Datasheet

DocID17977 Rev 2 27/46
ST1S14 Application information
Example:
–V
IN
= 24 V
–V
OUT
= 5 V
–I
OUT
= 3 A
R
DS(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.
T
SW_EQ
is approximately 12 ns.
I
Q
has a typical value of 2 mA @ V
IN
= 24 V.
The overall losses are:
Equation 27
The junction temperature of the device is:
Equation 28
where T
A
is the ambient temperature and Rth
J-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 (Rth
J-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
P
TOT
R
DSON
I
OUT
()⋅
2
DV
IN
I
OUT
T
SW
F
SW
V
IN
I
Q
""=⋅+⋅⋅⋅+⋅=
0.3 3()
2
0.137 24 3 12 10
9–
850 10
3–
24210
3–
⋅⋅+⋅⋅ ⋅ ⋅ ⋅+⋅⋅ 1.15W≅=
T
J
T
A
Rth
JA–
P
TOT
⋅+=
T
J
40 1.15 40 86° C≅⋅+=