tems to be customized for MOSFET gate capacitance

adjusted with the resistance connected from TIM to GND

Ω and 500kΩ. The

startup period has a default value of 9ms when TIM is left

Startup Sequence

There are two ways of completing the startup sequence.

Case A describes a startup sequence that slowly turns

on the MOSFETs by limiting the gate charge. Case B

uses the current-limiting feature and turns on the

MOSFETs as fast as possible while still preventing a high

determined by the longer of the two timings, case A and

to guarantee enough time for the output voltage to settle.

Case A: Slow Turn-On (without current limit)

There are two ways to turn on the MOSFETs without

reaching the fast-comparator current limit:

inrush current is low.

• If the gate capacitance is high, the MOSFETs turn

on slowly.

In both cases, the turn-on time is determined only by the

charge required to enhance the MOSFET. The small

gate-charging current of 100µA effectively limits the out-

put voltage dV/dt. Connecting an external capacitor

between GATE and GND extends turn-on time. The time

required to charge/discharge a MOSFET is as follows:

where:

(Figure 4).

In this case, the inrush current depends on the MOSFET

Example: Charging and Discharging times using the

Fairchild FDB7030L MOSFET

data sheet specifies that the FDB7030L has approxi-

MAX5918/MAX5919 have a 100µA gate-charging cur-

rent and a 3mA strong discharging current.

I

C

CC

II

INRUSH

BOARD

rss

GATE

GATE

LOAD

=

+

×+

t

CV

Q

I

GATE

GATE

GATE

GATE

=

×∆

+

R

t

pF

TIM

START

=

×

128 800

Low-Voltage, Dual Hot-Swap Controllers with

Independent ON/OFF Control

______________________________________________________________________________________

11

COMPONENT

MANUFACTURER

PHONE

WEBSITE

Dale-Vishay

402-564-3131

www.vishay.com

Sense Resistors

IRC

704-264-8861

www.irctt.com

Fairchild

888-522-5372

www.fairchildsemi.com

International Rectifier

310-233-3331

www.irf.com

MOSFETs

Motorola

602-244-3576

www.mot-sps.com/ppd

Table 3. Component Manufacturers

GATE

SENSE

GND

ON_

IN_

PGOOD_

MAX5918

MAX5919

Figure 4. Operating with an External Gate Capacitor