LTC3252

8

3252f

Output Capacitor Selection

The style and value of capacitors used with the LTC3252

determine several important parameters such as regulator

control loop stability, output ripple and charge pump

strength.

The switching nature of the LTC3252 minimizes output

noise significantly but not completely. What small ripple

that exists at an output is controlled by the value of output

capacitor directly. Increasing the size of the output capaci-

tor will proportionately reduce the output ripple. The ESR

(equivalent series resistance) of the output capacitor plays

the dominant role in output noise. When the LTC3252

switches between clock phases there is a period where all

switches are turned off. This “blanking period” shows up

as a spike at the output and is a direct function of the output

current times the ESR value. To reduce output noise and

ripple, it is suggested that a low ESR (<0.08

Ω) ceramic

capacitor be used for the output capacitor. Tantalum and

aluminum capacitors are not recommended because of

their high ESR.

Both the style and value of the output capacitors can

significantly affect the stability of the LTC3252. As shown

in the Simplified Block Diagram, the LTC3252 uses a

control loop to adjust the strength of each charge pump to

match the current required at the output. The error signal

of each loop is stored directly on each output capacitor.

Thus the output capacitors also serve to form the domi-

nant pole in each control loop. Figure 3 is a graph of the

recommended output capacitance, and minimum capaci-

tance required for good transient response (see the Ce-

ramic Capacitor Selection Guidelines section).

Likewise excessive ESR on the output capacitor will tend

to degrade the loop stability of the LTC3252. The closed

loop output impedance of the LTC3252 is approximately:

R

V

V

O

OUT

=Ω

008

08

.•

.

is 0.15

Ω, which produces a 38mV output change for a

250mA load current step. For stability and good load

transient response it is important for the output capacitor

to have 0.1

Ω or less of ESR. Ceramic capacitors typically

have exceptional ESR and combined with a tight board

layout should yield excellent stability and load transient

performance.

Further output noise reduction can be achieved by filtering

the LTC3252 outputs through a very small series inductor

as shown in Figure 4. A 10nH inductor will reject the fast

output transients caused by the blanking period, thereby

presenting a nearly constant output voltage. For economy

the 10nH inductor can be fabricated on the PC board with

about 1cm (0.4") of PC board trace.

OPERATIO

(Refer to Simplified Block Diagram)

Figure 3. Output Capacitance vs Output Voltage

Figure 4. 10nH Inductor Used for

Additional Output Noise Reduction

3252 F04

LTC3252

OUT

GND

4.7

µF

0.47

µF

10nH

The low noise, dual phase architecture used by the LTC3252

makes input noise filtering much less demanding than

conventional charge pump regulators. The LTC3252 input

each half cycle of the oscillator. The blanking period

this reason it is recommended that a low ESR 4.7

µF (1µF

Ceramic Capacitor Selection Guidelines section). Alumi-

num and tantalum capacitors can be used but are not

recommended because of their high ESR.

0.9

8

7

6

5

4

3

2

3252 F03

1

1.1

1.2

1.3

1.4

1.5

1.6

RECOMMENDED

CAPACITANCE

MINIMUM

CAPACITANCE