AN8015SH Datasheet, PDF (12/14 Pages) Panasonic Semiconductor – Single-channel step-down, step-up, or inverting use DC-DC converter control IC

AN8015SH
Voltage Regulators
s Application Notes (continued)
[5] Time constant setting method for short-circuit protection circuit with joint-use of soft start/timer latch type
(continued)
When the external capacitor C_S has been charged up to approximately 1.83 V, it sets the latch circuit, cuts off the
output drive transistor by enabling the low input voltage malfunction prevention circuit, and sets the dead-time at 100%.
The low input voltage malfunction prevention circuit is once enabled, the S.C.P. terminal voltage is discharged to
approximately 30 mV but the latch circuit is not reset unless the power is turned off.
The S.C.P. terminal voltage is inputted to the PWM comparator, and the soft start is actuated when the power turns
on. However, since at the beginning of power on, the error amplifier output is high level, and V_PE of S.C.P. terminal
voltage is released and charging begins; the external capacitor should be set so that the output voltage of DC-DC
convertor starts to rise before the latching circuit is set. The charging equation is as same as the above. The soft start
time t_SS until the input standby voltage V_STBY becomes 1.25 V is given in the following equations:
1.25 V = 0.03 V + I_CHG ×
t_SS
C_S
t_SS = 1.22 ×
C_S
I_CHG
[s]
I_CHG
1.25 V
Clamp
circuit
IN+ ^{8 Error amp.}
IN− ⁹
S.C.P. comp.
Q1
Q2
FB ¹⁰
1.83 V
SR
Latch
V_REF
PWM comparator input
R
U.V.L.O.
Output cut-off
C_S
Figure 6. Short-circuit protection circuit
12