Datasheet

Since the minimum intended V
IN
for the application will
result in the lowest I
TEST
during the load-probe test, this
V
IN,MIN
should be used to set the R
LP
. This voltage will
likely be near V
ON,FALLING
or V
OFF
for the application.
R
TEST
(Ω) = V
IN,MIN
/ I
TEST
= V
IN,MIN
x 90ms /
(C
LOAD(MAX)
x 220mV)
Example: V
IN
operating range = 36V to 72V, C
LOAD
=
10,000µF. First, find the R
LP
that will guarantee a suc-
cessful test of the load.
R
LP
= 36V x 90ms / (10,000µF x 220mV) = 1.472Ω⇒
1.47kΩ±1%
Next, evaluate the R
LP
at the maximum operating volt-
age to verify that it will not exceed the 1A current limit
for the load-probe test.
I
TEST,MAX
= V
IN,MAX
/ R
LP
= 72V / 1.47kΩ = 49.0mA
If the C
LOAD(MAX)
is increased to 190,000µF, the test
current will approach the limit. In this case, R
LP
will be
a much lower value and must include the internal
switch resistance. To find the external series resistor
value that will guarantee a successful test at the lowest
supply voltage, the maximum value for the load-probe
switch on-resistance of 11Ω should be used:
R
LP,TOT
= 36V x 90ms / (190,000µF x 220mV) =
90Ω = 11Ω + R
LP
R
LP
= 77.51Ω - 11Ω = 66.51Ω⇒66.5Ω±1%
Again R
LP
must be evaluated at the maximum operat-
ing voltage to verify that it will not exceed the 1A cur-
rent limit for the load-probe test. In this case, the
minimum value for the load-probe switch on-resistance
of 6Ω should be used:
I
TEST,MAX
= V
IN,MAX
/ R
LP,TOT
= 72V / (66.5Ω + 6Ω) =
993mA
Adjusting the V
OUT
Slew Rate
The default slew rate is set internally for 9V/ms. The slew
rate can be reduced by placing an external capacitor
from the drain of the power MOSFET to the GATE output
of the MAX5938. Figure 18 shows a graph of Slew Rate
vs. C
SLEW
. This graph shows that for C
SLEW
< 4700pF
there is very little effect to the addition of external slew-
rate control capacitance. This is intended so the GATE
output can drive large MOSFETs with significant gate
capacitance and still achieve the default slew rate. To
select a slew-rate control capacitor, go into the graph
with the desired slew rate and find the value of the Miller
Capacitance. When C
SLEW
> 4700pF, SR and C
SLEW
are inversely related. Given the desired slew rate, the
required C
SLEW
is found as follows:
C
SLEW
(nF) = 23 / SR (V/ms)
From the data sheet of the power MOSFET find the
reverse transfer capacitance (gate-to-drain capacitance)
above 10V. If the reverse transfer capacitance of the
external power MOSFET is 5% or more of C
SLEW
, then it
should be subtracted from C
SLEW
in the equation above.
Figure 19 gives an example of the external circuit for con-
trolling slew rate. Depending on the parasitics associated
with the selected power MOSFET, the addition of C
SLEW
may lead to oscillation while the MOSFET and GATE con-
MAX5938
-48V Hot-Swap Controller with V
IN
Step Immunity,
No R
SENSE
, and Overvoltage Protection
______________________________________________________________________________________ 19
Figure 18. MAX5938 Slew Rate vs. C
SLEW
SLEW RATE vs. C
SLEW
C
SLEW
(nF)
SLEW RATE (V/ms)
100101
0.1
1
10
0.01
0.1 1000
Figure 19. Adjusting the MAX5938 Slew Rate
MAX5938
V
OUT
C
LOAD
LOAD
GATEV
EE
GND
C
SLEW
R
GATE
-48V
GND