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LTC1550

Low Noise, Switched Capacitor Regulated Voltage Inverters

Linear Technology 

LTC1550/LTC1551

APPLICATIONS INFORMATION

noise – 4.1V generator which uses a ferrite bead output

filter to achieve better than 1mVP-P noise and output

ripple. The corresponding spectrum and spot noise plots

for this circuit are shown in the Typical Performance

Characteristics section.

Output Ripple vs Input Bypass Capacitor

The input bypass capacitor (CIN) can also have a fairly

significant impact on the output ripple. CIN provides most

of the LTC1550/LTC1551’s supply current while it is

charging the flying capacitor (C1). Inadequate input bypass

can cause the VCC supply to dip when the charge pump

switches, causing the output linear regulator to momentarily

stop regulating. CIN should be mounted as close to the

LTC1550/LTC1551 as possible and its value should be

significantly larger than C1. Tantalum capacitors with low

ESR generally provide adequate performance. Figure 6

shows the LTC1550/LTC1551 peak-to-peak output ripple

vs CIN, taken using the test circuit in Figure 2 with ILOAD set

at 5mA. COUT is a 10µF in parallel with a 0.1µF ceramic

capacitor.

A 4.7µF tantalum capacitor at VCC generally provides

adequate output ripple performance for most applica-

tions.

8

7

VCC = 6V

TA = 25°C

COUT = 10µF

6

5

4

3

2

1

0

0.1

1

10

100

INPUT CAPACITANCE (µF)

LTC1550/51 • F07

Figure 6. Output Ripple vs Input Bypass Capacitance

TYPICAL APPLICATION

– 4.1V Output GaAs FET Bias Generator

4.5V ≤ VCC ≤ 6.5V

+ CIN

4.7µF

1

SHDN

2

VCC

8

SENSE

7

CPOUT

LTC1550

3 C1+

GND 6

4

VOUT

C1– 5

CCP

0.1µF

– 4.1V BIAS

C1

0.1µF

COUT

10µF

CL

0.1µF

GaAs

TRANSMITTER

1550/51 TA02

9

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