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LTC3733/LTC3733-1

3733f

board trace and connector resistance losses are also

reduced by the reduction of the input ripple current in a

PolyPhase system. The required amount of input capaci-

tance is further reduced by the factor, N, due to the

effective increase in the frequency of the current pulses.

Ceramic capacitors are becoming very popular for small

designs but several cautions should be observed. “X7R”,

“X5R” and “Y5V” are examples of a few of the ceramic

materials used as the dielectric layer, and these different

dielectrics have very different effect on the capacitance

value due to the voltage and temperature conditions

applied. Physically, if the capacitance value changes due

to applied voltage change, there is a concommitant piezo

effect which results in radiating sound! A load that draws

varying current at an audible rate may cause an attendant

varying input voltage on a ceramic capacitor, resulting in

an audible signal. A secondary issue relates to the energy

flowing back into a ceramic capacitor whose capacitance

value is being reduced by the increasing charge. The

voltage can increase at a considerably higher rate than the

constant current being supplied because the capacitance

value is decreasing as the voltage is increasing! Ceramic

capacitors, when properly selected and used however, can

provide the lowest overall loss due to their extremely low

ESR.

The selection of COUT is driven by the required effective

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

ment is satisfied the capacitance is adequate for filtering.

The steady-state output ripple (

∆VOUT) is determined by:

∆∆

ESR

NfC

OUT

RIPPLE

OUT

≈+









where f = operating frequency of each stage, N is the

number of output stages, COUT = output capacitance and

∆IL = ripple current in each inductor. The output ripple is

highest at maximum input voltage since

∆IL increases

with input voltage. The output ripple will be less than 50mV

at max VIN with ∆IL = 0.4IOUT(MAX) assuming:

COUT required ESR < N • RSENSE

and

COUT > 1/(8Nf)(RSENSE)

The emergence of very low ESR capacitors in small,

surface mount packages makes very small physical imple-

mentations possible. The ability to externally compensate

the switching regulator loop using the ITH pin allows a

much wider selection of output capacitor types. The

impedance characteristics of each capacitor type is sig-

nificantly different than an ideal capacitor and therefore

requires accurate modeling or bench evaluation during

design.

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 and the Panasonic SP

surface mount types have a good (ESR)(size) product.

Once the ESR requirement for COUT has been met, the

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

requirement. Ceramic capacitors from AVX, Taiyo Yuden,

Murata and Tokin offer high capacitance value and very

low ESR, especially applicable for low output voltage

applications.

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. New special polymer sur-

face mount capacitors offer very low ESR also but have

much lower capacitive density per unit volume. In the case

of tantalum, it is critical that the capacitors are surge tested

for use in switching power supplies. Several excellent

choices are the AVX TPS, AVX TPSV, the KEMET T510

APPLICATIO S I FOR ATIO

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