The evaluation kit of the SD85 chip ultracapacitor provides a means to test the capacitor under a typical power loss protection scenarios. Features include:
- 2s2p ultracapacitor module daughter board
- 4.2V regulator for charging ultracapacitor bank
- 400mA constant current load for discharging ultracapacitor bank
- Manual power disconnect
- External trigger for power disconnect
- External source and load connections
- Sense connections for accurate voltage monitoring
- +85C ambient operational temperature for lifetime testing
Inputs / Outputs
The board contains numerous inputs and outputs to provide flexibility in testing the ultracapacitors module.
The POWER connections are the only required connections for testing the on-board functionality of the capacitor module under charging, discharging, and power loss. The input voltage, VIN, should be +5V with a minimum 2A current capability.
The SOURCE and LOAD connections directly connect to VCAP/GND of the capacitor module. They should be used to directly charge and discharge the ultracapacitor module. The capacitor module is connected in a 2s2p configuration with each capacitor rated for 2.1V at 85C. Charging the capacitors beyond 2.1V may result in accelerated degradation.
The SENSE connections are supplied for accurate voltage monitoring of the capacitor module. VCAP is the positive voltage of the top capacitor while MID is the middle voltage of the 2 series capacitors.
The EXT connection is a digital input supplied to toggle the disconnect circuit when in EXTERNAL mode. Set this pin low to connect the on-board regulator to the capacitor bank. Set this pin high (+5V) or open to disconnect the on-board regulator simulating a power loss scenario. The digital connection should be capable of sinking 0.5mA when set low.
External Source/Load Operation
External connections are available to customize charge/discharge cycles. When using an external charging source, set the DISCONNECT switch to EXTERNAL mode so that the load disconnect isolates the capacitors from the on-board regulator.
When using an external load, set the LOAD switch to OFF. It's possible to run the on-board load in parallel with an external load if desired. However, it's not recommended to do so when using the on-board regulator as the additional power dissipation may result in overheating.
Charging is accomplished via a low dropout linear regulator from the primary +5V rail. The TPS73801 has a built in current limit of 2A. Input sources should meet this requirement.
The on-board load is a fixed 400mA load current that when turned on is fully supported by the input regulator. The charging circuit is fully capable of supporting this load and frequent charge/discharge events up to +85C ambient temperature.
The capacitor will only charge via the on-board charger when the DISCONNECT switch is turned OFF.
The discharge load circuit is a simple linear current sink referenced from the +5V rail. A pair of resistors generate the positive reference voltage that's regulated over the pair of 1 Ohm load resistors. The result is a fixed 400mA draw on the capacitor module that's constant down to roughly 1V.
It's likely that real load circuits will behave similar to a constant power sink rather than a constant current sink. By measuring discharge with a constant current, however, energy and power estimations are simplified. External source and load connections are available to attach real loads and a more sophisticated constant power load.
The power disconnect is achieved through a highside bidirectional switch composed of two PFETs (IRLML6401). The open drain output of U4 drives both gates. When U4's positive input is high (>1.5V), it's output turns off and the 22k resistor pulls the gates of Q3 and Q4 high to turn them off, disconnecting the on-board charging circuit from the capacitor module and load.
The disconnect functions in two separate modes selected through switch S2.
EXTERNAL Mode: The circuit is by default disconnected through the 10k pullup resistor. When the external input is driven low the circuit switches to connected.
MANUAL Mode: The circuit is by default connected through the 22k pulldown resistor. When the S1 button is pressed, circuit transitions to connected.
External mode can therefore be used for automated testing such as accelerated lifetime testing to measure holdup times as the capacitor ages. Manual mode is suitable for asychronous testing to quickly measure or demonstrate performance across multiple loads or capacitor arrays.
Attached to U4's positive input is an LED driver. The LED is ON when the source is DISCONNECTED.
See the attached files for a full schematic and BOM.