Q
I'm using a 3.6V (28Ah) Lithium battery pack as the power source to power the
device. The load varies between
0.025uA in sleep mode and approx. 2A (when running the embedded GPRS modem for
data transmission). I'm switching the power with a ADP197 high side
switch to minimize power consumption. Since the current draw changes quite
drastically between
approx. 200mA and 2A (for bursts) I would like to know what kind of input and
output capacitors you recommend for such use? The datasheet recommends a
minimum 1uF, which seems quite low for such large current fluctuation.
Additionally, I would like to know if I could run all the power of the device
through a ADP197 switch. My idea is to use a key FOB magnet to turn on the
power of the device once its taken into use for the first time. The device is
hermetically sealed and does not have any buttons etc. on its enclosure.
Putting a simple Reed switch (NC) between the power source (battery) and a
ADP197 switch and the OUT pin attached to the ENABLE pin to keep the power
flowing from the battery to the switch continuously. What would the
implications of such use be? Would you recommend this kind of method for long
term (10+ years)? We have tested this in our lab and it works nicely, but I
would like to get your opinion on the design and the feasibility of running the
ADP197 continuously in enable mode.
A
The input and output capacitance recommended for the ADP195 is only a minimum
and can be increased depending on the allowable in-rush current at turn on and
the transient load response. The output capacitance required for a given
transient response depends on the size of the load step and the slew rate of
the load step. For example, if the load step is zero to 2A in 100nS, and the
maximum allowable transient load response is 100mV, then a 2uF capacitor for
the input and output is adequate. If the allowable load transient response is
10mV, then the capacitance should be 20uF, similarly, if the load step has a
10nS slew rate, the capacitance should be 20uF.
There should be no reliability issues with running all of the power through the
ADP197 as long as the total current does not exceed 3A at an ambient
temperature of 70C. If the temperature is 85C, the lifetime of the ADP197 is
reduced to 2190 hours. Note, the lifetime is calculated as hours of continuous
operation and is cumulative. So, if the device is carrying 3A at 85C and is on
for only 10% of the time, the lifetime is 21900 hours of calendar time. The
total operational lifetime remains 2190 hours.
