Data Sheet

LT7153SP

analog.com

Rev 0

20 of 31

more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly

depends on the price versus size requirements and any radiated field/EMI requirements.

Input Capacitor (CIN) Selection

The input capacitance, CIN, is needed to filter the square wave current at the drain of the top power MOSFET. To

prevent large voltage transients from occurring, use a low ESR input capacitor sized for the maximum RMS current.

The maximum RMS current is given by:

IRMS ≅ IOUT(MAX) •

VOUT

VIN

• √

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− 1

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

IRMS ≅

IOUT

2

This simple worst-case condition is commonly used for design because even significant deviations do not offer much

relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life, which

makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required.

Several capacitors may also be paralleled to meet size or height requirements in the design. For low input voltage

applications, sufficient bulk input capacitance is needed to minimize transient effects during output load changes.

Output Capacitor (COUT) Selection

The selection of COUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple

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:

∆VOUT < ∆IL • (

1

8 • fSW • COUT

+ ESR)

The output ripple is highest at maximum input voltage since ∆IL increases with input voltage. 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 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 are 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 and small footprints.

Since the ESR of a ceramic capacitor is so low, it is more useful to choose the output capacitor value to fulfill a charge

storage requirement. During a load step, the output capacitor must instantaneously supply the current to support

the load until the feedback loop raises the switch current enough to support the load. The time required for the

feedback loop to respond is dependent on the compensation and the output capacitor size. Typically, five 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 three times the linear drop of the first cycle. Thus, a good place to start with the output

capacitor value is approximately.

COUT = 3 • (

∆IOUT

fO•VDROOP

)