Datasheet

UCC284–5, UCC284–12, UCC284–ADJ, UCC384–5, UCC384–12, UCC384–ADJ
LOW-DROPOUT 0.5-A NEGATIVE LINEAR REGULATOR
SLUS234D – JANUARY 2000 – REVISED FEBRUARY 2002
13
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
VIN to VOUT Delay
Figure 11
VIN TO VOUT DELAY TIME
WITH QUICK START-UP CIRCUIT
Figure 12
VIN TO VOUT DELAY TIME
WITH CT CAPACITOR REMOVED
operation of the quick start-up circuit
During normal start-up, the UCC384 does not turn on until the voltage on the SD/CT pin reaches approximately
–1.6 V with respect to ground. It takes a certain amount of time for the CT capacitor to charge to this point. For
a circuit that has a very large load, the CT capacitor needs to be large in order for the overcurrent timing to work
properly. A large value of capacitance on the SD/CT pin increases the VIN to VOUT delay time.
The quick start-up circuit uses Q1 to quickly pull the SD/CT pin in a negative direction during start-up, thus
decreasing the VIN-to-VOUT delay time. When VIN is applied to the circuit, Q1 turns on and starts to charge
the CT capacitor. The current pulled through R4 determines the rate at which CT is charged. R4 can be
calculated as follows:
R4 +
V
IN
(V) T
D
seconds
1.6 CT (F)
ohms
(6)
t
D
is the approximate VIN-to-VOUT delay time desired.
Q1 needs to be turned off after a fixed time to prevent the SD/CT pin from going too far negative with respect
to GND. If the SD/CT pin is allowed to go too far negative with respect to GND, the output turns off again or
possibly even damages the SD/CT pin. The maximum amount of time that Q1 should be allowed to be on is
referred to as t
M
and can be calculated as follows:
t
M
+
2.6
1.6
t
D
seconds
(7)
R3 along with C2 set the time that Q1 is allowed to be on. Since t
M
is the maximum amount of time that Q1 should
be allowed to stay on, an added safety margin may be to use 0.9 × t
M
instead. This ensures that Q1 is turned
off in the proper amount of time. With a chosen value for C2, R3 can be calculated as follows: