Datasheet

LTC3226
12
3226f
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:
∆V
IN(HYS)
= 1.2+0.02 – 0.4
( )
V •
200kΩ
2MΩ
= 82mV
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
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
F LY
), 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= 1to4
R
S
+
1
f
OSC
• C
FLY
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.