MB39A106 Datasheet, PDF (19/46 Pages) Fujitsu Component Limited. – 2-Channel DC/DC Converter IC with Overcurrent Protection Symmetrical-Phase Type

MB39A106
■ ABOUT TIMER-LATCH PROTECTION CIRCUIT
1. Setting Timer-Latch Overcurrent Detection Current
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 using the
main-side FET ON resistor (RON), actuates the timer circuit, and starts charging the capacitor CSCP connected
to the CSCP terminal (pin 11). If the overcurrent remains flowing beyond the predetermined period of time, the
circuit sets the latch to turn off the FETs on the main side and synchronous rectification side of each channel
while setting the PWRGOOD terminal (pin 12) to the "L" level. The detection current value can be set by the
resistors (RLIM1 and RLIM2) connected between the main-side FET’s drain and the ILIM1 terminal (pin 25) and
between the drain and the ILIM2 terminal (pin 23), respectively.
The internal current (ILIM) can be set by the timing resistor (RT) connected to the RT terminal (pin 5).
Internal current value: ILIM
ILIM (µA) =:
5546
RT (kΩ)
Detection current value: IOCP
IOCP (A) =:
ILIM(A) × RLIM(Ω)
RON (Ω)
−
(VIN(V) − VO(V)) × VO(V)
2 × VIN(V) × fOSC(Hz) × L(H)
RLIM: Overcurrent detection resistor
RON: Main-side FET ON resistor
VIN: Input voltage
VO: DC/DC converter output voltage
fOSC: Oscillation frequency
L: Coil inductance
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 8) voltage to 1.7 V (Min) or less. It can also be reset by
setting both of the CTL1 terminal (pin 9) and CTL2 terminal (pin 10) to the "L" level.
VIN
M1
(VS2)
18 _VS1
28
Current
Protection
Logic
118 µA
(ILIM2)
23 _ILIM1
25
CSCP
11
10 µA
CTL
CTL1
CTL2
SR
Latch
Latch2
VREF
UVLO
PWRGOOD
12
<Overcurrent detection circuit>
19