LTC4211CS8 datasheet(17/36 Pages) LINER

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LTC4211

4211f

or by actively limiting the inrush current. The LTC4211

uses GATE voltage slew rate limiting when CLOAD is small

and/or the inrush current limit is set high. If GATE voltage

slew rate control is preferred with large CLOAD, an external

capacitor (CGX) can be used from GATE to ground, as

shown in Figure 7. According to Equation 3, adding CGX

slows the GATE voltage slew rate at the expense of slower

system turn-on and turn-off time. Should this technique

be used, values for CGX less than 150nF are recommended.

trips, the GATE pin is immediately pulled to ground, the

external N-channel MOSFET is quickly turned OFF and

FAULT is latched low.

The circuit breaker trips whenever the voltage across the

sense resistor exceeds two different levels, set by the

LTC4211’s SLOW COMP and FAST COMP thresholds (see

Block Diagram). The SLOW COMP trips the circuit breaker

if the voltage across the SENSE resistor (VCC – VSENSE =

VCB) is greater than 50mV for 20µs. There may be appli-

cations where this comparator’s response time is not long

enough, for example, because of excessive supply voltage

noise. To adjust the response time of the SLOW COMP, the

MS version of the LTC4211 is chosen and a capacitor is

used at the LTC4211’s FILTER pin (see section on Adjust-

ing SLOW Comp’s Response Time). The FAST COMP trips

the circuit breaker to protect against fast load overcurrents

if the transient voltage across the sense resistor is greater

than 150mV for 300ns. The response time of the LTC4211’s

FAST COMP is fixed.

The timing diagram of Figure 6 illustrates when the

LTC4211’s electronic circuit breaker is armed. After the

first timing cycle, the LTC4211’s FAST COMP is armed at

Time Point 5. Arming FAST COMP at Time Point 5 ensures

that the system is protected against a short-circuit

condition during the second timing cycle after CLOAD has

been fully charged. At Time Point 7, SLOW COMP is

armed when the internal control loop is disengaged.

The timing diagrams in Figures 8 and 9 illustrate the opera-

tion of the LTC4211 when the load current conditions exceed

the thresholds of the FAST COMP (VCB(FAST) > 150mV)

and SLOW COMP (VCB(SLOW) > 50mV), respectively.

RESETTING THE ELECTRONIC CIRCUIT BREAKER

Once the LTC4211’s circuit breaker is tripped, FAULT is

asserted low and the GATE pin is pulled to ground. The

LTC4211 remains latched OFF in this fault state until the

external fault is cleared. To clear the internal fault detect

circuitry and to restart the LTC4211, its ON pin must be

driven low (VON < 1.236V) for at least 150µs, after which

time FAULT goes high. Toggling the ON pin from low to

high (VON > 1.316V) initiates a restart sequence in the

LTC4211. The timing diagram in Figure 10 illustrates a

OPERATIO

Si4410DY

RSENSE

0.007

Ω

CGX*

CLOAD

4211 F07

VCC

SENSE

LTC4211**

GATE

36k

VOUT

VIN

15k

VALUES

≤150nF SUGGESTED

ADDITIONAL DETAILS OMITTED

FOR CLARITY

dVGATE

VGATE SLEW RATE CONTROL

µA

CGATE + CGX

()

Figure 7. Using an External Capacitor at GATE for

GATE Voltage Slew Rate Control and Large CLOAD

An external gate capacitor may also be useful to decrease

or eliminate current spikes through the MOSFET when

power is first applied. At power-up, the instantaneous in-

put voltage step attempts to pull the MOSFET gate up

through the MOSFET’s drain-to-gate capacitance. If the

MOSFET’s CGS is small, the gate can be pulled up high

enough to turn on the MOSFET, thereby allowing a current

spike to the output. This event occurs during the time that

the LTC4211 is coming out of UVLO and getting its intel-

ligence to hold the GATE pin low. An external capacitor

attenuates the voltage to which the GATE is pulled up and

eliminates the current spike. The value required is depen-

dent on the MOSFET capacitance specifications. In typical

applications, this capacitor is not required.

ELECTRONIC CIRCUIT BREAKER

The LTC4211 features an electronic circuit breaker func-

tion that protects against supply overvoltage, externally-

generated fault conditions and shorts or excessive load

current conditions on the supply. If the circuit breaker

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