LTC3226
13
3226fa
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 • 6) = 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 2s 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 47F ceramic
capacitor is recommended at the V
OUT
terminal. For any
output capacitance, C
OUT
, delay, ∆t, 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
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
C1
C2
C
SUP
3226 F03
CPO
LTC3226
VMID
GND
