Pulse Discharge Current For A Wireless End Device - Texas Instruments MSP430 User Manual

Pulse Discharge Current for a Wireless End Device

3.5 Pulse Discharge Current for a Wireless End Device
High current pulses place special demands on batteries. Repeated delivery of pulse currents exceeding
the recommended load current of a given chemistry diminishes the useful life of the cell. The effects can
be severe, depending on the amplitude of the current and the particular cell chemistry and construction.
Pulse currents of tens of milliamperes are common in wireless sensor systems during transmit and receive
modes. Moreover, the internal impedance of the cell often results in an internal voltage drop that
precludes the cell from delivering the pulse current at the voltage necessary to operate the external circuit.
One method of mitigating such effects is to place a low equivalent series resistance (ESR) capacitor
across the battery. The battery charges the capacitor between discharge pulses, and the capacitor
delivers the pulse current to the load. Specifying the capacitance for a given battery in an application is a
straightforward procedure once a few key parameters are known. The key parameters are:
• Battery impedance (at temperature and state-of-charge)
• Battery voltage (as a function of state-of-charge)
• Operating temperatures
• Pulse current amplitude
• Pulse current duration
• Allowable voltage drop during pulse discharge
Two equations are used to calculate two unknown parameters:
• The output capacitance needed to deliver the specified pulse current of a known duration
• The latency time that must be imposed between pulses to allow the capacitor to be recharged by the
battery
Both formulas assume that the capacitor ESR is sufficiently low to result in negligible internal voltage drop
while delivering the specified pulse current; consequently, only the battery resistance is considered in the
formula used to compute capacitor charging time, and only the load resistance is considered when
computing the capacitance needed to deliver the discharge current.
The first step in creating a battery-capacitor couple for pulse-current applications is to size the capacitance
using the following formula:
Discharge formula: C = t / R × [–l
Where:
C = output capacitance in parallel with battery
t = pulse duration
R = load resistance = V
V and V are determined by the combination of the battery voltage at a given state-of-charge and the
min max
operating voltage requirement of the external circuit.
Once the capacitance has been determined, the capacitor charging time can be calculated using the
following formula:
Charge formula: t = R × C × [–l
Where:
t = capacitor charging time from V
R = battery resistance
C = output capacitance in parallel with battery
Again, V and V
min max
resistance varies according to temperature and state-of-charge as described above. Worst-case
conditions are often applied to the calculations to ensure proper system operation over temperature
extremes, battery condition, capacitance tolerance, etc.
16
Solar Energy Harvester Module (SEH-01)
(V / V
n min max
(average) / I
OUT pulse
(1 – V / V
n min
to V
min max
are functions of the battery voltage and the circuit operating specifications. Battery
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)]
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max
SLAU273C – January 2009 – Revised May 2010
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