LTC4218

10

4218fb

R6

150k

R7

20k

ADC

R2

224k

C1

0.1

μF

R3

20k

12V

12V

4218 F01

R8

10k

R1

10

Ω

0.1

μF

330

μF

12V

3A

UV

FB

PG

GND

20k

2m

Ω

Q1

Si7108DN

20k

0.01

μF

1k

GATE SOURCE

LTC4218GN

OV

TIMER

FLT

+

R4

140k

R5

20k

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

prevents high frequency oscillations in Q1 and resistor

Several conditions must be present before the external

must exceed its undervoltage lockout level. Next, the

undervoltage threshold. This generates a 25μs power-

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

initializes internal latches.

After the power-on-reset pulse, the LTC4218 will go through

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

indicate that the input power is within the acceptable range.

All of these conditions must be satisfied for a duration

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

I

C

C

µA

INRUSH

L

GATE

=

•24

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

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

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

1.235V threshold. Additionally, several fault conditions will

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

pin) and overcurrent circuit breaker (SENSE pin). 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

t1

t2

SLOPE = 24

GATE

SOURCE

4218 F02