Power outages lasting just a few seconds can impact a system’s operating functions and also cause a loss of important data. In the past, a UPS (uninterruptible power supply) might have been used as a power backup, but it is overkill for short duration outages and requires constant attention to its batteries. Now, there is Linear Technology’s LTC3350, a mixed-signal power controller IC that can charge and monitor a series stack of one to four supercapacitors and then provide a several seconds of power backup (depending on the total capacitance and the load). In a typical system configuration, the LTC3350 mates with another circuit in order to provide it with dc backup power if it experiences a short duration power outage. The LTC3350 is available in a low profile 38-lead 5mm × 7mm × 0.75mm QFN surface mount package. Fig. 1 is a simplified schematic of an LTC3350 application.
Functions performed by LTC3350’s bi-directional switching controller include:
1. Charging one to four supercapacitors
2. In case of an associated circuit’s power failure, delivering dc voltage stored in the supercapacitors to the circuit input
3. Monitors system voltages, currents, and die temperature.
4. Balancing and providing overvoltage protection for the supercapacitors
5. Providing seamless power path control
6. Employing its I2C interface for data monitoring and capacitor charge control
The PFI pin is a Power-Fail Comparator input whose threshold voltage is programmed by an external resistor divider (R1 and R2) via the PFI pin. If VIN is above the externally programmable PFI threshold voltage, the synchronous controller operates in step-down mode and charges a stack of supercapacitors that store the maximum charge voltage. If VIN goes below the PFI threshold (for example, zero volts) then the synchronous controller runs in reverse as a step-up converter to deliver the stored supercapacitor stack voltage that provides the backup power.
In the charge mode, charging proceeds at a constant current until the supercapacitors reach their maximum charge voltage determined by the CAPFB servo voltage and the resistor divider between VCAP and CAPFB. The maximum charge current is determined by the value of sense resistor, RSNSC, in series with the inductor. The charge current loop servos the voltage across the sense resistor to 32mV. When charging begins, an internal soft-start ramp increases the charge current from zero to full current in 2 ms.
The LTC3350 provides constant power charging (for a fixed VIN) by limiting the input current drawn by the switching controller in step-down mode. The input current limit reduces charge current to limit the voltage across the input sense resistor, RSNSI, to 32 mV. If the combined system load plus supercapacitor charge current is large enough to cause the switching controller to reach the programmed input current limit, the input current limit loop will reduce the charge current by precisely the amount necessary to enable the external load to be satisfied.
IF input power is lost, the bidirectional switching controller acts as a step-up converter to provide power from the supercapacitors to VOUT, which becomes the backup power for the mating circuit. A resistor divider between VOUT and OUTFB sets VOUT regulation. The step-up mode can be used in conjunction with the output ideal diode. The VOUT regulation voltage can be set below the capacitor stack voltage. Upon removal of input power, power to VOUT will be provided from the supercapacitor stack via the output ideal diode. VCAP and VOUT will fall as the load current discharges the supercapacitor stack.
Two ideal diode controllers drive external N-channel MOSFETs (Q3 and Q4). The ideal diodes consist of a precision amplifier that drives the gates of N-channel MOSFETs whenever the voltage at VOUT is approximately 15 mV below the voltage at VIN or VCAP. Within the amplifier’s linear range, the small-signal resistance of the ideal diode will be quite low, keeping the forward drop near 15 mV. At higher current levels, the MOSFETs will be in full conduction.
The input ideal diode prevents the supercapacitors from back driving VIN during backup mode. A Fast-Off comparator shuts off the N-channel MOSFET if VIN falls 30 mV below VOUT. The PFI comparator also shuts off the MOSFET is there is a power failure.
The output ideal diode provides a path for the supercapacitors to power VOUT when VIN is unavailable. In addition to a Fast-Off comparator, the output ideal diode also has a Fast-On comparator that turns on the external MOSFET when VOUT drops 45 mV below VCAP.
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