Datasheet

LTC4253B
30
4253bf
R6
5.6k
R5
5.6k
D1
BZV85C43
R7
5.6k
PWRGD1
V
IN
V
IN
V
EE
LTC4253B
POWER
MODULE 1
PWRGD2
PWRGD3
EN3
EN2
OV
UV
RESET
DRAINSS
GATESQTIMER
SENSETIMER
EN
POWER
MODULE 2
EN
POWER
MODULE 3
EN
R
IN
2.5k
15k(1/4W)/6
R3
36.5k
EN3
4253b F15
EN2
R
C
10Ω
R
D
1M
R
S
0.02Ω
Q1
IRF530S
V
IN
C
C
18nF
R4
22
C
T
0.33µF
C1
10nF
C
SQ
0.1µF
C
SS
68nF
R8
POWER
MODULE 1
OUTPUT
POWER
MODULE 2
OUTPUT
V
IN
R9
+
C3
0.1µF
C2
100µF
C
IN
1µF
R2
402k
1%
R1
32.4k
1%
†
†
† †
†
RESET
(LONG PIN)
–48V RTN
(SHORT PIN)
–48V RTN
(LONG PIN)
–48V
(LONG PIN)
D
IN
††
DDZ13B*
*DIODES, INC.
†
MOC207
††
RECOMMENDED FOR HARSH ENVIRONMENTS.
applicaTions inForMaTion
Power Limit Circuit Breaker
Figure 15 shows the LTC4253B in a power limit circuit
breaking application. The SENSE pin is modulated by board
voltage V
SUPPLY
. The D1 Zener voltage, V
Z
, is set to be the
same as the lowest operating voltage, V
SUPPLY(MIN)
=43V.
If the goal is to have the high supply operating voltage,
V
SUPPLY(MAX)
= 82V give the same power as available at
V
SUPPLY(MIN)
, then resistors R3 and R4 are selected by:
R4
R3
=
V
CB
V
SUPPLY(MAX)
(16)
If R4 is 22Ω, then R3 is 36.5k. The peak circuit breaker
power limit is:
POWER(MAX)=
V
SUPPLY(MIN)
+ V
SUPPLY(MAX)
( )
2
4• V
SUPPLY(MIN)
• V
SUPPLY(MAX)
•POWER AT V
SUPPLY(MIN)
=1.108V •POWER AT V
SUPPLY(MIN)
(17)
when V
SUPPLY
= 0.5 • (V
SUPPLY(MIN)
+ V
SUPPLY(MAX)
)
= 62.5V
The peak power at the fault current limit occurs at the supply
overvoltage threshold. The fault current limited power is:
POWER(FAULT)=
V
SUPPLY
( )
R
S
• V
ACL
−(V
SUPPLY
− V
Z
) •
R4
R3






(18)
Circuit Breaker with Foldback Current Limit
Figure 16 shows the LTC4253B in a foldback current limit
application. When V
OUT
is shorted to the –48V RTN supply,
current flows through resistors R3 and R4. This results in
a voltage drop across R4 and a corresponding reduction
in voltage drop across the sense resistor, R
S
, as the ACL
amplifier servos the sense voltage between the SENSE
and V
EE
pins to about 100mV. The short-circuit current
through RS reduces as the V
OUT
voltage increases during
an output short-circuit condition. Without foldback current
limiting resistor R4, the current is limited to 5A during
analog current limit. With R4, the short-circuit current is
limited to 1.5A when V
OUT
is shorted to 82V.
Figure 15. Power Limit Circuit Breaker Application