LTC4218_15 datasheet(10/18 Pages) LINER

10 / 18 page

LTC4218

4218fg

For more information www.linear.com/LTC4218

applications inForMation

The typical LTC4218 application is in a high availability

system that uses a positive voltage supply to distribute

power to individual cards. The basic application circuit

is shown in Figure 1. External component selection is

discussed in detail in the following sections.

Figure 2. Supply Turn-On

Figure 1. 3A, 12V Card Resident Application

Turn-On Sequence

The power supply on a board is controlled by placing

an external N-channel pass transistor (Q1) in the power

path. Note the sense resistor (RS) detects current and

the capacitor (CGATE) controls gate slew rate. Resistor R1

prevents high frequency oscillations in Q1 and resistor

RGATE isolates CGATE during fast turn-off.

Several conditions must be present before the external

pass transistor can be turned on. First, the supply VDD

must exceed its undervoltage lockout level. Next, the

internally generated supply INTVCC must cross its 2.65V

undervoltage threshold. This generates a 25µs power-

on-reset pulse which clears the logic’s fault register and

initializes internal latches.

Afterthepower-on-resetpulse,theLTC4218willgothrough

the following sequence. First, the UV and OV pins must

indicate that the input voltage is within the acceptable

range. All of these conditions must be satisfied for a dura-

tion of 100ms to ensure that any contact bounce during

the insertion has ended.

The pass transistor is turned on by charging up the GATE

with a 24µA charge pump generated current source

(Figure 2).

The voltage at the GATE pin rises with a slope equal to

24µA/CGATE and the supply inrush current is set at:

IINRUSH =

CGATE

• 24µA

When the GATE voltage reaches the MOSFET threshold

voltage, the switch begins to turn on and the SOURCE

voltage follows the GATE voltage as it increases. Once

SOURCEreachesVDD,theGATEwillrampupuntilclamped

by the 6.15V zener between GATE and SOURCE.

As the SOURCE pin voltage rises, so will the FB pin which

is monitoring it. If the voltage across the current sense

resistor(RS)getstoohigh,theinrushcurrentwillbelimited

by the internal current limiting circuitry. Once the FB pin

crosses its 1.235V threshold and the GATE to SOURCE

voltage exceeds 4.2V, the PG pin will cease to pull low and

indicate that the power is good.

Turn-Off Sequence

The switch can be turned off by a variety of conditions. A

normal turn-off is initiated by the UV pin going below its

1.235Vthreshold.Additionally,severalfaultconditionswill

turn off the switch. These include an input overvoltage (OV

pin)andovercurrentcircuitbreaker(SENSEpin).Normally,

the switch is turned off with a 250µA current pulling down

the GATE pin to ground. With the switch turned off, the

SOURCE pin voltage drops which pulls the FB pin below

its threshold. The PG then pulls low to indicate output

power is no longer good.

150k

20k

ADC

226k

0.1µF

20k

12V

4218 F01

10k

0.1µF

330µF

VOUT

12V

VDD

GND

IMON

RSET

20k

Si7108DN

RMON

20k

ISET

CGATE

0.01µF

RGATE

GATE SOURCE

SENSE–

SENSE+

LTC4218GN

INTVCC

TIMER

FLT

140k

20k

SLOPE = 24µA/CGATE

GATE

SOURCE

VDD + 6.15

VDD

4218 F02

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