FUJITSU MB39A104 - Manual (Page 17)

MB39A104
17
■ ABOUT TIMER-LATCH PROTECTION CIRCUIT
1. Setting Timer-Latch Overcurrent Protection 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 FET’s drain and source using the external FET
ON resistor (R
ON
), 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, the circuit sets the
latch to fix OUT terminals (pin 3, 22) at “H” level and turn off the external FET. The detection current value can
be set by the resistors (R
LIM1
and R
LIM2
) connected between the FET’s drain and the ILIM1 terminal (pin 5) and
between the drain and the ILIM2 terminal (pin 20), respectively.
The internal current (I
LIM
) can be set by the timing resistor (R
T
) connected to the RT terminal (pin 12).
Time until activating timer circuit and setting latch is equal to short-circuit detection time in "2. Setting Time
Constant for Timer-Latch Short-Circuit Protection Circuit".
Internal current value: I
LIM
Detection current value: I
OCP
R
LIM
: Overcurrent detection resistor
R
ON
: External FET ON resistor
V
IN
: Input voltage
V
O
: DC/DC converter output voltage
f
OSC
: 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 24) to the "L" level to lower the VREF terminal (pin 17) voltage to 2.4 V (Min) or less.
I
LIM
(
µA) :=
2700
R
T
(k
Ω)
I
OCP
(A) :=
I
LIM
(A)
× R
LIM
(
Ω)
R
ON
( Ω)
(V
IN
(V)
− V
O
(V))
× V
O
(V)
2 × V
IN
(V) × f
OSC
(Hz) × L(H)
−
−
+
8
20
4
21
CSCP
Current
Protection
Logic
(1 µA)
VREF
UVLO
S
Latch
R
ILIM1
(R LIM )
(ILIM2)
VS1
(VS2)
V IN
Q1 L
5
V O
Each
Channel
Drive
• Overcurrent detection circuit