LTC4212IMS#TRPBF【PDF 20/24 页】IC CTRLR HOTSWAP TIMEOUT 10MSOP

20

LTC4212

4212f

OPERATIO

by the SLOW COMPs threshold, V

CB(SLOW)

= 50mV, and

occurs should a load current fault condition exist for more

than 18祍. The current level at which the electronic circuit

breaker trips is given by Equation 8:

I

V

R

mV

R

TRIP SLOW

CB SLOW

SENSE

(

)

(

)

=

50

(8)

The second trip point is set by the FAST COMPs threshold,

V

CB(FAST)

= 150mV, and occurs during fast load current

transients that exist for 500ns or longer. The current level

at which the circuit breaker trips in this case is given by

Equation 9:

I

V

R

mV

R

TRIP FAST

CB FAST

SENSE

(

)

(

)

=

150

(9)

As a design aid, the currents at which electronic circuit

breaker trips for common values for R

SENSE

are shown in

Table 4.

Table 4. I

TRIP(SLOW)

and I

TRIP(FAST)

vs R

SENSE

R

SENSE

I

TRIP(SLOW)

I

TRIP(FAST)

0.005&

10A

30A

0.006&

8.3A

25A

0.007&

7.1A

21A

0.008&

6.3A

19A

0.009&

5.6A

17A

0.01&

5A

15A

For proper circuit breaker operation, Kelvin-sense PCB

connections between the sense resistor and the LTC4212s

V

CC

and SENSE pins are strongly recommended. The

drawing in Figure 7 illustrates the correct way of making

connections between the LTC4212 and the sense resistor.

PCB layout should be balanced and symmetrical to mini-

mize wiring errors. In addition, the PCB layout for the

sense resistor should include good thermal management

techniques for optimal sense resistor power dissipation.

The power rating of the sense resistor should accommo-

date steady-state fault current levels so that the compo-

nent is not damaged before the circuit breaker trips.

Table 5 in the Appendix lists sense resistors that can be

used with the LTC4212s circuit breaker.

Calculating Circuit Breaker Trip Current

For a selected R

SENSE

value, the nominal load current that

trips the circuit breaker is given by Equation 10:

IRC-TT SENSE RESISTOR

LR251201R010F

OR EQUIVALENT

0.01&, 1%, 1W

CURRENT FLOW

TO LOAD

CURRENT FLOW

TO LOAD

TO

V

CC

TO

SENSE

TRACK WIDTH W:

0.03" PER AMP

ON 1 OZ COPPER

W

4212 F07

Figure 7. Making PCB Connections to the Sense Resistor

I

V

R

mV

R

TRIP NOM

CB NOM

SENSE NOM

(

)

(

)

(

)

(

)

=

50

(10)

The minimum load current that trips the circuit breaker is

given by Equation 11.

I

V

R

mV

R

TRIP MIN

CB MIN

SENSE MAX

( )

=

40

(11)

where

R

SENSE MAX

SENSE NOM

TOL

( )

(

)

"

=

+

?/DIV>

1

100

The maximum load current that trips the circuit breaker is

given in Equation 12.

I

V

R

mV

R

TRIP MAX

CB MAX

SENSE MIN

( )

=

60

(12)

where

R

SENSE MIN

SENSE NOM

TOL

( )

(

)

"

=

?/DIV>

1

100

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