ST1S14 datasheet(27/46 Pages) STMICROELECTRONICS

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ST1S14

Application information

Example:

–VIN = 24 V

–VOUT = 5 V

–IOUT = 3 A

RDS(on) has a typical value of 0.2 Ω @ 25 °C and increases to a maximum value of 0.4 Ω @

125 °C. We can consider a value of 0.3

Ω.

TSW_EQ is approximately 12 ns.

IQ has a typical value of 2 mA @ VIN = 24 V.

The overall losses are:

Equation 27

The junction temperature of the device is:

Equation 28

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 40 °C/W, and an ambient

temperature of about 40 °C:

7.4

Short-circuit protection

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

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

minimum value (approximately 90 nsec typical) to keep the inductor current limited. This is

the pulse by pulse current limitation to implement constant current protection feature.

For the ST1S14, the operation of the pulse by pulse current limitation out of the soft start

time depends on the FB voltage:

•

300 mV <VFB <1.22 V: the device operates at nominal switching frequency and the

current limitation value

•

VFB < 300mV: the switching frequency is decreased five times the nominal value (170

kHz = 850 kHz/5) if the current protection is triggered. The frequency foldback helps to

prevent the current diverging at low VOUT / high input voltage. The current foldback,

which is active out of the soft start time, reduces the stress of the embedded power

element and the external power components in case of persistent short circuit at the

output. The current foldback is disabled during the soft start time to provide full current

capability to charge the output capacitor at the power-up phase.The foldback peak

current value is to 1.45 A typical. In overcurrent condition, the duty cycle is strongly

TOT

DSON

OUT

()

⋅

OUT

⋅

⋅⋅

⋅

0.3

()

0.137

–

850

–

24210

–

⋅⋅

⋅

⋅⋅

1.15W

≅

Rth

–

TOT

⋅

1.15

° C

≅

⋅

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