Rainbow Electronics ATA6626 - Manual (Page 7)

7
4970A–AUTO–01/07
ATA6621/22/24 Development Board
3. Boosting the Voltage Regulator
For some applications there is a need for a higher current than the internal voltage regulator
can deliver (50 mA). So it is possible to boost the maximum current by using an external NPN
transistor. On the development board there is already a placeholder for this part, into which
would fit, for example, the MJD31C in a D-PAK package. In addition to the transistor itself
there are two more components to be placed on the development board, the resistor R7 (3.3
Ω
)
and the electrolytic capacitor C4 (2.2 µF). The jumper J1 has to be removed in this case.
Note that the output voltage is no longer short-circuit protected when boosting the output cur-
rent with an external NPN transistor.
The limiting parameter for the output current is the maximum power dissipation of the external
NPN transistor. In the version at this stage the thermal resistance of the MJD31C soldered on
the minimum pad size is 80 K/W, meaning the maximum possible output current in the case of
V
S
= 12V is approximately 230 mA at room temperature. It is not recommended to exceed this
limit, because the transistor could be damaged as a result of overtemperature. If a higher out-
put current is required, additional cooling of the external transistor has to be ensured (see
Figure 3-2 , Figure 3-3 and Figure 3-4 on page 8 ).
Figure 3-1. Boosting the Voltage Regulator
Place T1, R7 and C4 and re-
move jumper J1 when boost-
ing the output current