ST1S14 datasheet(28/46 Pages) STMICROELECTRONICS

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Application information

ST1S14

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reduced and, in most applications, this is enough to limit the switch current to the

active current threshold, nominal or foldback depending on the FB voltage.

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

•

ON phase

Equation 29

•

OFF phase

Equation 30

where VD is the voltage drop across the diode, DCRL is the series resistance of the inductor.

The pulse-by-pulse current limitation is effective in implementing constant current protection

when:

Equation 31

The overcurrent protection is operating over the entire output voltage, which goes from the

regulated output voltage (VO_SET) down to GND during heavy short circuit applied at the

output.

From Equation 29 and Equation 30 we can gather that the implementation of the constant

current protection becomes more critical the lower is the VOUT and the higher is VIN.

In fact, the voltage applied to the inductor during the OFF time becomes equal to the voltage

drop across parasitic components (typically the DCR of the inductor and the VFW of the free

wheeling diode) when VOUT is negligible, while during TON the voltage applied the inductor

is maximized and it is approximately equal to VIN. In general the worst case scenario is

heavy short-circuit at the output with maximum input voltage.

7.4.1

300 mV < VFB < 1.22 V

The nominal output voltage can be written as:

Equation 32

From Equation 32 the voltage can be expressed as:

Equation 33

L TON

OUT

–

DCR

DSON

() I

⋅

–

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

()

L TON

OUT

DCR

⋅

()

–

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

OFF

()

L TON

L TOFF

O_SET

-------

⎝⎠

⎛⎞

⋅

1.22

-------

⎝⎠

⎛⎞

⋅

-------

⎝⎠

⎛⎞

O_SET

1.22

-------------------

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