6

LTC1515 Series

Output Voltage Selection

The LTC1515-X versions have internal resistor networks

which set the output voltage. The 5/3 pin controls an

internal switch that shorts out a portion of the resistor

network to change the output voltage. A logic high on this

pin produces a 5V output and a low produces either a 3V

output or a 3.3V output.

The output voltage of the LTC1515 is selected using an

external resistor divider (see Figure 1). The output voltage

is determined using the following formula:

The total resistance of R1 and R2 should not exceed 2M,

Figure 1. LTC1515 Output Voltage Selection

SHDN

POR

GND

FB

LTC1515

1

2

3

4

8

7

6

5

LTC1515 • F01

R1

R2

The maximum output voltage and current available with

the LTC1515 can be calculated based on the effective

output resistance of the charge pump and the open circuit

output voltage. In step-up mode, the open circuit output

in Figure 3.

The following formulas can be used to find the maximum

output voltage that may be programmed using the LTC1515

for a given minimum input voltage and output current

load.

mode to step-up mode. In both step-up mode and step-

+

–

+

–

LT1515 • F02

Figure 2. Step-Up Mode Equivalent Circuit

INPUT VOLTAGE (V)

1

20

30

5

LT1515 • F03

10

0

2

3

4

40

maximum output current rating can be met.

Capacitor Selection

For best performance, low ESR capacitors are recom-

tantalum and should be 10

µForgreater.Iftheinputsource

impedance is very low (< 0.5

(8V or greater). A ceramic capacitor is recommended for

the flying capacitor C1 with a value of 0.1

µF or 0.22µF.

Smaller values may be used in low output current

applications.

Output Ripple

Normal LTC1515 series operation produces voltage ripple

parts to regulate. Low frequency ripple exists due to the

hysteresis in the sense comparator and propagation de-

lays in the charge pump enable/disable circuits. High

frequency ripple is also present mainly from the ESR