LED5000 datasheet(29/52 Pages) STMICROELECTRONICS

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LED5000

Application notes - buck conversion

Doc ID 023951 Rev 1

29/51

For the calculation we can estimate RDSON_HS = 300 mΩ as a consequence of Tj increase

during the operation.

TSW_EQ is approximately 12 ns.

IQ has a typical value of 2.4 mA at VIN = 48 V.

The overall internal losses are:

Equation 41

Equation 42

The junction temperature of device will be:

Equation 43

Where TA is the ambient temperature and RthJ-A is the thermal resistance junction-to-

ambient. The junction-to-ambient (RthJ-A) thermal resistance of the device assembled in

HSO8 package and mounted on the evaluation is about 40 °C/W.

Assuming the ambient temperature around 40 °C, the estimated junction temperature is:

5.12

Short-circuit protection

In overcurrent protection mode, when the peak current reaches the current limit threshold,

the device disables the power element and it is able to reduce the conduction time down to

the minimum value (approximately 100 nsec typical) to keep the inductor current limited.

This is the pulse-by-pulse current limitation to implement constant current protection feature.

In overcurrent condition, the duty cycle is strongly reduced and, in most applications, this is

enough to limit the switch current to the current threshold.

The inductor current ripple during ON and OFF phases can be written as:

●

ON phase

Equation 44

●

OFF phase

TOT

DSON_HS

OUT

()

⋅

OUT

⋅⋅

⋅

TOT

0.3

1.5

0.7

1.5

–

850

2.4

–

⋅⋅

⋅

⋅⋅

1.2W

≅

Rth

–

TOT

⋅

1.2W

° C

-------

⋅

110

° C

≅

⋅

L TON

OUT

–

DCR

DSON HS

() I

⋅

–

-------------------------------------------------------------------------------------------------- T

()

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