MB39A105 Datasheet, PDF (11/27 Pages) Fujitsu Component Limited. – 1-ch DC/DC Converter IC for low voltage

MB39A105
2. Power Control Function
A switch in series with a resistor connected with the RT terminal (pin 7) allows you to turn on or turn off the power.
On/off setting conditions of power supply
CTL
Power
L
OFF (standby)
H
ON (operating)
RT
ON/OFF CTL
(L : OFF, H : ON)
3. Protective Functions
(1) Timer-latch short-circuit protection circuit (SCP)
Short-circuit detection comparator detects the error amplifier output voltage level. If the load conditions for the
DC/DC converter are stable, the short-circuit protection comparator is kept in equilibrium condition because the
error amplifier is free from output variation. At this time the CSCP terminal (pin 2) is held at the soft-start end
voltage (about 0.8 V) . If the DC/DC converter output voltage falls and error amplifier output is over 0.9 V, the
timer circuits are actuated to start charging the external capacitor CSCP.
When the capacitor voltage reaches about 1.0 V, the latch is set and the circuit is turned off the external FET
and sets the dead time to 100 %. At this time, latch input is closed and the CSCP terminal is held at the “L” level.
To reset the actuated protection circuit, turn off and on the power supply again and set VCC terminal voltage
(pin 3) to 1.1 V (Min) or less. (See sSETTING TIME CONSTANT FOR TIMER-LATCH SHORT-CIRCUIT PRO-
TECTION CIRCUIT.)
(2) Under voltage lockout protection circuit (UVLO)
The transient state or a momentary decrease in supply voltage, which occurs when the power supply is turned
on, may cause the IC to malfunction, resulting in breakdown or degradation of the system. To prevent such
malfunctions, under voltage lockout protection circuit detects a decrease in internal reference voltage with respect
to the power supply voltage, turns off the output FET, and sets the dead time to 100% while holding the CSCP
terminal (pin 2) at the “L” level.
The circuit restores the output transistor to normal when the supply voltage reaches the threshold voltage of the
undervoltage lockout protection circuit.
(3) Short-circuit protection detection output function
Connecting the Pch MOS FET to SCPOD terminal (pin 4) turns off the Pch MOS FET when the short-circuit
protection is detected or under voltage lockout protection circuit operate. This allows you to prevent the short-
circuit between the input and output when the short-circuit protection is detected, thus preventing the input
voltage from occurring in the output region in the standby state.
(4) Protection circuit operating function table
This table refers to output condition when protection circuit is operating.
Operating circuit
SCPOD
OUT
Short-circuit protection circuit
High-Z
L
Under voltage lockout protection circuit
High-Z
L
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