LTC3226_12 Datasheet, PDF (12/16 Pages) Linear Technology – 2-Cell Supercapacitor Charger

LTC3226
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
R_PF2
R_PF1
V_IN V_OUT
LTC3226
PFI
PFO
R_H
D1
470k
3226 F02
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
12
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 R_S
S=1to 4
+
f_OSC
1
• C_FLY
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_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
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 –
6Ω
4.8V
=
33.3mA
3226fa