Datasheet

LTC3226
12
3226fa
APPLICATIONS INFORMATION
where V
F
is the forward voltage of the diode. As an ex-
ample, if R
PF1
= 200k, R
PF2
= 120k, R
H
= 2M, and V
F
=
0.4V, the additional hysteresis provided by this network
can be calculated using the above equation as follows:
ΔV
IN(HYS)
= 1.2+0.02 – 0.4
()
V•
200kΩ
2MΩ
= 82mV
GND is needed to set V
OUT
and the reset threshold 7.5%
below the V
OUT
programmed voltage. However, the reset
threshold can be set independent of V
OUT
by an additional
resistor divider.
Effective Open-Loop Output Resistance (R
OL
) of the
Charge Pump
The effective open-loop output resistance (R
OL
) of a charge
pump determines the strength of a charge pump. The value
of this parameter depends on many factors such as the
oscillator frequency (f
OSC
), value of the flying capacitor
(C
FLY
), the nonoverlap time, the internal switch resistances
(R
S
), and the ESR of the external capacitors. A first order
approximation of R
OL
is given below:
R
OL
≅ 2 Σ
S=1to 4
R
S
+
1
f
OSC
•C
FLY
For the LTC3226 charge pump, the sum of the switch
resistances is approximately 2.5 in a typical applica-
tion where V
IN
= 3.3V and V
CPO
= 5V. For C
FLY
= 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
1M Hz • 1µ F
= 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
≅ 6)
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
6Ω
= 33.3mA
3226 F02
LTC3226
D1
V
IN
V
OUT
PFO
PFI
470k
R
PF1
R
H
R
PF2
Figure 2. Increasing PFI Comparator Hysteresis Externally
Programming the LDO Output Voltage (V
OUT
)
The LDO output voltage in backup mode can be pro-
grammed for any voltage between 2.5V to 5.3V by using a
resistor divider from the V
OUT
pin to GND via the LDO_FB
pin such that:
V
OUT
= V
LDO_FB
•1+
R
FB1
R
FB2
⎛
⎝
⎜
⎞
⎠
⎟
where V
LDO_FB
is 0.8V. See the Block Diagram in Figure 1.
Typical values for R
FB1
and R
FB2
are in the range of 40k to
500k. Too small a resistor will result in a large quiescent
current whereas too large a resistor coupled with LDO_FB
pin capacitance will create an additional pole and may
cause loop instability.
Programming the Reset Threshold
The threshold for the reset comparator can be programmed
by using a resistor divider from the V
OUT
pin to GND via
the RST_FB pin such that:
V
OUT
= V
RST _ FB
•1+
R
FB1
R
FB2
⎛
⎝
⎜
⎞
⎠
⎟
where V
RST_FB
is 0.74V. See Figure 1. Typical values for
R
FB1
and R
FB3
are in the range of 40k to 5M. In most ap-
plications, the LDO_FB and RST_FB pins can be shorted
together and only one resistor divider between V
OUT
and