EnerChip Solar Energy Harvesting Demo Kit

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DS-72-08 Rev18

Page 5 of 11

Designing for Pulse Discharge Currents in Wireless Devices

Pulse currents of tens of milliamperes are common in wireless sensor systems during transmit and receive

modes. Pulse discharge currents place special demands on energy storage devices. Repeated delivery of

pulse currents exceeding the recommended load current of a given chemistry will diminish 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. 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 main energy storage device. The storage device charges the capacitor between discharge pulses and the

capacitor delivers the pulse current to the load. Specifying the capacitance for a given energy storage device in

an application is a straightforward procedure, once a few key parameters are known. The key parameters are:

Storage cell impedance (at temperature and state-of-charge)

»

Storage cell voltage (as a function of state-of-charge)

»

Operating temperature

»

Pulse current amplitude

»

Pulse current duration

»

Allowable voltage droop during pulse discharge

»

Two equations will be used to calculate two unknown parameters:

1) the output capacitance needed to deliver the specified pulse current of a known duration;

2) the latency time that must be imposed between pulses to allow the capacitor to be recharged by the

main energy storage device such as the EnerChip solid state storage cell.

Both formulae will assume that the capacitor ESR is sufficiently low to result in negligible internal voltage

drop while delivering the specified pulse current; consequently, only the EnerChip device resistance will be

considered in the formula used to compute capacitor charging time and only the load resistance will be

considered when computing the capacitance needed to deliver the discharge current. The first step in creating

an EnerChip-capacitor couple for pulse current applications is to size the capacitance using the following

formula:

Discharge formula: C = t / [ R * ln (Vmax / Vmin) ]

where:

C = output capacitance, in parallel with EnerChip;

t = pulse duration;

R = load resistance = Vout(average) / Ipulse

Vmin and Vmax are determined by the combination of the EnerChip voltage at a given state-of-charge and the

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 * ln [ (Vmax - Vchg) / (Vmin - Vchg) ]

where:

t = capacitor charging time, from Vmin to Vmax

R = EnerChip resistance

C = output capacitance, in parallel with EnerChip

Vmax = final voltage to which the capacitor must be charged prior to delivering the next current pulse

Vmin = initial voltage on the capacitor when charging begins

Vchg = applied charging voltage on the capacitor