Datasheet

LTC3105
11
3105fa
APPLICATIONS INFORMATION
Step-Up Converter Feedback Configuration
A resistor divider connected between the V
OUT
and FB pins
programs the step-up converter output voltage, as shown
in Figure 2. An optional 22pF feedforward capacitor, C
FF1
,
can be used to reduce output ripple and improve load
transient response. The equation for V
OUT
is:
V
OUT
= 1.004V •
R1
R2
+ 1






LDO Regulator Feedback Configuration
Two methods can be used to program the LDO output
voltage, as shown in Figure 3. A resistor divider connected
between the LDO and FBLDO pins can be used to program
the LDO output voltage. The equation for the LDO output
voltage is:
V
LDO
= 1.004V •
R3
R4
+ 1






Alternatively, the FBLDO pin can be connected directly to
GND. In this configuration, the LDO is internally set to a
nominal 2.2V output.
Figure 2. FB Configuration
3105 F02
LTC3105
C
FF1
R1
R2
FB
V
OUT
Figure 3. FBLDO Configuration
3105 F03
LDO
LTC3105
R3
2.2V
R4
LDO
FBLDO FBLDO
LTC3105
MPPC Threshold Configuration
The MPPC circuit controls the inductor current to main-
tain V
IN
at the voltage on the MPPC pin. The MPPC pin
voltage is set by connecting a resistor between the MPPC
pin and GND, as shown in Figure 4. The MPPC voltage is
determined by the equation:
V
MPPC
= 10µA • R
MPPC
In photovoltaic cell applications, a diode can be used to
set the MPPC threshold so that it tracks the cell voltage
over temperature, as shown in Figure 5. The diode should
be thermally coupled to the photovoltaic cell to ensure
proper tracking. A resistor placed in series with the diode
can be used to adjust the DC set point to better match
the maximum power point of a particular source if the
selected diode forward voltage is too low. If the diode is
located far from the converter inputs, a capacitor may be
required to filter noise that may couple onto the MPPC
pin, as shown in Figure 5. This method can be extended
to stacked cell sources through use of multiple series
connected diodes.
Figure 4. MPPC Configuration
Figure 5. MPPC Configuration with Temperature Adjustment
3105 F04
MPPC
LTC3105
R
MPPC
10µA
3105 F05
MPPC
LTC3105
C6
10nF
V
FWD
R
MPPC
10µA
+
–