LTC4222CG#PBF【PDF 19/32 页】IC CTRLR DUAL HOT SWAP 36-SSOP

The MOSFET is sized to handle the power dissipation dur-

ing inrush when output capacitor C

OUT

is being charged.

A method to determine power dissipation during inrush

is based on the principle that:

Energy in C

L

= Energy in Q1

This uses:

Energy in C

L

=

1

2

CV

2

=

1

2

(0.33mF)(12)

2

or 0.024 joules. Calculate the time it takes to charge up

C

OUT

:

t

STARTUP

=

C

L

V

DD

I

INRUSH

I

INRUSH

=

0.33mF 12V

1A

= 4ms

The power dissipated in the MOSFET:

P

DISS

=

Energy in C

L

t

STARTUP

=6W

The SOA (safe operating area) curves of candidate MOSFETs

must be evaluated to ensure that the heat capacity of the

package tolerates 6W for 4ms. The SOA curves of the

Fairchild FDC653N provide for 2A at 12V (24W) for 10ms,

satisfying this requirement. Since the FDC653N has less

than 8nF of gate capacitance and we are using a GATE

RC network, the short-circuit stability of the current limit

should be checked and improved by adding a capacitor

from GATE to SOURCE if needed.

The inrush current is set to 1A using C1:

C1=

C

L

I

GATE

I

INRUSH

C1=

0.33mF 12礎

1A

or C1=3.9nF

The inrush dI/dt is set to 10A/ms using C

SS

:

C

SS

=

I

SS

dl/dt

A

s

?/DIV>

0.0429

1

R

SENSE

=

10礎 0.0429 1

10000 0.01?/DIV>

= 4.3nF choose 4.7nF

For a start-up time of 4ms with a 2x safety margin we

choose:

C

TIMER

=

2 t

STARTUP

12.3ms/礔

+C

SS

2

C

TIMER

=

8ms

12.3ms/礔

+4.7nF 2=0.68礔

Note the minimum value of C

TIMER

is 10nF .

The UV and OV resistor string values can be solved in the

following method. First pick R3 based on I

STRING

being

1.235V/R3 at the edge of the OV rising threshold. Then

solve the following equations:

R2=

V

OV(OFF)

V

UV(ON)

R3

UV

TH(RISING)

OV

TH(FALLING)

R3

R1=

V

UV(ON)

(R3+R2)

UV

TH(RISING)

R3R2

In our case we choose R3 to be 3.4k to give a resistor

string current below 100礎.

Then solving the equations results in R2 = 1.16k and

R1 = 34.6k.

The FB divider is solved by picking R8 and solving for R7,

choosing 3.57k for R8 we get:

R7=

V

PWRGD(UP)

R8

FB

TH(RISING)

R8

Resulting in R7 = 30k

A 0.1礔 capacitor, C

F

, is placed on the UV pins to prevent

supply glitches from turning off the GATE via UV or OV .

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