Datasheet

MAX5921/MAX5939
-48V Hot-Swap Controllers with External
R
SENSE
and High Gate Pulldown Current
14 ______________________________________________________________________________________
Component Selection Procedure:
• Determine load capacitance:
C
L
= C2 + C3 + module input capacitance
• Determine load current, I
LOAD
.
• Select circuit-breaker current, for example:
I
CB
= 2 x I
LOAD
• Calculate R
SENSE
:
Realize that I
CB
varies ±20% due to trip-voltage tol-
erance.
• Set allowable inrush current:
• Determine value of C2:
• Calculate value of C1:
• Determine value of R3:
•Set R2 = 10Ω.
• If an optocoupler is utilized as in Figure 14, deter-
mine the LED series resistor:
Although the suggested optocoupler is not specified for
operation below 5mA, its performance is adequate for
36V temporary low-line voltage where LED current
would then be ≈2.2mA to 3.7mA. If R7 is set as high as
51kΩ, optocoupler operation should be verified over
the expected temperature and input voltage range to
ensure suitable operation when LED current ≈0.9mA for
48V input and ≈0.7mA for 36V input.
If input transients are expected to momentarily raise the
input voltage to >100V, select an input transient-voltage-
suppression diode (TVS) to limit maximum voltage on the
MAX5921/MAX5939 to less than 100V. A suitable device
is the Diodes Inc. SMAT70A telecom-specific TVS.
Select Q1 to meet supply voltage, load current, efficien-
cy, and Q1 package power-dissipation requirements:
BV
DSS
≥ 100V
I
D(ON)
≥ 3x I
LOAD
DPAK, D
2
PAK, or TO-220AB
R
VV
ImA
IN NOMINAL
LED
7
2
35
()
=
−
≤≤
R
s
C
3
150
2
=
µ
CCCx
VV
V
gd
IN MAX GS TH
GS TH
12=+
−






()
() ()
()
C
AxC
I
L
INRUSH
2
45
=
µ
Ix
mV
R
Ior
II xI
INRUSH
SENSE
LOAD
INRUSH LOAD CB MIN
≤−
+≤
08
40
08
.
.
()
R
mV
I
SENSE
CB
=
50
MAX5921A
MAX5921E
MAX5939A
MAX5939E
V
EE
OV
UV
SENSE GATE DRAIN
PWRGD
V
DD
GND
-48V
C3
100µF
100V
C2
15nF
100V
R3
1kΩ
5%
R2
10Ω
5%
Q1
IRF530
R1
0.02Ω
5%
R4
549kΩ
1%
R5
6.49kΩ
1%
R6
10kΩ
1%
PWRGD
*DIODES INC. SMAT70A
R7
51kΩ
5%
-48V RTN
(SHORT PIN)
*
Figure 13. Using PWRGD to Drive an Optoisolator