7

LTC3202

3202fa

In this configuration the feedback factor (

be very near unity since the small signal LED impedance

will be considerably less than the current setting resistor

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

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 0.35

Ω with a

gain of 1, the closed-loop output resistance will be about

1

Ω when using a gain of 3.

Figure 2. Voltage Control Mode

Charge Pump Strength

Figure 3 shows how the LTC3202 can be modeled as a

Thevenin equivalent circuit to determine the amount of

Figure 3. Equivalent Open-Loop Circuit

3202 F02

LTC3202

FB

GND

)

R1

R2

1

µF

R1

R2

5, 11

3

2

3202 F03

+

–

–

+

OPERATIO

(Refer to Simplified Block Diagram)

From Figure 3 the available current is given by:

I

VV

R

OUT

IN

OUT

OL

=

15

.–

shown in Figure 4.

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 k

Ωs up to 1MΩ.

TEMPERATURE (

°C)

–40

4.8

4.6

4.4

4.2

4.0

3.8

60

3202 F04

–15

10

35

85

C1 = C2 = 1

µF