LTC7138 datasheet(13/24 Pages) LINER

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LTC7138

7138f

For more information www.linear.com/LTC7138

applicaTions inForMaTion

at light load can be approximated by:

∆VOUT ≈

IPEAK

–ILOAD









•

4•10–6

COUT

VOUT

160

The output ripple is a maximum at no load and approaches

lower limit of VOUT/160 at full load. Choose the output

capacitor COUT to limit the output voltage ripple ∆VOUT

using the following equation:

COUT ≥

IPEAK •2•10–6

∆VOUT –

VOUT

160

Thevalueoftheoutputcapacitormustalsobelargeenough

to accept the energy stored in the inductor without a large

change in output voltage during a single switching cycle.

Setting this voltage step equal to 1% of the output voltage,

the output capacitor must be:

COUT >

•

IPEAK

VOUT









•

100%

Typically, a capacitor that satisfies the voltage ripple re-

quirement is adequate to filter the inductor ripple. To avoid

overheating, the output capacitor must also 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. Multiple capacitors placed in parallel

may be needed to meet the ESR and RMS current handling

requirements.

Dry tantalum, special polymer, aluminum electrolytic,

and ceramic capacitors are all available in surface mount

packages. Special polymer capacitors offer very low ESR

but have lower capacitance density than other types.

Tantalum capacitors have the highest capacitance density

but it is important only to use types that have been surge

tested for use in switching power supplies. Aluminum

electrolytic capacitors have significantly higher ESR but

can be used in cost-sensitive applications provided that

consideration is given to ripple current ratings and long-

term reliability. Ceramic capacitors have excellent low ESR

characteristics but can have high voltage coefficient and

audible piezoelectric effects. The high quality factor (Q)

of ceramic capacitors in series with trace inductance can

also lead to significant input voltage ringing.

Input Voltage Steps

Iftheinputvoltagefallsbelowtheregulatedoutputvoltage,

thebodydiodeoftheinternalMOSFETwillconductcurrent

from the output supply to the input supply. If the input

voltage falls rapidly, the voltage across the inductor will be

significant and may saturate the inductor. A large current

will then flow through the MOSFET body diode, resulting

in excessive power dissipation that may damage the part.

If rapid voltage steps are expected on the input supply, put

a small silicon or Schottky diode in series with the VIN pin

to prevent reverse current and inductor saturation, shown

below as D1 in Figure 4. The diode should be sized for a

reverse voltage of greater than the regulated output volt-

age, and to withstand repetitive currents higher than the

maximum peak current of the LTC7138.

Figure 4. Preventing Current Flow to the Input

INPUT

SUPPLY

LTC7138

COUT

7138 F04

CIN

VOUT

VIN

Ceramic Capacitors and Audible Noise

Higher value, lower cost ceramic capacitors are now be-

coming available in smaller case sizes. Their high ripple

current, high voltage rating, and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

output. When a ceramic capacitor is used at the input and

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