8

LTC1872

Figure 4. Setting Output Voltage

3

LTC1872

R1

1872 F04

R2

Setting Output Voltage

The LTC1872 develops a 0.8V reference voltage between

the feedback (Pin 3) terminal and ground (see Figure 4). By

selecting resistor R1, a constant current is caused to flow

through R1 and R2 to set the overall output voltage. The

regulated output voltage is determined by:

VV

R

R









08

1

2

1

.

tor.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest ESR (size)

product of any aluminum electrolytic at a somewhat

higher price. The output capacitor RMS current is approxi-

mately equal to:

IDC

DC

2

duty cycle.

When using electrolytic output capacitors, if the ripple and

ESR requirements are met, there is likely to be far more

capacitance than required.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the ESR or RMS current

handling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available in

surface mount configurations. An excellent choice of

tantalum capacitors is the AVX TPS and KEMET T510

series of surface mount tantalum capacitors. Also,

ceramic capacitors in X5R pr X7R dielectrics offer excel-

lent performance.

Low Supply Operation

Although the LTC1872 can function down to approxi-

mately 2.0V, the maximum allowable output current is

amount of change as the supply is reduced down to 2V.

goes below 2.3V.

INPUT VOLTAGE (V)

2.0

105

100

95

90

85

80

75

2.2

2.4

2.6

2.8

1872 F03

3.0