LTC4212CMS datasheet(13/24 Pages) LINER

13 / 24 page

LTC4212

4212f

OPERATIO

VGS = 10V. From Equation 6, the slew rate is calculated to

be 3.03V/ms.

The inrush current being delivered to the load while the

GATE pin is ramping depends on CLOAD and CGATE. The

external N-channel MOSFET acts as a source follower so

that its source (load) voltage ramps up at the same rate as

the GATE pin. The output current component for capacitor

charging is given by Equation 7:

IINRUSH = CLOAD • dVGATE/dt

(7)

=10

µA • CLOAD/CGATE

where, CLOAD is the total capacitance at the load side of the

MOSFET. For example, if CGATE = 3300pF and

CLOAD = 2000µF, the inrush current charging CLOAD is

6.06A. Note that the soft-start circuit will servo the inrush

to ILIMIT(SOFTSTART) or 5A in this example and dVGATE/dt

will be lower than calculated from Equation 6.

Frequency Compensation at Soft-Start

If the external MOSFET’s gate input capacitance (CISS) is

greater than 600pF, no external gate capacitor is required

at GATE to stabilize the internal current-limiting loop

during soft-start. Otherwise, connect a gate capacitor

between the GATE pin and ground to increase the total gate

capacitance to be equal to or above 600pF. The servo loop

that controls the external MOSFET during current limiting

has a unity-gain frequency of about 105kHz and phase

margin of 80

° for external MOSFET gate input capaci-

tances of up to 2.5nF.

Electronic Circuit Breaker

The LTC4212 features an electronic circuit breaker func-

tion that protects against supply overvoltage, externally-

generated fault conditions, shorts or excessive load current

conditions and power good faults. If the circuit breaker

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

LTC4212’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 18µs. 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 500ns.

The timing diagram of Figure 2 illustrates when the

LTC4212’s electronic circuit breaker is armed. After the

first timing cycle, the LTC4212’s FAST COMP is armed at

Time Point 6. This 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 8,

SLOW COMP is armed when the internal control loop is

disengaged.

The timing diagram in Figure 4 illustrates the operation of

the LTC4212 when the load current conditions exceed the

threshold of SLOW COMP (VCB(SLOW) > 50mV).

Circuit Breaker Reset

Referring to the Block Diagram, the ON pin drives two

internal comparators, COMP1 and COMP2. COMP1 is

referenced to 1.236V and has a hysterisis of 80mV.

COMP2 is referenced to 0.5V and has a hysterisis of 45mV.

The outputs of the two comparators drive an internal flip-

flop to generate a typical high and low ON pin threshold of

1.31V and 0.455V respectively.

If the voltage at the ON pin is driven below 0.455V for more

than 10

µs, all internal control logic except the circuit

breaker is reset. A 200

µA pull-down current source is

connected to the GATE pin to pull it down gradually.

Holding the ON pin below 0.455V for 120

µs or longer,

resets the circuit breaker. Following reset, the ON pin must

be taken above 1.316V to start a power-up sequence.

Normal Operating Sequence

Figure 2 illustrates the normal power-up sequence for two

different applications. The PGI (RST) and PGF (RST)

waveforms are valid for applications which use the PGI pin

to monitor the RST output of a supply monitor IC. The PGI

(PGOOD) and PGF (PGOOD) waveforms refer to applica-

tions that tie the PGI pin to the PGOOD output

of a DC/DC converter. All other waveforms in Figure 2

are common to both applications. The PGI and PGF

waveforms for applications that connect PGI pin to the

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