LTC3226
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
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.
external FET between V and V , and a PFI comparator
IN
OUT
todecidewhethertoactivatethechargepumptochargethe
supercapacitor stack or to activate the LDO to supply the
loadwhenV fallsbelowanexternallyprogrammedvalue.
IN
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 to VOUT through the external FET
and the internal charge pump stays on to top off the
supercapacitor 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.
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
voltage across the top capacitor (VMID-V ) ever gets
CPO
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
stackvoltagereachesitsprogrammedvalue. Ifthevoltage
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
maximumallowablechargecurrentwhichisapproximately
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.
CHARGE PUMP
One of the principal circuit components of the LTC3226
is a dual mode low noise constant frequency (0.9MHz)
regulated charge pump which transfers charge from V
IN
and stores it onto the supercapacitor stack at the CPO pin.
The target or termination voltage on the CPO pin is pro-
grammedbyanexternalresistordividerusingtheCPO_FB
pin. The input current limit to the charger is programmed
by an external resistor between the PROG pin and ground.
ThechargepumpturnsonwhenV exceedstheexternally
IN
programmablePFIthreshold.Atthebeginningofthecharge
Leakage Balancer
cycle when the CPO pin voltage is less than V , the charge
IN
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
supplyvoltageisabovethePFIthreshold. Theinternalbal-
ancer eliminates the need for external balancing resistors.
pumpisin1xmode(linearmode)inwhichthechargepump
actsasapasselementandchargesthesupercapacitorwith
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(doublermode)inwhichtheaveragechargecurrent
isapproximatelyequaltohalftheinputcurrentlimit.Asthe
CPOvoltageexceedsthetargetvaluebyapproximately1%,
the charge pump switches turn off and the charge pump
3226fa
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Linear Systems [ Linear Systems ]
