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LTC1874

short-circuited. Under this condition the diode must safely

handle IPEAK at close to 100% duty cycle. Therefore, it is

important to adequately specify the diode peak current and

average power dissipation so as not to exceed the diode

ratings.

Under normal load conditions, the average current con-

ducted by the diode is:

OUT

−









The allowable forward voltage drop in the diode is calcu-

lated from the maximum short-circuit current as:

SC MAX

≈

()

where PD is the allowable power dissipation and will be

determined by efficiency and/or thermal requirements.

Schottky diodes are a good choice for low forward drop

and fast switching times. Remember to keep lead length

short and observe proper grounding (see Board Layout

Checklist) to avoid ringing and increased dissipation.

CIN and COUT Selection

In continuous mode, the source current of the P-channel

MOSFET is a square wave of duty cycle (VOUT + VD)/

(VIN + VD). To prevent large voltage transients, a low ESR

input capacitor sized for the maximum RMS current must

be used. The maximum RMS capacitor current is given by:

MAX

OUT

Required IRMS ≈

−

()

[]12/

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. Several capacitors may be paralleled to

meet the size or height requirements in the design. Due to

the high operating frequency of the controller, ceramic

capacitors can also be used for CIN. Always consult the

manufacturer if there is any question.

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 output ripple (

∆VOUT) is approximated by:

∆V

ESR

OUT

RIPPLE

OUT

≈+









where f is the operating frequency, COUT is the output

capacitance and IRIPPLE is the ripple current in the induc-

tor. The output ripple is highest at maximum input voltage

since

∆IL increases with input voltage.

Once the ESR requirement for COUT has been met, the

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

requirement. Multiple capacitors may have to be paral-

leled to meet the ESR or RMS current handling require-

ments of the application. Aluminum electrolytic and dry

tantalum capacitors are both available in surface mount

configurations. An excellent choice of tantalum capacitors

are the AVX TPS and KEMET T510 series of surface mount

tantalum capacitors.

APPLICATIO S I FOR ATIO

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