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LTC4211

4211fb

OPERATION

SENSE RESISTOR CONSIDERATIONS

The fault current level at which the LTC4211’s internal

electronic circuit breaker trips is determined by a sense

resistor connected between the LTC4211’s VCC and SENSE

pins and two separate trip points. The first trip point is

set by the SLOW COMP’s threshold, VCB(SLOW) = 50mV,

and occurs should a load current fault condition exist for

more than 20μs. The current level at which the electronic

circuit breaker trips is given by Equation 8:

ITRIP(SLOW) =

VCB(SLOW)

RSENSE

50mV

RSENSE

(8)

The second trip point is set by the FAST COMP’s threshold,

VCB(FAST) = 150mV, and occurs during fast load current

transients that exist for 300ns or longer. The current level

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

Equation 9:

ITRIP(FAST) =

VCB(FAST)

RSENSE

150mV

RSENSE

(9)

As a design aid, the currents at which the electronic circuit

breaker trips for common values for RSENSE are shown

in Table 3.

Figure 11. Making PCB Connections to the Sense Resistor

Table 2. tSLOWCOMP vs CFILTER

CFILTER

tSLOWCOMP

100pF

82μs

220pF

156μs

330pF

224μs

470pF

310μs

680pF

440μs

820pF

527μs

1000pF

638μs

Table 3. ITRIP(SLOW) and ITRIP(FAST) vs RSENSE

RSENSE

ITRIP(SLOW)

ITRIP(FAST)

0.005Ω

10A

30A

0.006Ω

8.3A

25A

0.007Ω

7.1A

21A

0.008Ω

6.3A

19A

0.009Ω

5.6A

17A

0.01Ω

15A

For proper circuit breaker operation, Kelvin-sense PCB

connections between the sense resistor and the LTC4211’s

VCC and SENSE pins are strongly recommended. The

drawing in Figure 11 illustrates the correct way of making

connections between the LTC4211 and the sense resis-

tor. PCB layout should be balanced and symmetrical to

minimize 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 accommodate

steady-state fault current levels so that the component is

not damaged before the circuit breaker trips. Table 4 in

the Appendix lists sense resistors that can be used with

the LTC4211’s circuit breaker.

IRC-TT SENSE RESISTOR

LR251201R010F

OR EQUIVALENT

0.01, 1%, 1W

CURRENT FLOW

TO LOAD

CURRENT FLOW

TO LOAD

VCC

SENSE

TRACK WIDTH W:

0.03" PER AMP

ON 1 OZ COPPER

4211 F11

CALCULATING CIRCUIT BREAKER TRIP CURRENT

For a selected RSENSE value, the nominal load current that

trips the circuit breaker is given by Equation 10:

ITRIP(NOM) =

VCB(NOM)

RSENSE(NOM)

50mV

RSENSE(NOM)

(10)

The minimum load current that trips the circuit breaker is

given by Equation 11.

ITRIP(MIN) =

VCB(MIN)

RSENSE(MAX)

40mV

RSENSE(MAX)

(11)

where

RSENSE(MAX) = RSENSE(NOM)

RTOL

100

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