max5938 Maxim Integrated Products, Inc., max5938 Datasheet - Page 19

• Manufacturer Part Number: max5938
• Description: Max5938 -48v Hot-swap Controller With Vin Step Immunity, No Rsense, And Overvoltage Protection
• Manufacturer: Maxim Integrated Products, Inc.

Since the minimum intended V
result in the lowest I
V
likely be near V
Example: V
10,000µF. First, find the R
cessful test of the load.
Next, evaluate the R
age to verify that it will not exceed the 1A current limit
for the load-probe test.
If the C
current will approach the limit. In this case, R
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:
Again R
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:
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
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
are inversely related. Given the desired slew rate, the
required C
-48V Hot-Swap Controller with V
I
R
TEST,MAX
IN,MIN
I
TEST,MAX
LP
R
LP,TOT
R
SLEW
= 36V x 90ms / (10,000µF x 220mV) = 1.472Ω ⇒
R
TEST
LP
1.47kΩ ±1%
LOAD(MAX)
should be used to set the R
LP
= 77.51Ω - 11Ω = 66.51Ω ⇒ 66.5Ω ±1%
. This graph shows that for C
SLEW
(Ω) = V
= V
must be evaluated at the maximum operat-
IN
= 36V x 90ms / (190,000µF x 220mV) =
= V
993mA
90Ω = 11Ω + R
C
operating range = 36V to 72V, C
IN,MAX
ON,FALLING
SLEW
IN,MAX
is found as follows:
(C
Adjusting the V
IN,MIN
______________________________________________________________________________________
LOAD(MAX)
is increased to 190,000µF, the test
TEST
LP
(nF) = 23 / SR (V/ms)
/ R
SLEW
/ R
at the maximum operating volt-
LP,TOT
No R
during the load-probe test, this
/ I
LP
LP
TEST
or V
= 72V / 1.47kΩ = 49.0mA
> 4700pF, SR and C
LP
that will guarantee a suc-
x 220mV)
IN
OFF
= 72V / (66.5Ω + 6Ω) =
= V
for the application will
for the application.
IN,MIN
SENSE
LP
OUT
. This voltage will
SLEW
x 90ms /
Slew Rate
< 4700pF
LP
LOAD
will be
, and Overvoltage Protection
SLEW
=
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
should be subtracted from C
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
may lead to oscillation while the MOSFET and GATE con-
Figure 18. MAX5938 Slew Rate vs. C
Figure 19. Adjusting the MAX5938 Slew Rate
-48V
GND
0.01
0.1
10
GND
V
1
EE
IN
0.1
MAX5938
Step Immunity,
GATE
SLEW RATE vs. C
1
V
OUT
R
GATE
C
SLEW
SLEW
10
C
SLEW
(nF)
SLEW
in the equation above.
SLEW
100
C
LOAD
1000
SLEW
LOAD
, then it
SLEW
19