LTC4232

9

4232fb

For more information www.linear.com/LTC4232

APPLICATIONS INFORMATION

This gate slope is designed to charge up a 1000µF capaci-

tor to 12V in 40ms, with an inrush current of 300mA. This

allows the inrush current to stay under the current limit

threshold(1.5A)forcapacitorslessthan1000µF.Included

in the Typical Performance Characteristics section is a

graph of the Safe Operating Area for the MOSFET. It is

evident from this graph that the power dissipation at 12V,

300mA for 40ms is in the safe region.

pin will lower the inrush current below the default value

set by the INRUSH circuit. The GATE is charged with an

24µA current source (when INRUSH circuit is not driving

the GATE). The voltage at the GATE pin rises with a slope

• 24µA

When the GATE voltage reaches the MOSFET threshold

voltage, the switch begins to turn on and the OUT volt-

age follows the GATE voltage as it increases. Once OUT

6.15V Zener between GATE and OUT.

As the OUT voltage rises, so will the FB pin which is moni-

toring it. Once the FB pin crosses its 1.235V threshold

and the GATE to OUT voltage exceeds 4.2V, the PG pin

will cease to pull low and indicate that the power is good.

Parasitic MOSFET Oscillation

When the N-channel MOSFET ramps up the output during

power-up it operates as a source follower. The source

follower configuration may self-oscillate in the range of

25kHz to 300kHz when the load capacitance is less than

Figure 2. Supply Turn-On

Figure 1. 2A, 12V Card Resident Application

The typical LTC4232 application is in a high availability

system that uses a positive voltage supply to distribute

power to individual cards. A complete application circuit

is shown in Figure 1. External component selection is

discussed in detail in the following sections.

t1

t2

SLOPE = 0.3V/ms

GATE

OUT

4232 F02

Turn-On Sequence

Several conditions must be present before the internal

exceed its undervoltage lockout level. Next the internally

agethreshold.Thisgeneratesa25µspower-on-resetpulse

whichclearsthefaultregisterandinitializesinternallatches.

After the power-on-reset pulse, the LTC4232 will go

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

must indicate that the input voltage is within the accept-

able range. All of these conditions must be satisfied for

the duration of 100ms to ensure that any contact bounce

during the insertion has ended.

The MOSFET is turned on by charging up the GATE with

a charge pump generated current source whose value is

adjusted by shunting a portion of the pull-up current to

ground. The charging current is controlled by the INRUSH

circuit that maintains a constant slope of GATE voltage

versus time (Figure 2). The voltage at the GATE pin rises

with a slope of 0.3V/ms and the supply inrush current is

set at:

ADC

R1

226k

C1

1µF

R2

20k

12V

4232 F01

0.1µF

330µF

12V

2A

UV

OUT

FB

PG

GND

20k

20k

0.1µF

100k

GATE

LTC4232

OV

TIMER

FLT

+

R3

140k

R4

20k

R7

10k

R6

20k

R5

150k

Z1

Z1: DIODES INC. SMAJ22A

3.3nF