4
dc1886af
DEMO MANUAL DC1886A
Quick start proceDure
Inrush into capacitive load: One of the main functions of
a hot swap circuit is to provide controlled ramp-up into a
capacitive load to avoid disturbing the input power supply.
For the following tests, ensure that the AUTORETRY jumper
is set to OFF and the TIMER DURATION jumper is set to
12ms. To guarantee that current limit is never reached
during ramp-up into a capacitor, the current must be less
than the minimum current limit threshold of 1.2A when
the gate ramp rate is at its maximum value of 0.55V/ms.
(Note that the minimum current limit threshold occurs
when the FB pin is at zero.)
C = I/(dV/dt) = 1.2A/(0.55V/ms) = 2182µF
Thus, the circuit will always power up successfully with a
2000µF capacitor at V
OUT
, and observation of the TIMER
turret post will show that it never begins to ramp. Figure 2
shows successful ramp-up into a 1200µF capacitor. Inrush
current is determined by ramp rate alone.
It is more complicated to find the smallest capacitor that is
guaranteed to prevent successful ramp-up, since the cur-
rent into the load capacitor is a function of the scaled output
voltage at the FB pin. As a starting point, the minimum
ramp rate of 0.15V/ms should produce a current greater
than the maximum current limit threshold of 1.8A (when
FB is at zero) to ensure that the timer begins to ramp as
soon as the output begins to rise.
C = I/(dV/dt) = 1.8A/(0.15V/ms) = 12000µF
Figure 2. Successful Ramp-Up into 1200µF
I
OUT
1A/DIV
V
OUT
5V/DIV
TIMER
1V/DIV
