Power path control and the diode
The LTC3226 contains a diode controller which controls the gate of an external PFET connected between the input, VIN
, and the output, VOUT
, through the GATE pin. Refer to Figure 2
for details. Under normal operating conditions, this external FET constitutes the main power path from input to output. For very light loads, the controller maintains a 15mV delta across the FET between the input and output voltage. In the event VIN
suddenly drops below VOUT
, the controller quickly turns the FET completely off to prevent any reverse conduction from VOUT
back to the input supply.
Figure 2. LTC3226 Block diagram
The LTC3226 has two modes of operation: normal and backup. If VIN
is above an externally programmable PFI threshold voltage, the part is in normal mode in which power flows from VIN
through the external FET and the internal charge pump stays on to top off the super capacitor stack. If VIN
is below this PFI threshold, the part is in backup mode. In this mode, the internal charge pump is turned off, the external FET is turned off and the LDO is turned on to supply the load current from the stored charge. See Figure 3
Figure 3: LTC3226 Normal-to-backup mode switching transient waveform
Voltage clamp circuitry
The LTC3226 charge pump is equipped with circuitry to limit the voltage across any supercapacitor in the stack to a maximum allowable preset voltage of 2.65V. If the voltage across the top capacitor (VMID-VCPO) ever gets to 2.65V before the CPO pin reaches the target voltage, the charge pump stops charging the top of the stack via the CPO pin, switches to 1x mode and delivers charge directly to the bottom capacitor via the VMID pin until the stack voltage reaches its programmed value. If the voltage across the bottom capacitor reaches 2.65V before the stack gets to its target value, the charge pump continues to deliver charge to the top of the stack via the CPO pin and a shunt regulator turns on to bleed charge off of the bottom capacitor and prevents the VMID pin voltage from rising any further. The shunt regulator is able to shunt the maximum allowable charge current which is approximately 315mA (in 1x mode). In the event both capacitors exceed 2.65V, the charge pump enters sleep mode by turning off most of its circuitry.
Leakage balancing circuitry
The LTC3226 is equipped with an internal leakage balancing amplifier which serves the VMID pin voltage to exactly half of the CPO pin voltage. However, it has limited source (~4.5mA) and sink (~5.5mA) capability. It is designed to handle slight mismatch of the supercapacitors due to leakage currents; not to correct any gross mismatch due to defects. The balancer is only active as long as the input supply voltage is above the PFI threshold. The internal balancer eliminates the need for external balancing resistors.
shows a comparison of Linear Technology’s family of supercapacitor chargers.
Table 2: Comparison of Linear Technology supercap chargers
(Click on image to enlarge)
Supercapacitors are now being used in applications where batteries were once the norm. Initial applications were low current, but technology has advanced and supercaps are now found in a variety of medium and high power applications in both consumer and non-consumer segments. Supercaps have many inherent advantages over batteries such as higher peak power delivery, longer cycle life and smaller form factor. However, product designers using supercaps are faced with problems such as cell balancing and potential over-voltage damage to supercap cells. Fortunately, Linear Technology has addressed these needs by continuously adding to its supercap charger IC family. The LTC3226 is a charge pump based supercap charger with seamless PowerPath control featuring automatic cell balancing, voltage clamping, reverse current protection, various operation modes, low current consumption, and up to 2A of backup current. The LTC3226 offers useful features in a small footprint, reducing overall solution size and in turn enabling more compact, simpler designs.
About the author:
Steve Knoth is senior product marketing engineer, Power Products Group, Linear Technology Corp.
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