9

LTC1622

∆V

I

ESR

fC

OUT

RIPPLE

OUT

≈+









1

8

tor. The output ripple is highest at maximum input voltage

since

The choice of using a smaller output capacitance in-

creases the output ripple voltage due to the frequency

dependent term, but can be compensated for by using

capacitors of very low ESR to maintain low ripple voltage.

optimized to provide stable, high performance transient

response regardless of the output capacitors selected.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest ESR (size)

product of any aluminum electrolytic at a somewhat

met, the RMS current rating generally far exceeds the

In surface mount applications, multiple capacitors may

have to be paralleled to meet the ESR or RMS current

handling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available in

surface mount configurations. In the case of tantalum, it is

critical that the capacitors are surge tested for use in

switching power supplies. An excellent choice is the AVX

TPS, AVX TPSV and KEMET T510 series of surface mount

tantalum, available in case heights ranging from 2mm to

4mm. Other capacitor types include Sanyo OS-CON, Sanyo

POSCAP, Nichicon PL series and the Panasonic SP series.

Low Supply Operation

Although the LTC1622 can function down to 2V, the

decreases below 3V. Figure 3 shows the amount of change

as the supply is reduced down to 2V. Also shown in

The allowable forward voltage drop in the diode is calcu-

lated from the maximum short-circuit current as:

V

P

I

F

D

SC MAX

≈

()

determined by efficiency and/or thermal requirements.

A fast switching diode must also be used to optimize

efficiency. Schottky diodes are a good choice for low

forward drop and fast switching times. Remember to keep

lead length short and observe proper grounding (see

Board Layout Checklist) to avoid ringing and increased

dissipation.

In continuous mode, the source current of the P-channel

input capacitor sized for the maximum RMS current must

be used. The maximum RMS capacitor current is given by:

CI

VV

V

V

IN

MAX

OUT

IN

OUT

IN

−

()

used for design because even significant deviations do not

offer much relief. Note that capacitor manufacturer’s

ripple current ratings are often based on 2000 hours of life.

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may be paralleled to meet the

size or height requirements in the design. Due to the high

operating frequency of the LTC1622, ceramic capacitors

if there is any question.

series resistance (ESR). Typically, once the ESR require-

ment is satisfied, the capacitance is adequate for filtering.

The output ripple (