Datasheet

LTC4252B-1/LTC4252B-2
LTC4252C-1/LTC4252C-2
15
4252b12f
APPLICATIONS INFORMATION
and R2 = 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 74.4V. 1% divider
resistors are recommended to preserve threshold accuracy.
The R1-R2 divider values shown in the Typical Application
set a standing current of slightly more than 100µA and
define an impedance at UV/OV of 30kΩ. In most applica-
tions, 30kΩ impedance
coupled with 300mV UV hysteresis
makes the LTC4252B insensitive to noise. If more noise
immunity is desired, add a 1nF to 10nF filter capacitor
from UV/OV to V
EE
.
Separate UV and OV pins are available in the 10-pin MS
package and can be used for a different operating range
such as 35.5V to 76V as shown in Figure 3. Other combi-
nations are possible with
different resistor arrangements.
UV/OV COMPARATORS (LTC4252C)
A UV hysteretic comparator detects undervoltage condi-
tions at the UV pin, with the following thresholds:
UV low-to-high (V
UV
) = 3.08V
UV high-to-low (V
UV
– V
UVHST
) = 2.756V
An OV hysteretic comparator detects overvoltage condi-
tions at the OV pin, with the following thresholds:
OV low-to-high (V
OV
) = 5.09V
OV high-to-low (V
OV
– V
OVHST
) = 4.988V
The UV and OV trip point ratio is designed to match the
standard telecom operating range of 43V to 71V when
connected together as in Figure 2. A divider (R1, R2) is
used to scale the supply voltage. Using R1 = 390k and R2
= 30.1k gives a typical operating range of 43V to 71V. The
undervoltage shutdown and overvoltage recovery thresh
-
olds are then 38.5V and 69.6V respectively. 1% divider
resistors are recommended to preserve threshold accuracy.
The R1-R2 divider values shown in Figure 2 set a standing
current of slightly more than 100µA and define an impedance
at UV/OV of 28kΩ. In most applications, 28kΩ impedance
coupled with 324mV UV hysteresis makes the LTC4252C
insensitive to noise. If more noise immunity is desired, add
a 1nF
to 10nF filter capacitor from UV/OV to V
EE
.
The UV and OV pins can be used for a wider operat-
ing range such as 35.5V to 76V as shown in Figure 4.
Other combinations are possible with different resistor
arrangements.
UV/OV OPERATION
A low input to the UV comparator will reset the chip and pull
the GATE and TIMER pins low. A low-to-high
UV transition
will initiate an initial timing sequence if the other interlock
4252B12 F04
–48RTN
UV
OV
V
EE
V
IN
SENSESS
TIMER GATE
PWRGD
DRAIN
LTC4252C-1
R1
464k
1%
R3
34k
1%
R2
10k
1%
C
T
0.68µF
C
SS
68nF
C
C
10nF
–48V
R
S
0.02Ω
Q1
IRF530S
R
C
10Ω
R5
2.2k
Q2
R
IN
10k
1/2W
1
9
8
10
3
2
7
6
4
5
C2
10nF
C
IN
1µF
C
L
100µF
–48RTN
(SHORT PIN)
+
R
D
1M
LOAD
EN
R4
22k
*
* M0C207
Q2: MMBT5551LT1
**DIODES, INC
†
RECOMMENDED FOR HARSH ENVIRONMENTS
D
IN
†
DDZ13B
**
Figure 4. –48V/2.5A Application with Wider Input Operating Range