Datasheet

LTC3226
13
3226f
APPLICATIONS INFORMATION
For the LTC3226 charge pump, the sum of the switch
resistances is approximately 2.5Ω in a typical applica-
tion where V
IN
= 3.3V and V
CPO
= 5V. For C
F LY
= 1µF and
f
OSC
= 1MHz, the effective open-loop output resistance
of the charge pump can be approximated from the above
equation as follows:
R
OL
≅ 2 • 2.5Ω+
1
1MHz• 1µF
= 6Ω
Maximum Available Charge Current
In the absence of any internal current limit, the maximum
available current out of a charge pump in 2x mode can be
calculated from the charge pump input and output voltage
and the effective open-loop output resistance R
OL
using
the following equation:
I
CHRG
=
2V
IN
– V
CPO
R
OL
For example, if the LTC3226 charge pump (R
OL
≅ 6Ω)
has to charge a supercapacitor to 5V from 2.5V input,
the charge current available when V
CPO
= 4.8V can be
calculated as follows:
I
CHRG
=
2 • 2.5V – 4.8V
6
Ω
= 33.3mA
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
•
∆V
∆t
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