R5973D datasheet(26/39 Pages) STMICROELECTRONICS

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Application information

R5973D

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DocID026876 Rev 1

Example 2:



VIN = 12 V



VOUT = 3.3 V



IOUT = 2 A

RDS(on) has a typical value of 0.25 at 25 °C and increases up to a maximum value of 0.5 at

150 °C. We can consider a value of 0.4

.

TSW is approximately 70 ns.

IQ has a typical value of 2.5 mA at VIN = 12 V.

The overall losses are:

Equation 25

The junction temperature of device will be:

Equation 26

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

ambient. Considering that the device is mounted on board with a good ground plane, that it

has a thermal resistance junction to ambient (RthJ-A) of about 42 °C/W, and an ambient

temperature of about 70 °C:

Equation 27

8.4

Short-circuit protection

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

reduces the TON down to its minimum value (approximately 250 nsec) and the switching

frequency to approximately one third of its nominal value even when synchronized to an

external signal (see Section 5.4: Current protection on page 11). In these conditions, the

duty cycle is strongly reduced and, in most applications, this is enough to limit the current to

ILIM. In any event, in case of heavy short-circuit at the output (VO = 0 V) and depending on

the application conditions (Vcc value and parasitic effect of external components) the current

peak could reach values higher than ILIM. This can be understood considering the inductor

current ripple during the ON and OFF phases:



ON phase

Equation 28



OFF phase

PTOT

RDSON IOUT





DVIN IOUT



TSW



FSW VIN IQ=



0.4 2



0.3 12 2



–



250



–

12 2.5



–



0.93W



TA RthJA

–

PTOT



70 0.93 42 110

C





IL TON



VIN Vout

–

DCRL RDSON

 I



–

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



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