LTC4253/LTC4253A
15
425353aff
For more information www.linear.com/4253
APPLICATIONS INFORMATION
An OV hysteretic comparator detects overvoltage condi-
tions at the OV pin, with the following thresholds:
OV low-to-high (V
OVHI
) = 6.150V
OV high-to-low (V
OVLO
) = 5.850V
The UV and OV trip point ratio is designed to match the
standard telecom operating range of 43V to 82V when
connected together as in the Typical Application. A resistive
divider is used to scale the supply voltage. Using 402k and
32.4k gives a typical operating range of 43.2V to 82.5V .
The undervoltage shutdown and overvoltage recovery
thresholds are then 39.2V and 78.4V . 1% divider resistors
are recommended to preserve threshold accuracy.
The resistive divider values shown set a standing current
of slightly more than 100礎 and define an impedance at
UV/OV of 30k? In most applications, 30k?impedance
coupled with 300mV UV hysteresis make the LTC4253
insensitive to noise. If more noise immunity is desired,
add a 1nF to 10nF filter capacitor from UV/OV to V
EE
.
pulse. To protect V
IN
against damage from higher am-
plitude spikes, clamp V
IN
to V
EE
with a 13V Zener diode.
Star connect V
EE
and all V
EE
-referred components to the
sense resistor Kelvin terminal as illustrated in Figure 3,
keeping trace lengths between V
IN
, C
IN
, D
IN
and V
EE
as
short as possible.
INTERNAL UNDERVOLTAGE LOCKOUT (UVLO)
A hysteretic comparator, UVLO, monitors V
IN
for undervolt-
age. The thresholds are defined by V
LKO
and its hysteresis
V
LKH
. When V
IN
rises above V
LKO
, the chip is enabled;
below (V
LKO
V
LKH
), it is disabled and GATE is pulled low.
The UVLO function at V
IN
should not be confused with the
UV and OV pins. These are completely separate functions.
UV/OV COMPARATORS (LTC4253)
A UV hysteretic comparator detects undervoltage condi-
tions at the UV pin, with the following thresholds:
UV low-to-high (V
UVHI
) = 3.225V
UV high-to-low (V
UVLO
) = 2.925V
Figure 3. 48V/2.5A Application for the LTC4253A
(Refer to Block Diagram)
R5
2.2k
R4
2.2k
R6
2.2k
PWRGD1
V
IN
V
EE
LTC4253A
POWER
MODULE 1
PWRGD2
PWRGD3
EN3
EN2
UV
OV
RESET
DRAIN
SS
GATE
SQTIMER
SENSE
TIMER
EN
POWER
MODULE 2
EN
POWER
MODULE 3
EN
R
IN
10k
20k(1/4W)/2
PUSH
RESET
EN3
4253 F03
EN2
R
C
10
R
D
1M
R
S
0.02
Q1
IRF530S
Q2
FZT857
V
IN1
C
C
10nF
C
T
0.68礔
C1
10nF
C
SQ
0.1礔
C
SS
33nF
R7
POWER
MODULE 1
OUTPUT
POWER
MODULE 2
OUTPUT
V
IN1
R8
+
C
L
100礔
C
IN
1礔
R1
30.1k
1%
R2
392k
1%
R9
47k
R3 22k
V
IN1
48V RTN
(SHORT PIN)
48V RTN
(LONG PIN)
48V
(LONG PIN)
D
IN
DDZ13B*
*DIODES, INC.
MOC207
RECOMMENDED FOR HARSH ENVIRONMENTS.
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