LTC1701 datasheet(6/12 Pages) LINER

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LTC1701

Most LTC1701 circuits will be well served by either an

MBR0520L or an MBRM120L. An MBR0520L is a good

choice for IOUT(MAX) ≤ 500mA, as long as the output

doesn’t need to sustain a continuous short.

Input Capacitor (CIN) Selection

In continuous mode, the input current of the converter is

a square wave with a duty cycle of approximately VOUT/

VIN. To prevent large voltage transients, a low equivalent

series resistance (ESR) input capacitor sized for the maxi-

mum RMS current must be used. The maximum RMS

capacitor current is given by:

RMS

MAX

OUT

≈

−

()

where the maximum average output current IMAX equals

the peak current (1 Amp) minus half the peak-to-peak

ripple current, IMAX = 1 – ∆IL/2.

This formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT/2. This simple worst-case is commonly used to

design because even significant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours life-

time. This makes it advisable to further derate the capaci-

tor, or choose a capacitor rated at a higher temperature

than required. Several capacitors may also be paralleled to

meet the size or height requirements of the design. An

additional 0.1

µF to 1µF ceramic capacitor is also recom-

mended on VIN for high frequency decoupling.

Output Capacitor (COUT) Selection

The selection of COUT is driven by the required ESR.

Typically, once the ESR requirement is satisfied, the

capacitance is adequate for filtering. The output ripple

(

∆VOUT) is determined by:

∆≈ ∆









I ESR

OUT

where f = operating frequency, COUT = output capacitance

and

∆IL = ripple current in the inductor. With ∆IL = 0.4

IOUT(MAX) the output ripple will be less than 100mV with:

ESRCOUT < 100mΩ

Once the ESR requirements for COUT have been met, the

RMS current rating generally far exceeds the IRIPPLE(P-P)

requirement.

When the capacitance of COUT is made too small, the

output ripple at low frequencies will be large enough to trip

the ITH comparator. This causes Burst Mode operation to

be activated when the LTC1701 would normally be in

continuous mode operation. The effect can be improved at

higher frequencies with lower inductor values.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirement of the application. Alumi-

num electrolyte and dry tantulum capacitors are both

available in surface mount configurations. The OS-CON

semiconductor dielectric capacitor available from Sanyo

has the lowest ESR(size) product of any aluminum elec-

trolytic at a somewhat higher price. In the case of tanta-

lum, 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 tantalums, avalable in case heights ranging from

2mm to 4mm. Other capacitor types include Nichicon PL

series, Sanyo POSCAP and Panasonic SP.

Ceramic Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. These are tempt-

ing for switching regulator use because of their very low

ESR. Unfortunately, the ESR is so low that it can cause

loop stability problems. Solid tantalum capacitor ESR

generates a loop “zero” at 5kHz to 50kHz that is instrumen-

tal in giving acceptable loop phase margin. Ceramic ca-

pacitors remain capacitive to beyond 300kHz and usually

resonate with their ESL before ESR becomes effective.

Also, ceramic caps are prone to temperature effects which

requires the designer to check loop stability over the

operating temperature range.

For these reasons, most of the input and output capaci-

tance should be composed of tantalum capacitors for

stability combined with about 0.1

µF to 1µF of ceramic

capacitors for high frequency decoupling.

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