Copyright © 2015 Robot Circuits, LLC
9
RCAT-1A Rev A3 Designer’s manual
Serious Power for the Serious Designer
Optionally, you can connect a shorting wire in place of the switch if you will be controlling the RCAT™ power from the
power supply itself.
The RCAT™ can be fitted with an optional onboard 5Vdc regulator (U4) for cases where you do not have 5Vdc already
available. As shipped the RCAT™ does not include this regulator. This is something you would add yourself. The chip
required is the LM7805C – TO220 and can be purchased
here
. Cost is around 68 cents. Figure 3 shows the land patterns
where this chip installs.
Figure 3
The RCAT™ draws about 500mA when it isn’t driving external peripherals. When driving other peripheral devices such as
devices utilizing the full capability of the mosfets (which can draw up to 400mA each), and sonar ranging modules, etc.,
the current requirements increase accordingly. The LM7805 is good for 1A which will (with adequate heatsinking) suffice
for moderate current requirements, but for most applications we recommend using an external 5Vdc supply rated for
3A which should cover the most extreme cases. Of course, your particular design requirements will drive this decision.
As shown in Figure 2, there exists an onboard 3.3Vdc regulator (U5) that supplies power to the onboard devices
requiring 3.3Vdc. These include the accelerometer (about 10mA max), the USB bridge chip (about 26mA max), and the
5V to 3.3V voltage translators (about 100mA max) for a total consumption around 136mA. The regulator employed is a
LM2937-3.3 that is capable of up to 500mA. Under normal conditions, 136mA does not require a heat sink. However, if
you plan to tap into the 3.3Vdc to power external devices you may need to fit U5 with an appropriate heat sink – not
doing so can damage the RCAT™ and will void your warranty.
