L9305A
APPLICATION INFORMATIONS (refer to application circuit)
D1 and D2 diodes are required only for reverse po-
larity protection.
V
S MAX – VD1 – 18
< IDZ MAX
RS
If VS may be higher than VZS aresistor RS is neces-
sary to limit the zener current IZS. In order to deter-
mineRS valuethe following equationscan be used :
where IDZ MAX is the maximum allowed DZ current.
VA voltage cannot be higher than 20 V otherwise
output overvoltage protection may be activated.
Morever VA must be less than 16 V if short circuit
protection is required.
V
S MAX – VD1 – VZS min
1)
< IZS MAX
RS
2) VS min – VD1 – RS – ISON MAX > VST min
DZ2 =22 to 24 V is a mandatoryfor output 7 protec-
tion if VS may be higher than 26 V.
where from Tamb = 25 °C :
- VS MAX and VS min are the maximum and minimum
values of power supply voltage
MOUNTING INSTRUCTION
- VD1 is the forward diode D1 voltage drop
- VZS min = 20 V
TheL9305Aisassembledin a new plasticpackage,
the Powerdip, in which 8 pins (from 9 to 16) are at-
tachedto the frame and remover the heatproduced
by the chip.
- IZS MAX = 30 mA for d.c. mode and IZS MAX =80 mA
for pulsed mode (see Absolute maximum ratings)
- ISOM MAX = 30 mA
Figure 2 and 3 show two ways of heatsinking.In the
first case, a PC board copper area is used as a
heatsink I = 65 mm. While in the second case, the
device is soldered to an external heatsink. In both
examples, the thermal resistance junction-ambient
is 35 °C/W.
- VST min = 3.5 V
If no RS value can satisfy the system 1), 2) a more
powerfull external zener DZ = 18 V is required.
Then 1) becomes:
Figure 2 : Example of Heatsink Using PC Board
Figure 3 : Example of an External Heatsink.
Copper (I = 65 mm).
4/6
STMICROELECTRONICS [ ST ]
