CalAmp SMC-GPRS Series, User Manual

Page: 60 / 84

001-0004-829 Rev00
Page 60 of 84
SMC-GPRS Power Supply Considerations
Ideally, the external 5VDC supply should be able to provide the 1.4A current required by the SMC-
GPRS modem when the TX bursts occur. When this is not a practical design alternative, a mix
between a linear regulator and capacitor supply may be an alternative approach. Figure 3 shows the
basic block diagram.
Vin Vout 5VDC
External Regulator
SMC-GPRS
GND
GND
C
Figure 3: Regulator/Capacitor Block Diagram
In a mixed power supply solution there are two components that can supply the current for the
module if operated in parallel. These components are usually a big capacitor and a power supply
circuit. With this solution power is supplied from both the capacitor and the external regulator.
C = ( Icap x dT ) / dU
Icap Amps, current required by capacitor
dU Volts, voltage change across capacitor
dT 577µs, discharge time (TX burst timing)
Ipeak = I Icap (1.4A TX burst current)
For example, if the regulator supply is designed to provide 500mA to cover the average power needs
of the SMC-GPRS modem, the capacitor will need to supply the 900mA needed during TX bursts.
Assuming a 400mV change across the capacitor we have;
C = (0.9A x 577µs) / 0.4V
C = 1298µF
The table below shows the required capacitor values for different capacitor current requirements
using the same voltage drop across the capacitor.
Isupply,
Amps
Icap,
Amps
C, µF
0.5 0.9 1298
0.6 0.8 1154
0.7 0.7 1010
0.8 0.6 866
0.9 0.5 721
1.0 0.4 577