about 0.68 V, the circuit turns off the output transistor and sets the dead
time to 100 .
Once the protection circuit is actuated, it can be reset by turning the power supply off and on again. (See
“SETTING THE TIME CONSTANT FOR TIMER-LATCH SHORT-CIRCUIT PROTECTION CIRCUIT”.)
The transient state or a momentary drops in supply voltage, which occurs when the power supply is turned on,
may cause the control IC to malfunction, resulting in breakdown or degradation of the system. To prevent such
malfunctions, the undervoltage lockout circuit detects the internal reference voltage level with respect to the
power supply voltage, turns off the output transistor, and sets the dead time to 100
terminal (pin 25) at the “L” level.
System operation is restored when the supply voltage reaches the threshold voltage of the undervoltage lockout
circuit.
The overcurrent protection circuit is actuated upon completion of the soft-start period. When an overcurrent
flows, the circuit detects the increase in the voltage between the main-side FET’s drain and source by the main-
side FET ON resistor and sets the latch to fix the main and synchronous rectification outputs of the relevant
channel at the “L” level. The detection current value can be set by resistor R
side FET’s drain and the ILIM1 terminal (pin 3) and resistor R
pin (pin 28) . (See “SETTING THE OVERCURRENT DETECTION CURRENT”.)
When the overvoltage detection comparator (OVP Comp.) provided for each channel detects the output voltage
level exceeding its threshold voltage, the overvoltage protection circuit sets the latch to fix only the main output
of the relevant channel at the “L” level and the synchronous-rectification output at the “H” level. (See “SETTING
THE OVERVOLTAGE DETECTION VOLTAGE”.)
CTL1
H
H
L
L
CTL terminal voltage level
Channel On/Off Setting Conditions
CTL2
H
H
L
L
LIM2
CH1
OFF
OFF
ON
ON
connected between the drain and the ILIM2
Channel output state
LIM1
connected between the main-
while holding the CSCP
MB3886
CH2
OFF
OFF
ON
ON
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