inductor for the application. 10µH is a more widely available

standard value, and very close to the optimum value too, and

would therefore be a good choice too. Note that inductances

larger than 15µH are not recommended in general.

To more accurately predict how the selected off-the-shelf

part will actually perform in the real application, the designer

is referred to AN-1197. The procedure contained therein

could greatly help in correctly choosing the lowest accept-

able current/energy rating of the inductor, and thereby reduc-

ing its size further.

Input Capacitor Selection

At the input, the first requirement is a high frequency (pref-

erably ceramic) decoupling capacitor, of value 0.1µF, placed

very close to, and between the V

of the IC. This provides the triangular pulsed current wave-

form that flows through the switch. In addition, a bulk capaci-

tor is also required, which replenishes the decoupling ca-

pacitor, and may be placed slightly further away if necessary.

The rating and selection of this capacitor is discussed below.

In general a standard low-esr aluminum electrolytic is rec-

ommended at the input (’esr’ refers to the equivalent series

resistance hereafter). There are several reasons for this.

Firstly, tantalum capacitors have inherent input surge-current

limitations. So when the input surge current comes from a

very low impedance source (such as a high current lab DC

power supply), there is a chance that the capacitor may not

survive several such repeated high dV/dt events. In any

case, even using ’surge-tested’ tantalums (like TPS series

from AVX) a 50% voltage derating is recommended in such

conditions. Therefore hypothetically, a 35V tantalum must be

used for the preceding example, in which the maximum input

was 16V. The second reason for avoiding very low esr input

capacitors is that there is a possibility of severe input oscil-

lations. The elements involved in this resonance are the

inductance of the input leads, the input capacitance and the

(negative) input impedance of the switching stage. It is

known that the esr of the input capacitor actually serves a

useful purpose in damping out these oscillations.

These oscillations can only be seen clearly under lab condi-

tions if the output of the lab DC power supply is ON/Output-

Enabled and then the lead from the converter stage is physi-

cally connected to the output terminals of the DC power

supply. Just turning the DC power supply ON/OFF (or with

an Output-Enable button) does not generate the high dV/dt

required to provoke these oscillations. Under a real situation,

input oscillations can become severe enough to cause the

maximum voltage rating of the IC to be exceeded. The

ringing can in turn, also feed in to the Analog sections of the

LM1572, causing strange behavior and possibly device fail-

ure.

The designer needs to therefore monitor the input ramp

close to the input of the converter, preferably with a digitizing

oscilloscope set to about 10-20µs/div and using the single

acquisition mode. Once the ramp is being captured, it will be

seen that large input capacitances ’slow’ the dV/dt consider-

ably, thereby reducing the overshoot and the input oscilla-

tions. However, besides the capacitance itself, the esr of the

input capacitor is a major contributor too. Therefore, in a

typical comparison of a 10µF aluminum electrolytic vs. a

10µF tantalum electrolytic (tantalum has lower esr), it was

seen that there was an almost 50% overshoot in the peak

input voltage for the tantalum capacitor (accompanied by

severe ringing), whereas for the aluminum capacitor, the

overshoot was only about 10% (plus the waveform was that

of a well damped system). The designer should also be

aware that some older DC power supplies actually exacer-

bate the problem, while apparently trying to ’correct’ the

output voltage. The situation gets even worse if the DC

power supply has a remote-sense which is being used to

apparently ’correct’ the input voltage at the input of the

converter. Therefore, it is always a good idea to try out

another available DC power supply to see how severe the

problem is in reality, or whether it is just a ’bad’ lab supply.

If because of size constraints the designer must use tantal-

ums, a minimum capacitance of 22µF is recommended for

any application, irrespective of input/output conditions. This

’softens’ up the input dV/dt significantly and reduces the

ringing.

The basic electrical criterion for selecting an input capacitor

is the input RMS current. The equation for this is

where ’r’ is the current ripple ratio. It is given by

where L is in µH and f in Hz. This calculation should be done

at the worst case condition for this parameter, which corre-

sponds to 50% duty cycle. If the application never ’sees’ 50%

duty cycle over its entire operating range, then the worst

case is simply the closest duty cycle to 50%. This can, in

general, occur at either of the input voltage extremes, and

therefore both ends must then be examined. In the example,

it was seen that the duty cycle varies from 34.4% to 65%. So

it is clear that there does exist an input voltage point within

the range, at which the duty cycle is 50%. At this worst-case

condition, for the chosen inductor, ’r’ at D=0.5 is

r = 0.45

At this point the input RMS current in the capacitor is

I

Therefore, this is also the minimum required RMS current

rating of any input capacitor to be used. Now, a typical 25V

aluminum capacitor would need to be around 470-1000µF

just to be able to handle this current. It would also take up

valuable space on the board. Therefore for the example, the

choice is tantalum 22µF/35V TPS series AVX capacitor, Part

Number TPSE226K035S0200, rated for 0.812A at 85˚C.

Though it is also possible to use a Panasonic surface mount

aluminum

470µF/25V

FK

series,

Part

Number

EEVFK1E471P, rated for 0.85A at 105˚C.

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