LTC3240-3.3/LTC3240-2.5

9

3240fb

amount of output ripple for a given load current. Increasing

of higher minimum turn-on time. The peak-to-peak output

ripple is approximately given by the expression:

V

I

fC

RIPPLE P P

OUT

OSC

OUT

()

•

−

≅

2

Also, the value and style of the output capacitor can sig-

nificantly affect the stability of the LTC3240. As shown

in the Block Diagram, the LTC3240 uses a linear control

loop to adjust the strength of the charge pump to match

the current required at the output. The error signal of this

loop is stored directly on the output storage capacitor.

This output capacitor also serves to form the dominant

pole of the control loop. To prevent ringing or instability

on the LTC3240, it is important to maintain at least 2µF

of capacitance over all conditions.

Excessive ESR on the output capacitor can degrade the

loop stability of the LTC3240. The closed-loop output

resistance of the LTC3240 is designed to be 0.5

Ω. For a

100mA load current change, the output voltage will change

by about 50mV. If the output capacitor has 0.5

Ω or more

of ESR, the closed-loop frequency response will cease to

roll off in a simple one-pole fashion and poor load transient

response or instability could result. Ceramic capacitors

typically have exceptional ESR performance and combined

with a tight board layout should yield very good stability

and load transient performance.

input current to the LTC3240 is relatively constant during

the input charging phase and the output charging phase

but drops to zero during the nonoverlap times. Since the

nonoverlap time is small (~25ns), these missing notches

result in a small perturbation on the input power supply

line. A higher ESR capacitor such as tantalum will have

higher input noise than a low ESR ceramic capacitor.

Therefore, ceramic capacitors are again recommended

for their exceptional ESR performance.

Further input noise reduction can be achieved by power-

ing the LTC3240 through a very small series inductor as

shown in Figure 3. A 10nH inductor will reject the fast

current notches, thereby presenting a nearly constant

current load to the input power supply. For economy, the

10nH inductor can be fabricated on the PC board with

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

Flying Capacitor Selection

Warning: A polarized capacitor such as tantalum or alumi-

num should never be used for the flying capacitor since

its voltage can reverse upon start-up of the LTC3240.

Low ESR ceramic capacitors should always be used for

the flying capacitor.

The flying capacitor controls the strength of the charge

pump. A 1µF or greater ceramic capacitor is suggested

for the flying capacitor. For the LTC3240-3.3 operating

necessary to have at least 0.5µF of capacitance for the

flying capacitor in order to achieve the maximum rated

current of 40mA.

For very light load applications, the flying capacitor may

be reduced to save space or cost. From the first order

Resistance” section, the theoretical minimum output

resistance of a voltage doubling charge pump can be

expressed by the following equation:

R

VV

If

C

OL MIN

IN

OUT

OUT

OSC

FLY

()

–

•

=≅

21

is the value of the flying capacitor. The charge pump

will typically be weaker than the theoretical limit due

Figure 3. 10nH Inductor Used for

Additional Input Noise Reduction

APPLICATIO S I FOR ATIO

LTC3240-3.3/

LTC3240-2.5

0.22

µF

2.2

µF

GND

1cm OF WIRE

10nH

1

2

3240 F03