MCP18480 datasheet(35/64 Pages) MICROCHIP

35 / 64 page

 2003 Microchip Technology Inc.

DS20091B-page 35

MCP18480

TIMER

Current Limit Timer.

The value of the external capacitor (CTIMER) connected to the TIMER

pin sets the two time periods used during a current-limit event. These

are:

• The time that the GATE pin will limit the current through the

external FET

• The time that the GATE pin will disable the external FET

During current limit, a pull-up current source charges up the external

capacitor. Until the voltage on the TIMER pin reaches VREFIN/2, the

GATE pin is driven to maintain a reduced current flow determined by

the VDS of the external FET.

While the capacitor is being discharged by the pull-down current (pull-

up current is off), the GATE pin is at VNEG and the PWRGOOD pin is

deasserted. When the TIMER voltage falls below approximately

100 mV, the GATE pin turns on, if the RESTART pin is low, to reset the

internal fault latch. If the RESTART pin is high, the GATE pin remains

off until the ENABLE pin is forced low. It is then forced high or the

RESTART pin is forced low (asserted).

The PWRGOOD pin reasserts after the voltages on the DRAINTH and

GATE pins meet the appropriate conditions.

The TIMER pin pull-up current is proportioned to the IISET current

(approximately a multiple of 16).

VNEG

Negative supply input.

The negative voltage applied to the board by the backplane (typically

the most negative voltage in the system).

RDISCH

External MOSFET activation delay.

An external resistor (RRDISCH) is connected between the RDISCH pin

and the VNEG pin and is used to set the delay between the deactivation

and activation of the external pass MOSFET during a current-limit

event. The delay is set by the values of the external capacitor (CTIMER)

and the external resistor (RRDISCH). The formulas are:

TDEACT = (CTIMER x RISET) / 16

TACT = (9.2 x RRDISCH x CTIMER)

TABLE 3-1:

MCP18480 PIN DESCRIPTIONS (CONTINUED)

Pin Name

Number

Direction

Buffer

Type

Description

SSOP

Legend: TTL = TTL compatible input

ST = Schmitt Trigger input with CMOS levels

I = Input

O = Output

P = Power

CMOS = CMOS-compatible input

A = Analog

D = Digital

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