LTC4210-2CS6 datasheet(13/20 Pages) LINER

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LTC4210-1/LTC4210-2

421012f

Circuit Breaker Timer Operation

When a current limit fault is encountered at time point A in

Figure 5, the circuit breaker timing is activated with a 60

µA

pull-up. The circuit breaker trips at time point B if the fault

is still present and the TIMER pin voltage reaches the

COMP2 threshold and the LTC4210 shuts down. For a

continuous fault, the circuit breaker delay is:

CBDELAY

TIMER

.•

(13)

Intermittent overloads may exceed the current limit as in

Figure 6, but if the duration is sufficiently short, the TIMER

pin may not reach the COMP2 threshold and the LTC4210

will not shut down. To handle this situation, the TIMER

discharges with 2

µA whenever (VCC – SENSE) voltage is

below the 50mV limit and the TIMER voltage is between

the COMP1 and COMP2 thresholds. When the TIMER

voltage falls below the COMP1 threshold, the TIMER pin is

discharged with an equivalent 7k resistor (normal mode,

100

µA source) when (VCC – SENSE) voltage is below the

50mV limit. If the TIMER pin does not drop below the

COMP1 threshold, any intermittent overload with an ag-

gregate duty cycle of more than 3.8% will eventually trip

the circuit breaker. Figure 7 shows the circuit breaker

response time in seconds normalized to 1

µF. The asym-

metric charging and discharging of TIMER is a fair gauge

of MOSFET heating.

TIMER

)

.•

•–

µ=

() µ

(14)

When the circuit breaker trips, the GATE pin is pulled low.

The TIMER enters latchoff mode with a 5

µA pull-up for the

LTC4210-2 (latched-off version), while an autoretry “cool-

off” cycle begins with a 2

µA pull-down for the LTC4210-1

(autoretry version). An autoretry cool-off delay of the

LTC4210-1 between COMP2 and COMP1 thresholds takes:

COOLOFF

TIMER

.•

(15)

APPLICATIO S I FOR ATIO

VTIMER

NORMAL

MODE

FAULT

MODE

100

µA

COMP1

CIRCUIT BREAKER

TRIPS

LATCHED OFF (5

µA PULL-UP)

OR RETRY (2

µA PULL-DOWN)

COMP2

µA

4210 F05

Figure 5. A Continuous Fault Timing

OVERLOAD DUTY CYCLE, D (%)

4210 F07

0.01

0.1

100

(s/

µF) =

CTIMER

1.3V • 1

µF

µA • D – 2µA

Figure 7. Circuit Breaker Timer Response

for Intermittent Overload

~50mV/RSENSE

µA

CIRCUIT BREAKER

TRIPS

COMP1

4210 F06

COMP2

ILOAD

VTIMER

VGATE

µA

FAULT

µA

LATCHED OFF (5

µA PULL-UP)

OR RETRY (2

µA PULL-DOWN)

µA

Figure 6. Mulitple Intermittent Overcurrent Conditon

LTC4210-2CS6 Datasheet(PDF) 13 Page - Linear Technology

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