LTC3250 datasheet(7/12 Pages) LINER

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LTC3250-1.5/LTC3250-1.2

3250fa

current. For an ideal 2 to 1 step-down charge pump the

power efficiency is given by:

η≡

OUT

•

The switching losses and quiescent current of the

LTC3250-1.5/LTC3250-1.2 are designed to minimize effi-

ciency loss over the entire output current range, causing

only a couple % error from the theoritical efficiency. For

example with VIN = 3.6V, IOUT = 100mA and VOUT regulat-

ing to 1.5V the measured efficiency is 80.6% which is in

close agreement with the theoretical 83.3% calculation.

VOUT Capacitor Selection

The ESR and value of capacitors used with the LTC3250-

1.5/LTC3250-1.2 determine several important parameters

such as regulator control loop stability, output ripple, and

charge pump strength.

The value of COUT directly controls the amount of output

ripple for a given load current. Increasing the size of COUT

will reduce the output ripple.

To reduce output noise and ripple, it is suggested that a

low ESR (<0.1

Ω) ceramic capacitor (4.7µF or greater) be

used for COUT. Tantalum and aluminum capacitors are not

recommended because of their high ESR.

Both ESR and value of the COUT can significantly affect the

stability of the LTC3250-1.5/LTC3250-1.2. As shown in

the block diagram, the LTC3250-1.5/LTC3250-1.2 use a

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 charge storage

capacitor. Thus the charge storage capacitor also serves

to form the dominant pole for the control loop. To prevent

ringing or instability it is important for the output capacitor

to maintain at least 2.5

µF of capacitance over all condi-

tions (see “Ceramic Capacitor Selection Guidelines” sec-

tion).

Likewise excessive ESR on the output capacitor will tend

to degrade the loop stability of the LTC3250-1.5/LTC3250-

1.2. The closed-loop output resistance is designed to be

0.15

Ω for the LTC3250-1.5 and 0.12Ω for the

LTC3250-1.2. For a 250mA load current change the output

voltage will change by about 37mV for the LTC3250-1.5

and by 30mV for the LTC 3250-1.2. If the ESR of the output

capacitor is greater than the closed-loop-output imped-

ance the part will cease to roll-off in a simple one-pole

fashion and poor load transient response or instability

could result. Ceramic capacitors typically have excep-

tional ESR performance 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 LTC3250-1.5/LTC3250-1.2 output through a very small

series inductor as shown in Figure 1. A 10nH inductor will

OPERATIO

(Refer to Simplified Block Diagram)

reject the fast output transients, 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.

VIN Capacitor Selection

The constant frequency architecture used by the

LTC3250-1.5/LTC3250-1.2 makes input noise filtering

much less demanding than conventional charge pump

regulators. On a cycle by cycle basis, the LTC3250-1.5/

LTC3250-1.2 input current will go from IOUT/2 to 0mA.

Lower ESR will reduce the voltage steps caused by chang-

ing input current, while the absolute capacitor value will

determine the level of ripple. For optimal input noise and

ripple reduction, it is recommended that a low ESR 1

µF or

greater ceramic capacitor be used for CIN (see “Ceramic

Capacitor Selection Guidelines” section). Aluminum and

tantalum capacitors are not recommended because of

their high ESR.

Figure 1. 10nH Inductor Used for

Additional Output Noise Reduction

3250 F01

LTC3250-1.5/

LTC3250-1.2

VOUT

GND

4.7

µF

0.22

µF

VOUT

10nH

(TRACE INDUCTANCE)

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