LTC3226

9

3226fa

OPERATION

The LTC3226 is a 2-cell series supercapacitor charger

designed to back up a Li-ion battery or any system rail

in the range of 2.5V to 5.3V. It has four principal circuit

components: a dual mode (1x/2x) charge pump with an

integrated balancer and a voltage clamp, an LDO to supply

the load current from the charge stored on the superca-

pacitor, an ideal diode controller to control the gate of the

to decide whether to activate the charge pump to charge the

supercapacitor stack or to activate the LDO to supply the

The LTC3226 has two modes of operation: normal and

threshold voltage, the part is in normal mode in which

and the internal charge pump stays on to top off the

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.

CHARGE PUMP

One of the principal circuit components of the LTC3226

is a dual mode low noise constant frequency (0.9MHz)

and stores it onto the supercapacitor stack at the CPO pin.

The target or termination voltage on the CPO pin is pro-

grammed by an external resistor divider using the CPO_FB

pin. The input current limit to the charger is programmed

by an external resistor between the PROG pin and ground.

programmable PFI threshold. At the beginning of the charge

pump is in 1x mode (linear mode) in which the charge pump

acts as a pass element and charges the supercapacitor with

a charge current that is limited by the programmed input

current limit. As the CPO voltage rises to within 200mV of

the input supply voltage, the charge pump switches to 2x

mode (doubler mode) in which the average charge current

is approximately equal to half the input current limit. As the

CPO voltage exceeds the target value by approximately 1%,

the charge pump switches turn off and the charge pump

enters the sleep mode. In sleep mode, most of the charge

pump control circuitry is turned off to minimize quiescent

current. As the supercapacitor discharges due to leakage

and internal quiescent current load, the CPO pin voltage

slowly drops. When the CPO pin voltage drops 1% below

the programmed voltage, the charge pump turns on to

replenish charge on the supercapacitor and the cycle con-

tinues. The charge pump can be turned off by pulling the

EN_CHG pin below 0.4V. However, by default, the charge

pump is always enabled via an internal low current pull-up

circuit if the EN_CHG pin is left floating.

Voltage Clamp

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

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 Balancer

The LTC3226 is equipped with an internal leakage balanc-

ing amplifier which servos 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 bal-

ancer eliminates the need for external balancing resistors.