LTC4244IGN datasheet(23/28 Pages) LINER

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LTC4244/LTC4244-1

42441f

the PC board. For 1 ounce copper foil plating, a general rule

is 1 ampere of DC current per via making sure the via is

properly dimensioned so that solder completely fills the

void. For other plating thicknesses, check with your PCB

fabrication facility.

Design Example

As a design example, consider a CPCI Hot Swap applica-

tion with the following power supply requirements:

Table 4. Design Example Power Supply Requirements

VOLTAGE

MAXIMUM DC

LOAD

SUPPLY

SUPPLY CURRENT

CAPACITANCE

12V

450mA

100

µF

2200

µF

3.3V

2200

µF

–12V

100mA

100

µF

The first step is to select the appropriate values of RSENSE

for the 5V and 3.3V supplies. Calculating the value of

RSENSE is based on ILOAD(MAX) and the lower limit for the

circuit breaker threshold voltage (47mV for both the 5V

and 3.3V circuit breakers). If a 1% tolerance is assumed

for the sense resistors, then 5m

Ω and 7mΩ resistor

values yield the following minimum and maximum ITRIP

values:

Table 5. ITRIP vs RSENSE

RSENSE (1% RTOL)

ITRIP(MIN)

ITRIP(MAX)

Ω

9.3A

11.5A

Ω

6.6A

8.2A

So sense resistor values of 7m

Ω and 5mΩ should suffice

for the 5V and 3.3V supplies, respectively.

The second step is to select MOSFETs for the 5V and 3.3V

supplies. The IRF7457’s on resistance is less than 10.5m

Ω

for VGS > 4.5V and a junction temperature of 25°C. Since

the maximum load current requirement for the 3.3V sup-

ply is 7A, the steady-state power the device may be

required to dissipate is 514mW. The IRF7457 has a

junction-to-ambient thermal resistance of 50

°C/Watt. If a

maximum ambient temperature of 50

°C is assumed, this

yields a junction temperature of 75.7

°C. According to the

IRF7457’s Normalized On-Resistance vs Junction Tem-

perature curve, the device’s on-resistance can be expected

to increase by about 20% over its room temperature value.

Recalculation of the steady-state values of RON and junc-

tion temperature yields approximately 12.6m

Ω and 81°C,

respectively. The I • R drop across the 3.3V sense resistor

and series MOSFET at maximum load current under these

conditions will be less than 124mV.

The next step is to select appropriate values for C1 and

CTIMER. Assuming that the total current for the 5V supply

is constrained to less than 6A during power-up (6

× 5V

medium length connector pins at 1A per pin), then the

inrush current shouldn’t exceed:

IINRUSH < 6A – ILOAD(5VOUT) = 6A – 5A = 1A

(12)

This yields:

GATE MAX

INRUSH MAX

2200

100

2200

220

⇒>

µµ =

()

•

(13)

Hence a C1 value of 330nF

±10% should suffice. The value

of CTIMER for this design example will be constrained by

the duration of the 12V supply inrush current, which

according to Equation 2 is:

VOUT

LOAD

LIMIT MIN

LOAD MAX

VOUT

()

(

)

()

••

–

••

–

212

2 100

550

450

⇒<

(14)

In order to guarantee that the LTC4244’s TIMER fault

inhibit period is greater than 24ms, the value of CTIMER

should be:

ms I

TIMER

TIMER MAX

TIMER

⇒>

µ =

1 9

61 8

•

–

•

–.

()

(15)

So a value of 82nF (

±10%) should suffice.

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