Datasheet

LM26400Y

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SNVS457C –FEBRUARY 2007–REVISED APRIL 2013

POWER LOSS ESTIMATION

The total power loss in the LM26400Y comprises of three parts - the power FET conduction loss, the power FET

switching loss and the IC's housekeeping power loss. Use the following equation to estimate the conduction loss.

(6)

where T

is the junction temperature or the target junction temperature if the former is unknown. R

is the ON

resistance of the internal FET at room temperature. Use 180mΩ for R

if the actual value is unknown.

Use the following equation to estimate the switching loss.

(7)

Another loss in the IC is the housekeeping loss. It is the power dissipated by circuitry in the IC other than the

power FETs. The equation is:

(8)

The 15mW is gate drive loss. Do the calculation for both channels and find out the total power loss in the IC.

(9)

The power loss calculation can help estimate the overall power supply efficiency.

Example:

= 12V, V

OUT1

= 1.2V, I

OUT1

= 2A, V

OUT2

= 2.5V, I

OUT2

= 2A. Target junction temperature is 90°C.

So conduction loss in Channel 1 is:

(10)

Conduction loss in Channel 2 is:

(11)

Switching loss in either channel is:

(12)

House keeping loss is:

(13)

Finally the total power loss in the LM26400Y is:

(14)

PROGRAMMING OUTPUT VOLTAGE

First make sure the required maximum duty cycle in steady state is less than 80% so that the regulator will not

lose regulation. The datasheet lower limit for maximum duty cycle is about 90% over temperature (see Electrical

Characteristics table for the accurate value). The maximum duty cycle in steady state happens at low line and full

load.

The output voltage is programmed through the feedback resistors R1 and R2, as illustrated in Figure 35.

Product Folder Links: LM26400Y