LTC4218

11

4218fb

APPLICATIONS INFORMATION

its threshold. The PG then pulls low to indicate output

power is no longer good.

drops below 2.5V for greater than 1μs, a fast shutdown

of the switch is initiated. The GATE is pulled down with a

170mA current to the SOURCE pin.

Overcurrent Fault

The LTC4218 features an adjustable current limit with

foldback that protects the MOSFET when excessive load

current happens. To protect the switch during active cur-

rent limit, the available current is reduced as a function

of the output voltage sensed by the FB pin. A graph in the

Typical Performance Characteristics shows the current

limit versus FB voltage.

An overcurrent fault occurs when the current limit circuitry

has been engaged for longer than the time-out delay set by

the TIMER. Current limiting begins when the current sense

3.75mV to 15mV (depending on the foldback). The GATE

pin is then brought down with a 170mA GATE-to-SOURCE

current. The voltage on the GATE is regulated in order to

limit the current sense voltage to less than 15mV. At this

point, a circuit breaker time delay starts by charging the

external timing capacitor from the TIMER pin with a 100μA

pull-up current. If the TIMER pin reaches its 1.2V thresh-

old, the external switch turns off (with a 250μA current

from GATE to ground). Next, the FLT pin is pulled low to

indicate an overcurrent fault has turned off the MOSFET.

For a given the circuit breaker time delay, the equation for

setting the timing capacitor’s value is as follows:

After the switch is turned off, the TIMER pin begins dis-

charging the timing capacitor with a 2μA pull-down current.

When the TIMER pin reaches its 0.2V threshold, the switch

is allowed to turn on again if the overcurrent fault has been

cleared. Bringing the UV pin below 0.6V and then high will

switch is allowed to turn on again (after an internal 100ms

delay) if the overcurrent fault is cleared.

Tying the FLT pin to the UV pin allows the part to self-clear

the fault and turn the MOSFET on as soon as TIMER pin has

ramped below 0.2V. In this auto retry mode, the LTC4218

repeatedly tries to turn on after an overcurrent at a period

determined by the capacitor on the TIMER pin.

The waveform in Figure 3 shows how the output latches

off following a short circuit. The drop across the sense

resistor is 3.75mV as the timer ramps up.

Figure 3. Short-Circuit Waveform

Current Limit Adjustment

The default value of the active current limiting signal

threshold is 15mV. The current limit threshold can be

pin (via the clamp circuit) sets the CS amplifier’s built-in

offset voltage. This offset voltage directly determines the

voltage. This voltage is set to 0.618V which corresponds

to a 15mV current limit threshold.

forms a resistive divider with the internal 20k sourcing

pin and therefore lower the current limit threshold. The

overall current limit threshold precision is reduced to ±11%

when using a 20k resistor to half the threshold.

1ms/DIV

4218 F03

10V/DIV

2A/DIV

10V/DIV

TIMER

2V/DIV