LTC4230IGN datasheet(23/36 Pages) LINER

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LTC4230

4230f

–

SLOW

COMP

VCCn

VOUTn

VCBn

SENSE

RSENSE

*ADDITIONAL DETAILS

OMITTED FOR CLARITY

4230 F12

LTC4230*

VCB(MAX) = 60mV

VCB(NOM) = 50mV

VCB(MIN) = 40mV

ILOAD(MAX)

VCCn

–

Figure 12. Circuit Breaker Equivalent Circuit

for Calculating RSENSE

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 4 in the Appendix lists suggested sense resistors

that can be used with the LTC4230’s circuit breaker.

For example:

If a sense resistor with 7m

Ω ±5% RTOL is used for current

limiting, the nominal trip current ITRIP(NOM) = 7.1A. From

Equations 11 and 12, ITRIP(MIN) = 5.4A and ITRIP(MAX) = 9A

respectively.

For proper operation and to avoid the circuit breaker

tripping unnecessarily, the minimum trip current

(ITRIP(MIN)) must exceed the circuit’s maximum operating

load current. For reliability purposes, the operation at the

maximum trip current (ITRIP(MAX)) must be evaluated

carefully. If necessary, two resistors with the same RTOL

can be connected in parallel to yield an RSENSE(NOM) value

that fits the circuit requirements.

IRC-TT SENSE RESISTOR

LR251201R010F

OR EQUIVALENT

0.01

Ω, 1%, 1W

CURRENT FLOW

TO LOAD

CURRENT FLOW

TO LOAD

VCCn

SENSEn

TRACK WIDTH W:

0.03" PER AMP

ON 1 OZ COPPER

4230 F11

Figure 11. Making PCB Connections to the Sense Resistor

CALCULATING CIRCUIT BREAKER TRIP CURRENT

For a selected RSENSE value, the nominal load current that

trips the circuit breaker is given by Equation 10:

TRIP NOM

CB NOM

SENSE NOM

()

(10)

The minimum load current that trips the circuit breaker is

given by Equation 11.

TRIP MIN

CB MIN

SENSE MAX

()

(11)

where

SENSE MAX

SENSE NOM

TOL

()

(

) •

=+ 



















100

The maximum load current that trips the circuit breaker is

given in Equation 12.

TRIP MAX

CB MAX

SENSE MIN

()

(12)

where

SENSE MIN

SENSE NOM

TOL

()

(

) •–





















100

POWER MOSFET SELECTION CRITERIA

To start the power MOSFET selection process, choose the

maximum drain-to-source voltage, VDS(MAX), and the

maximum drain current, ID(MAX) of the MOSFET. The

VDS(MAX) rating must exceed the maximum input supply

voltage (including surges, spikes, ringing, etc.) and the

ID(MAX) rating must exceed the maximum short-circuit

current in the system during a fault condition. In addition,

consider three other key parameters: 1) the required gate-

source (VGS) voltage drive, 2) the voltage drop across the

drain-to-source ON resistance, RDS(ON) and 3) the maxi-

mum junction temperature rating of the MOSFET.

Power MOSFETs are classified into two categories: stan-

dard MOSFETs (RDS(ON) specified at VGS = 10V) and logic-

level MOSFETs (RDS(ON)specifiedatVGS=5V).Theabsolute

APPLICATIO S I FOR ATIO

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