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LTC3226

3226fa

APPLICATIONS INFORMATION

So even if the charge pump input current limit is pro-

grammed for 315mA (RPROG = 33.2k), the actual charge

current will be considerably less than 157.5mA (half of

programmed limit) in 2x mode for very low input supply.

For VIN = 2.5V, the CPO voltage above which the charge

current will decrease from the programmed value of

157.5mA (RPROG = 33.2k) can be calculated from the

previous equation as follows:

VCPO = (2 • 2.5V – 157.5mA • 6Ω) = 4.055V

Choosing the LDO Output Capacitor

In the event VIN falls below the programmed PFI threshold,

the PFI comparator turns off the charge pump and turns

on the internal LDO to supply the load from the backed-

up supercapacitor storage. However, due to the delay

associated with the PFI comparator and LDO circuitry, it

could be up to 2μs before the LDO is capable of supplying

the load demand at VOUT. In order to prevent VOUT from

drooping too much during this transition, a 47μF ceramic

capacitor is recommended at the VOUT terminal. For any

output capacitance, COUT, delay, Δt, and load current,

ILOAD, the drop in VOUT, ∆V, can be calculated using the

following equation:

LOAD = COUT •

ΔV

Δt

For example, if VOUT can not tolerate more than 100mV

drop under a maximum load of 2A during this transition,

the minimum capacitance required at the LDO output can

be calculated using the above equation as follows:

OUT(MIN) = 2A •

2µs

100mV

= 40µF

Charging a Single Supercapacitor

The LTC3226 can also be used to charge a single super-

capacitor by connecting two series-connected matched

ceramic capacitors with a minimum capacitance of 100μF

in parallel with the supercapacitor as shown in Figure 3.

Supercapacitor Manufacturers

Refer to the following table for supercapacitor manu-

facturers.

Table 1. Supercapacitor Manufacturers

CAP-XX

www.cap-xx.com

NESS CAP

www.nesscap.com

Maxwell

www.maxwell.com

Bussmann

www.cooperbussmann.com

AVX

www.avx.com

Illinois Capacitor

www.illinoiscapacitor.com

Tecate Group

www.tecategroup.com

Board Layout Considerations

Due to high switching frequency and high transient cur-

rents produced by the LTC3226 charge pump, careful

board layout is necessary for optimum performance. A true

ground plane and short connections to all of the external

capacitors will improve performance.

Also, to be able to deliver maximum load current from

the LDO under all conditions, it is critical that the exposed

metal pad on the backside of the QFN package has a good

thermal contact to the PC board ground plane. Lack of

proper thermal contact can cause the junction temperature

to exceed the threshold for thermal shutdown.

Figure 3. Charging a Single Supercapacitor

CSUP

3226 F03

CPO

LTC3226

VMID

GND

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