LTC3202
OPERATIO
In this configuration the feedback factor (∆V
FB
/∆V
OUT
) will
be very near unity since the small signal LED impedance
will be considerably less than the current setting resistor
R
X
. Thus, this configuration will have the highest
loop gain
giving it the lowest closed-loop output resistance. Like-
wise it will also require the largest amount of output
capacitance to preserve stability.
For fixed voltage applications, the output voltage can be
set by the ratio of two resistors and the feedback control
voltage as shown in Figure 2. The output voltage is given
by the set point voltage times the gain factor 1 + R
1
/R
2
.
Note that the closed-loop output resistance will increase in
proportion to the loop gain consumed by the resistive
divider ratio. For example, if the resistor ratio is 2:1 giving
a gain of 3, the closed-loop output resistance will be about
3 times higher than its nominal gain of 1 value. Given that
the closed-loop output resistance is about 0.35Ω with a
gain of 1, the closed-loop output resistance will be about
1Ω when using a gain of 3.
3
V
OUT
= V
FB
(1 +
R1
)
R2
V
OUT
LTC3202
FB
GND
5, 11
3202 F02
OUTPUT RESISTANCE (Ω)
Figure 2. Voltage Control Mode
When using the LTC3202 in voltage control mode, any of
the three voltage settings (0.2V, 0.4V or 0.6V) can be used
as the set point voltage. For optimum noise performance
and lowest closed-loop output resistance the highest
voltage setting will likely be the most desirable.
Typical values for total voltage divider resistance can
range from several kΩs up to 1MΩ.
U
(Refer to Simplified Block Diagram)
Charge Pump Strength
Figure 3 shows how the LTC3202 can be modeled as a
Thevenin equivalent circuit to determine the amount of
current available from the effective input voltage, 1.5V
IN
and the effective open-loop output resistance, R
OL
.
R
OL
+
V
OUT
+
–
1.5V
IN
–
3202 F03
Figure 3. Equivalent Open-Loop Circuit
From Figure 3 the available current is given by:
I
OUT
=
1.5V
IN
– V
OUT
R
OL
Typical values of R
OL
as a function of temperature are
shown in Figure 4.
4.8
V
FB
= 0
I
L
= 100mA
C1 = C2 = 1µF
R
OL
= (1.5V
IN
– V
OUT
)/I
L
R1
2
1µF
R2
4.6
4.4
V
IN
= 2.7V
4.2
V
IN
= 3.6V
4.0
3.8
–40
–15
35
10
TEMPERATURE (°C)
60
85
3202 F04
Figure 4. Typical R
OL
vs Temperature
3202fa
7
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