Application information
8 Application information
8.1
STCS05A
Reverse polarity protection
STCS05A must be protected from reverse connection of the supply voltage. Since the current sunk from V
CC
pin is in the range of 450 µA a small diode connected to V
CC
is able to protect the chip. Care must be taken for the whole application circuit, especially for the
LEDs, in fact, in case a negative voltage is applied between V
IN
and GND, a negative voltage will be applied to the LED string that must have a total breakdown voltage higher than the negative applied voltage in order to avoid any damage.
Figure 15. Reverse polarity condition
8.2 Thermal considerations
The STCS05A is able to control a LED current up to 500 mA and able to sustain a voltage on the drain pin up to 40 V. Those operating conditions are however limited by thermal constraints, the thermal resistances shown in the thermal data section is the typical ones.
The power dissipation in the device can be calculated as follow:
P
D
= (V
DRAIN
- V
FB
) x I
LED
+ (V
CC x I
CC
) basing on this and on the thermal resistance and ambient temperature, the junction temperature can be calculated as:
T
J
= R thJA
x P
D
+ T
A
A typical application could be:
– Input voltage: 12 V;
– 3 white LEDs with an typical V
F
= 3.6 V;
– LEDs current: 350 mA;
– Package: SO-8;
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STCS05A Application information
– T
A
= 50 °C;
In this case V
DRAIN
= 12 - 3 x 3.6 = 1.2 V
P
D
= (1.2 - 0.1) x 0.35 + 12 x 0.5 x 10
-3
= 0.385 + 6 x 10
-3
= 391 mW
The junction temperature will be:
T
J
= 100 x 0.391 + 50 = 89 °C
For a correct operation of the chip, the junction temperature must not exceed 110 °C.
The following pictures show the maximum power dissipation according to the ambient temperature:
Figure 16. Maximum power dissipation vs T
A
for SO-8
25 35 45 55 65 75 85 95 105 115 125
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