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LTC4244/LTC4244-1

42441f

OUTPUT VOLTAGE (V)

4244 F16

5V MOSFET

5V RSENSE = 0.007Ω

3.3V RSENSE = 0.005Ω

3.3V MOSFET

Figure 16. Worst-Case 5V and 3.3V MOSFET

Dissipated Power vs Output Voltage

The next step is to verify that the thermal ratings of the

external 5V and 3.3V MOSFETs aren’t being exceeded

during power-up cycles into the designed loads or into a

short circuit.

The amount of heating in the 5V and 3.3V MOSFETs during

a normal power cycle depends on the LTC4244’s GATE pin

current (refer to Gate Current vs Temperature plot in the

Typical Performance Characteristics section). The magni-

tude of the off-on power pulse that results in maximum

heating of the MOSFETs is given by Equation 10 as:

OUT

INRUSH MIN

LOAD VOUT

OFF-ON =

()

•

()

(

)

(16)

where

C MAX

INRUSH MIN

LOAD

GATE MIN

()

•

(17)

The duration of the power-pulse is given by Equation 11

as:

INRUSH

LOAD

OUT

INRUSH MIN

•

()

(18)

Solving these equations for the 5V and 3.3V supplies

yields:

Table 6

POFF-ON

tINRUSH(MAX)

5V MOSFET

12.8W

90ms

3.3V MOSFET

11.8W

60ms

Under these conditions, the IRF7457 datasheet’s Thermal

Response vs Pulse Duration curve indicates that the

junction-to-ambient temperature will increase by 60

°C for

the 5V MOSFET and 46

°C for the 3.3V MOSFET.

The duration and magnitude of the power pulse that

results during a short-circuit condition on either the 5V or

3.3V outputs are a function of the TIMER capacitor and the

LTC4244’s foldback current limit. Figure 16 shows the

worst-case power dissipated in the 5V and 3.3V external

FETs vs V5VOUT and V3.3VOUT, respectively. In the case of

the 3.3V external MOSFET, the maximum dissipated power

is 24 Watts (V3.3VOUT= 0.9V). For the 5V external MOSFET,

the maximum dissipated power is 22 Watts (V5VOUT =

1.75V). The maximum duration of the short-circuit power-

pulse is given by Equation 19 as:

tms

PULSE

TIMER MAX

TIMER MIN

PULSE

⇒<

() (

)

⇒<

()

•

–

.•

– .

8 2

1 3

60 3

(19)

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