Texas Instruments UCC3918EVM Hot Swap Power Manager Evaluation Module UCC3918EVM UCC3918EVM 데이터 시트

2

The maximum sourcing current is programmed by
selection of R3, connected from pin 9 (I

MAX

) to

GND. R3 can be calculated as:

R

k

I

MAX

3

126

=

Ω

(1)

For the UCC3918 evaluation kit R3 is set to 42.2k

Ω

for I

MAX

= 3A typical. Please refer to the UCC3918

data sheet for further discussion of I

MAX

.

The I

FAULT

pin allows linear programming of the

fault level current from 0 to 5A. When operating be-
low the fault current the internal FET will be fully
enhanced providing low on resistance. When out-
put currents exceed I

FAULT

the output remains on

but the fault timer starts charging CT. R2 sets the
current fault level. R2 can be calculated as:

R

k

I

FAULT

2

105

=

Ω

(2)

For the UCC3918 evaluation kit R2 is set to 52.3k

Ω

for I

FAULT

= 2A typical.

The UCC3918 evaluation kit incorporates a 0.1

µ

F

timing capacitor, C2 (CT) to set the fault time. The
fault and shutdown times can be found by using the
following equations.

T

k C

FAULT

=

•

27 8

2

.

(3)

T

X

C

SHUTDOWN

=

•

0 833 10

2

6

.

(4)

Where C2 is in Farads and T is in Seconds.

For the evaluation board T

FAULT

= 2.78mSec and

T

SHUTDOWN

=83.3mSec.

The maximum load capacitance that can be
charged depends on several factors; the nature of
the load (constant current or resistive), the maxi-
mum sourcing current, and the allowed fault time.

For a constant current load, the maximum capaci-
tance can be estimated from:

C

I

I

X

C

V

Farads

OUT

MAX

LOAD

OUT

=

•

•













(

–

)

.

27 8 10

2

3

(5)

For a resistive load of value RL, C

OUT

can be esti-

mated from:

C

x

C

RL

n

V

I

RL

Far

OUT

OUT

MAX

=

•

•

−

•

























27 8 10

2

1

1

3

.

l

ads

(6)

Although the UCC3918 is designed to provide sys-
tem protection for all fault conditions, all integrated
circuits can ultimately fail short. For this reason, if
the UCC3918 is intended for use in safety critical
applications where UL or some other safety agency
rating is required, a redundant safety device such
as a fuse should be placed in series with the power
MOSFET device. The UCC3918 will prevent the
fuse from blowing for virtually all fault conditions,
increasing system reliability and reducing mainte-
nance cost, in addition to providing the hot swap
benefits of the device.

Note: Capacitor C1 is located on the component side of
the input switch and will contribute to the inrush current.

For more complete information, pin descriptions and
specifications for the UCC3918 Low On-Resistance Hot
Swap Power Manager, please refer to the UCC3918 data
sheet or contact your Unitrode Field Applications Engi-
neer at (603) 424-2410.