LTC4215CUFD datasheet(12/24 Pages) LINER

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LTC4215

4215fb

APPLICATIO S I FOR ATIO

is brought high or if a serial bus turn-on command is sent

by setting bit A3.

The MOSFET is turned on by charging up the GATE with

a 20µA current source. When the GATE voltage reaches

the MOSFET threshold voltage, the MOSFET begins to

turn on and the SOURCE voltage then follows the GATE

voltage as it increases.

When the MOSFET is turning on, it ramps inrush current

up linearly at a dI/dt rate selected by capacitor CSS. Once

the inrush current reaches the limit set by the FB pin, the

dI/dt ramp stops and the inrush current follows the foldback

proﬁle as shown in Figure 2. The TIMER pin integrates at

100µA during start-up and once it reaches its threshold

of 1.235V, the part checks to see if it is in current limit,

which indicates that it has started up into a short-circuit

condition. If this is the case, the overcurrent fault bit, D2

in Table 5, is set and the part turns off. If the part is not in

current limit, the 25mV circuit breaker is armed and the

current limit is switched to 75mV. Alternately an internal

100ms start-up timer may be selected by tying the TIMER

pin to INTVCC.

As the SOURCE voltage rises, the FB pin follows as set

by R7 and R8. Once FB crosses its 1.235V threshold, and

the start-up timer has expired, the GPIO pin, in its default

conﬁguration, ceases to pull low and indicates that power

is now good.

VDD + 6V

VGATE

VOUT

GPIO

(POWER GOOD)

ILOAD • RSENSE

VDD

VSENSE

25mV

10mV

LIMITED

4215 F02

TIMER

EXPIRES

Figure 2. Power-Up Waveforms

If R6 and C1 are employed for a constant current during

start-up, which produces a constant dV/dt at the output,

a 20µA pull-up current from the gate pin slews the gate

upwards and the part is not in current limit. The start-up

TIMER may expire in this condition and an OC fault is not

generated even though start-up has not completed. Either

the sense voltage increases to the 25mV CB threshold and

generates an OC fault, or the FB pin voltage crosses its

1.235V power good threshold and the GPIO pin signals

power good.

GATE Pin Voltage

A curve of GATE-to-SOURCE drive vs VDD is shown in the

Typical Performance Characteristics. At minimum input

supply voltage of 2.9V, the minimum GATE-to-SOURCE

drive voltage is 5V. The GATE-to-SOURCE voltage is

clamped below 6.5V to protect the gates of logic level

N-channel MOSFETs.

Turn-Off Sequence

The GATE is turned off by a variety of conditions. A normal

turn-off is initiated by the ON pin going low or a serial bus

turn-off command. Additionally, several fault conditions

turn off the GATE. These include an input overvoltage

(OV pin), input undervoltage (UV pin), overcurrent circuit

breaker (SENSE– pin), or EN transitioning high. Writing

a logic one into the UV, OV or OC fault bits (D0-D2 in

Table 5) also latches off the GATE if their auto-retry bits

are set to false.

Normally the MOSFET is turned off with a 1mA current

pulling down the GATE pin to ground. With the MOSFET

turned off, the SOURCE and FB voltages drop as CL dis-

charges. When the FB voltage crosses below its threshold,

GPIO pulls low to indicate that the output power is no

longer good.

If the VDD pin falls below 2.74V for greater than 2µs or

INTVCC drops below 2.60V for greater than 1µs, a fast shut

down of the MOSFET is initiated. The GATE pin is pulled

down with a 450mA current to the SOURCE pin.

Overcurrent Fault

The LTC4215 features an adjustable current limit that

protects against short circuits or excessive load current.

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