Datasheet

17
LTC1645
1645fa
APPLICATIO S I FOR ATIO
WUUU
Using the LTC1645 as a Linear Regulator
This application uses the LTC1645 to Hot Swap one
primary supply and generate a secondary low dropout
regulated supply. Figure 19 shows how to switch a 5V
supply and create a 3.3V supply using the spare compara-
tor and one additional transistor. The COMP
+
pin is used
to monitor the 3.3V output. As the voltage on the gate of
Q2 increases, the 3.3V output increases. At the 3.3V
threshold the spare comparator trips. The COMPOUT pin
goes high which turns on Q3. This lowers the voltage on
the gate of Q2. This feedback loop is compensated by
capacitors C1 and C2 and resistor R1. When power is first
applied, the FB pin is low and RESET holds one side of C2
low, slowing the ramp-up of V
OUT2
. As V
OUT2
exceeds
2.75V, RESET releases to allow improved loop transient
response. Figure 20 shows the load transient response
and voltage ripple of the generated supply.
Q1
IRF7413
0.01Ω*
0.01Ω*
Q2
IRFZ24
10Ω
10k
10Ω
R1
200k
1M
Q3
PN2222
1.5k
1%
12.1k
1%
10k
1%
2.49k
1%
C2
0.1µF
25V
C1
0.033µF
0.1µF
25V
0.33µF
470µF**
6V
×2
C
LOAD1
12
1
2
3
8
9
6
BOTH CURRENT LIMITS: 5A
5
11
7
1314
10
4
V
CC1
ON
FAULT
SENSE1
TIMER GND
GATE2
V
CC2
V
IN
5V
V
OUT2
3.3V
2.5A
V
OUT1
5V
2.5A
LTC1645
(14-LEAD)
COMP
+
COMPOUT
FB
SENSE2GATE1
1645 F19
RESET
+
+
LRF1206-01-R010-J (IRC)
T510X477K006AS (KEMET)
*
**
Figure 19. Switching 5V and Generating 3.3V
V
OUT2
0.1V/DIV
I
OUT2
1A/DIV
2.5A
0.5A
Figure 20. Load Transient Response and Voltage Ripple