MAX5938LEEE+【PDF 19/26 页】IC HOT-SWAP CTRLR -48V 16-QSOP

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Since the minimum intended V

for the application will

result in the lowest I

TEST

during the load-probe test, this

IN,MIN

should be used to set the R

. This voltage will

likely be near V

ON,FALLING

or V

OFF

for the application.

TEST

(&) = V

IN,MIN

/ I

TEST

= V

IN,MIN

x 90ms /

LOAD(MAX)

x 220mV)

Example: V

operating range = 36V to 72V, C

LOAD

10,000礔. First, find the R

that will guarantee a suc-

cessful test of the load.

= 36V x 90ms / (10,000礔 x 220mV) = 1.472& ?/DIV>

1.47k& ?%

Next, evaluate the R

at the maximum operating volt-

age to verify that it will not exceed the 1A current limit

for the load-probe test.

TEST,MAX

= V

IN,MAX

/ R

= 72V / 1.47k& = 49.0mA

If the C

LOAD(MAX)

is increased to 190,000礔, the test

current will approach the limit. In this case, R

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 11& should be used:

LP,TOT

= 36V x 90ms / (190,000礔 x 220mV) =

90& = 11& + R

= 77.51& - 11& = 66.51& ?66.5& ?%

Again R

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 6& should be used:

TEST,MAX

= V

IN,MAX

/ R

LP,TOT

= 72V / (66.5& + 6&) =

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:

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-

-48V Hot-Swap Controller with V

Step Immunity,

No R

SENSE

, and Overvoltage Protection

______________________________________________________________________________________ 19

Figure 18. MAX5938 Slew Rate vs. C

SLEW

SLEW RATE vs. C

SLEW

(nF)

100

0.1

0.01

0.1

1000

Figure 19. Adjusting the MAX5938 Slew Rate

MAX5938

OUT

LOAD

GATE

GND

SLEW

GATE

-48V

GND

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