LTC1474CS8-3.3 datasheet(9/20 Pages) LINER

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LTC1474/LTC1475

APPLICATIONS INFORMATION

CIN and COUT Selection

At higher load currents, when the inductor current is

continuous, the source current of the P-channel MOSFET

is a square wave of duty cycle VOUT/VIN. To prevent large

voltage transients, a low ESR input capacitor sized for the

maximum RMS current must be used. The maximum

capacitor current is given by:

OUT

Required I

RMS

MAX

−

()

[]12/

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

IRMS = IOUT/2. This simple worst-case condition is com-

monly 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. Do not underspecify this component. An addi-

tional 0.1

µF ceramic capacitor is also required on VIN for

high frequency decoupling.

The selection of COUT is driven by the required effective

series resistance (ESR) to meet the output voltage ripple

and line regulation requirements. The output voltage ripple

during a burst cycle is dominated by the output capacitor

ESR and can be estimated from the following relation:

25mV <

∆VOUT, RIPPLE = ∆IL • ESR

where

∆IL≤ IPEAK and the lower limit of 25mV is due to the

voltage comparator hysteresis. Line regulation can also

vary with COUT ESR in applications with a large input

voltage range and high peak currents.

ESR is a direct function of the volume of the capacitor.

Manufacturers such as Nichicon, AVX and Sprague should

be considered for high performance capacitors. The

OS-CON semiconductor dielectric capacitor available from

SANYO has the lowest ESR for its size at a somewhat

higher price. Typically, once the ESR requirement is satis-

fied, the capacitance is adequate for filtering. For lower

current applications with peak currents less than 50mA,

µF ceramic capacitors provide adequate filtering and

are a good choice due to their small size and almost

negligible ESR. AVX and Marcon are good sources for

these capacitors.

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 series of surface mount tantalums, available in case

heights ranging from 2mm to 4mm. Other capacitor types

include SANYO OS-CON, Nichicon PL series and Sprague

595D series. Consult the manufacturer for other specific

recommendations.

To avoid overheating, the output capacitor must be sized

to handle the ripple current generated by the inductor. The

worst-case ripple current in the output capacitor is given

by:

IRMS = IPEAK/2

Once the ESR requirement for COUT has been met, the

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

requirement.

Efficiency Considerations

The efficiency of a switching regulator is equal to the

output power divided by the input power times 100%. It is

often useful to analyze individual losses to determine what

is limiting efficiency and which change would produce the

most improvement. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, three main sources usually account for most of the

losses in LTC1474/LTC1475 circuits: VIN current, I2R

losses and catch diode losses.

1. The VIN current is due to two components: the DC bias

current and the internal P-channel switch gate charge

current. The DC bias current is 9

µA at no load and

increases proportionally with load up to a constant

100

µA during continuous mode. This bias current is so

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