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Applications Information
Sense Resistor
The circuit-breaker current-limit threshold is set to 50mV
(typically). Select a sense resistor that causes a drop
equal to or above the current-limit threshold at a current
level above the maximum normal operating current.
Typically, set the overload current to 1.5 to 2.0 times the
nominal load current plus the load-capacitance charging
current during startup. Choose the sense resistor power
rating to be greater than (V
CL
)
2
/ R
SENSE
.
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
tolerance.
Set allowable inrush current:
Determine value of C2:
Calculate value of C1:
Determine value of R3:
If an optocoupler is utilized as in Figure 14, deter-
mine the LED series resistor:
V
mA
3
≤
Set R2 = 10
.
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
51k
, 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-volt-
age-suppression diode (TVS) to limit maximum voltage
on the MAX5920 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)
≥
3 x I
LOAD
DPAK, D
2
PAK, or TO-220AB
The lowest practical R
DS(ON)
, within budget constraints
and with values from 14m
to 540m
, are available at
100V breakdown.
Ensure that the temperature rise of Q1 junction is not
excessive at normal load current for the package select-
ed. Ensure that I
CB
current during voltage transients
does not exceed allowable transient-safe operating-area
limitations. This is determined from the SOA and tran-
sient-thermal-resistance curves in the Q1 manufacturer
’
s
data sheet.
Example 1:
I
LOAD
= 2.5A, efficiency = 98%, then V
DS
= 0.96V is
acceptable, or R
DS(ON)
≤
384m
at operating temper-
ature is acceptable. An IRL520NS 100V NMOS with
R
DS(ON)
≤
180m
and I
D(ON)
= 10A is available in
D
2
PAK. (A Vishay Siliconix SUD40N10-25 100V NMOS
with R
DS(ON)
≤
25m
and I
D(ON)
= 40A is available in
DPAK, but may be more costly because of a larger die
size).
Using the IRL520NS, V
DS
≤
0.625V even at +80
°
C so
efficiency
≥
98.6% at 80
°
C. P
D
≤
1.56W and junction
temperature rise above case temperature would be 5
°
C
due to the package
θ
JC
= 3.1
°
C/W thermal resistance.
Of course, using the SUD40N10-25 would yield an effi-
ciency greater than 99.8% to compensate for the
increased cost.
R
V
I
mA
5
IN NOMINAL
(
LED
7
2
)
≤
=
R
s
C
typically k
(
3
2
150
1
≤
μ
)
C
C
(
C
x
V
V
V
gd
IN MAX
(
GS TH
(
GS TH
(
1
2
=
+
)
)
)
)
C
A x C
μ
I
L
INRUSH
2
45
=
I
x
mV
R
I
or
I
I
x I
INRUSH
SENSE
≤
LOAD
INRUSH
LOAD
CB MIN
(
≤
+
0 8
.
40
0 8
.
)
R
mV
I
SENSE
CB
=
50
M
-48V Hot-Swap Controller
with External R
SENSE
______________________________________________________________________________________
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