LTC3226_12 Datasheet, PDF (13/16 Pages) Linear Technology – 2-Cell Supercapacitor Charger

LTC3226
APPLICATIONS INFORMATION
So even if the charge pump input current limit is pro-
grammed for 315mA (R_PROG = 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 V_IN = 2.5V, the CPO voltage above which the charge
current will decrease from the programmed value of
157.5mA (R_PROG = 33.2k) can be calculated from the
previous equation as follows:
V_CPO = (2 • 2.5V – 157.5mA • 6Ω) = 4.055V
Choosing the LDO Output Capacitor
In the event V_IN 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 V_OUT. In order to prevent V_OUT from
drooping too much during this transition, a 47μF ceramic
capacitor is recommended at the V_OUT terminal. For any
output capacitance, C_OUT, delay, Δt, and load current,
I_LOAD, the drop in V_OUT, ∆V, can be calculated using the
following equation:
I_LOAD
=
C_OUT
•
ΔV
Δt
For example, if V_OUT 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:
C_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
CPO
LTC3226
VMID
GND
C1
C_SUP
C2
3226 F03
Figure 3. Charging a Single Supercapacitor
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.
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