LTC1550IGN datasheet(9/12 Pages) LINER

9 / 12 page

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 4.7

µF tantalum capacitor at VCC generally provides

adequate output ripple performance for most applica-

tions.

– 4.1V Output GaAs FET Bias Generator

Figure 6. Output Ripple vs Input Bypass Capacitance

INPUT CAPACITANCE (

µF)

0.1

100

LTC1550/51 • F07

VCC = 6V

TA = 25°C

COUT = 10µF

TYPICAL APPLICATION

4.5V

≤ VCC ≤ 6.5V

SENSE

CPOUT

SHDN

VCC

C1+

VOUT

LTC1550

– 4.1V BIAS

GND

C1–

COUT

µF

1550/51 TA02

CCP

0.1

µF

GaAs

TRANSMITTER

0.1

µF

CIN

4.7

µF

0.1

µF

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