LTC7150S datasheet(13/22 Pages) LINER

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LTC7150S

7150sfa

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OPERATION

Output Capacitor (COUT) Selection

The selection of COUT is determined by the effective series

resistance(ESR)thatisrequiredtominimizevoltageripple

and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

the load transient response. The output ripple, ∆VOUT, is

determined by:

ΔVOUT < ΔIL

8 • f • COUT

+ESR

⎛

⎝⎜

⎞

⎠⎟

The output ripple is highest at maximum input volt-age

since ∆IL increases with input voltage. Multiple capaci-

tors 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 are very low ESR but have

lower capacitance density than other types. Tantalum

capacitors have the highest capacitance density but it is

important to only use types that have been surge tested

foruseinswitchingpowersupplies.Aluminumelectrolytic

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

and small footprints.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming 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 the power is supplied by a wall adapter through long

wires, a load step at the output can induce ringing at the

VIN input. At best, this ringing can couple to the output and

be mistaken as loop instability. At worst, a sudden in-rush

of current through the long wires can potentially cause

a voltage spike at VIN large enough to damage the part.

When choosing the input and output ceramic capacitors,

choose the X5R and X7R dielectric formulations. These

dielectrics have the best temperature and voltage char-

acteristics of all the ceramics for a given value and size.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement.Duringaloadstep,theoutputcapacitormust

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

to support the load. Typically, 5 cycles are required to

respond to a load step, but only in the first cycle does the

output voltage drop linearly. The output droop, VDROOP,

is usually about 3 times the linear drop of the first cycle.

Thus, a good place to start with the output capacitor value

is approximately:

COUT = 3

ΔIOUT

fO • VDROOP

More capacitance may be required depending on the duty

cycle and load step requirements. In most applications,

the input capacitor is merely required to supply high

frequency bypassing, since the impedance to the supply

is very low. A 47μF ceramic capacitor is usually enough

for these conditions. Place this input capacitor as close

to the PVIN pin as possible.

Inductor Selection

Given the desired input and output voltages, the inductor

valueandoperatingfrequencydeterminetheripplecurrent:

IL =

VOUT

f •L

VOUT

VIN

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a trade-off between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 50% of IOUT(MAX). To guarantee that ripple

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