FUJITSU MB39A104 - Manual (Page 12)

MB39A104
12
2. Control Function
When CTL terminal (pin 24) is “L” level, IC becomes the standby mode. The power supply current is 10 µA (Max)
at the standby mode.
On/Off Setting Conditions
3. Protective Functions
(1) Timer-latch overcurrent protection circuit block (OCP)
The timer-latch 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 FET’s drain and source using the
external FET ON resistor, actuates the timer circuit, and starts charging the capacitor C
SCP
connected to the
CSCP terminal (pin 8). If the overcurrent remains flowing beyond the predetermined period of time, latch is set
and OUT terminals (pin 3,22) of each channel are fixed at “H” level. And the circuit sets the latch to turn off the
external FET. The detection current value can be set by resistor R
LIM1
connected between the FET’s drain and
the ILIM1 terminal (pin 5) and resistor R
LIM2
connected between the drain and the ILIM2 terminal (pin 20).
Changing connection enables to detect overcurrent at current sense resistor.
To reset the actuated protection circuit, either the power supply turn off and on again or set the CTL terminal
(pin 6) to the “L” level to lower the VREF terminal (pin 17) voltage to 2.4 V (Min) or less. (Refer to “1. Setting
Timer-Latch Overcurrent Protection Detection Current” in “
■ABOUT TIMER-LATCH PROTECTION CIRCUIT”.)
(2) Timer-latch short-circuit protection circuit (SCP Logic, SCP Comp.)
The short-circuit detection comparator (SCP Comp.) detects the output voltage level of Error Amp, and if the
error amp output voltage of any channel falls below the short-circuit detection voltage (3.1 V Typ), the timer
circuits are actuated to start charging the external capacitor C
SCP
connected to the CSCP terminal (pin 8).
When the capacitor voltage reaches about 0.73 V, the circuit is turned off the output transistor and sets the dead
time to 100 %.
To reset the actuated protection circuit, either the power supply turn off and on again or set the CTL terminal
(pin 24) to the “L” level to lower the VREF terminal (pin 17) voltage to 2.4 V (Min) or less. (Refer to “2. Setting
Time Constant for Timer-Latch Short-Circuit Protection Circuit” in “
■ABOUT TIMER-LATCH PROTECTION
CIRCUIT”.)
(3) 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 transistor, and sets the dead time to 100% while holding the
CSCP terminal (pin 8) 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.
(4) Protection circuit operating function table
This table refers to output condition when protection circuit is operating.
CTL Power
L OFF (Standby)
H ON (Operating)
Operating circuit CS1 CS2 OUT1 OUT2
Overcurrent protection circuit L L H H
Short-circuit protection circuit L L H H
Under-voltage lockout L L H H