ST1CC40 datasheet(26/37 Pages) STMICROELECTRONICS

26 / 37 page

Application information

ST1CC40

26/37

DocID18279 Rev 5

The overall losses are:

Equation 32

Equation 33

The junction temperature of the device is:

Equation 34

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 board is about 40 °C/W.

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

Equation 35

7.4

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 typ.) to keep the inductor current limited. This

is the pulse-by-pulse current limitation to implement the 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 36



OFF phase

Equation 37

where DCRL is the series resistance of the inductor.

PTOT

RDS(on)_HS IOUT





2 DR

DS(on)_LS

IOUT





–





VIN IOUT fSW TSW





VIN IQ



PTOT

0,14 0,7

2 0,6 0,1 0,72 0,4



0,7 12 10

–

850 10

12 1,5 10

–







205mW



TA RthJA

–

PTOT



60 0,205 40 68

C





IL TON



VIN VOUT

–

DCRL RDS(on) HS

 I



–

------------------------------------------------------------------------------------------------- TON



IL TON



VOUT

DCRL RDS(on) LS

 I





–

----------------------------------------------------------------------------------------- TOFF



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