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LTC1515

Step-Up/Step-Down Switched Capacitor DC/DC Converters with Reset

Linear Technology 

LTC1515 Series

APPLICATIONS INFORMATION

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:

VOUT = (1.232V)[1 + (R1/R2)]

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

otherwise excess ripple may appear at VOUT.

ROUT

+– 2VIN

+

VOUT

–

COUT

IOUT

LT1515 • F02

Figure 2. Step-Up Mode Equivalent Circuit

40

30

IOUT = 20mA

IOUT = 50mA

20

10

1 SHDN

R1

2 POR

VOUT 8

7

VIN

3

FB

LTC1515

C1+

6

R2

4

GND

C1– 5

LTC1515 • F01

Figure 1. LTC1515 Output Voltage Selection

Maximum VOUT and IOUT Calculations for the LTC1515

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

voltage is approximately 2VIN (see Figure 2). In step-down

mode, the open circuit output voltage equals VIN. The

relationship between ROUT and VIN in step-up mode is shown

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.

Step-Up Mode: Max VOUT = (2)(Min VIN) – (IOUT)(ROUT)

Step-Down Mode: Max VOUT = (Min VIN) – (IOUT)(ROUT)

When VIN – (IOUT)(ROUT) is less than the programmed

VOUT, the part will automatically switch from step-down

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

0

1

2

3

4

5

INPUT VOLTAGE (V)

LT1515 • F03

Figure 3. Step-Up Mode ROUT vs Input Voltage

down mode, ROUT is internally adjusted to ensure that the

maximum output current rating can be met.

Capacitor Selection

For best performance, low ESR capacitors are recom-

mended for both CIN and COUT to reduce noise and ripple.

The CIN and COUT capacitors should be either ceramic or

tantalum and should be 10µF or greater. If the input source

impedance is very low (< 0.5Ω) CIN may not be needed.

Increasing the size of COUT to 22µF or greater will reduce

output voltage ripple — particularly with high VIN voltages

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

the flying capacitor C1 with a value of 0.1µF or 0.22µF.

Smaller values may be used in low output current

applications.

Output Ripple

Normal LTC1515 series operation produces voltage ripple

on the VOUT pin. Output voltage ripple is required for the

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

6

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