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LTC3823

3823fd

does not exceed a speciﬁed maximum, the inductance

should be chosen according to:

OUT

LMAX

OUT

IN MAX

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟ −

⎛

⎝

⎜⎜

⎞

⎠

Δ ()

()

⎟⎟⎟

Once the value for L is known, the type of inductor must

be selected. High efﬁciency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy

or Kool Mμ

® cores. A variety of inductors designed for

high current, low voltage applications are available from

manufacturers such as Sumida, Panasonic, Coiltronics,

Coilcraft and Toko.

Schottky Diode D1 Selection

The Schottky diode D1 shown in Figure 12 conducts dur-

ing the dead time between the conduction of the power

MOSFET switches. It is intended to prevent the body diode

of the bottom MOSFET from turning on and storing charge

during the dead time, which can cause a modest (about

1%) efﬁciency loss. The diode can be rated for about one

half to one ﬁfth of the full load current since it is on for

only a fraction of the duty cycle. In order for the diode

to be effective, the inductance between it and the bottom

MOSFET must be as small as possible, mandating that

these components be placed adjacently. The diode can

be omitted if the efﬁciency loss is tolerable.

CIN and COUT Selection

The input capacitance CIN is required to ﬁlter the square

wave current at the drain of the top MOSFET. Use a low ESR

capacitor sized to handle the maximum RMS current.

RMS

OUT MAX

OUT

≅

()

–1

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

IRMS = IOUT(MAX)/2. This simple worst-case condition is

commonly used for design because even signiﬁcant de-

viations 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 derate

the capacitor.

The selection of COUT is primarily determined by the

ESR required to minimize voltage ripple and load step

transients. The output ripple ΔVOUT is approximately

bounded by:

ΔΔ

I ESR

OUT

≤+

⎛

⎝

⎜

⎞

⎠

⎟

Since ΔIL increases with input voltage, the output ripple

is highest at maximum input voltage. Typically, once the

ESR requirement is satisﬁed, the capacitance is adequate

for ﬁltering and has the necessary RMS current rating.

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 pack-

ages. 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 to only 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 providing that consideration

is given to ripple current ratings and long term reliability.

Ceramic capacitors have excellent low ESR characteris-

tics but can have a high voltage coefﬁcient and audible

piezoelectric effects. The high Q of ceramic capacitors with

trace inductance can also lead to signiﬁcant ringing. When

used as input capacitors, care must be taken to ensure that

ringing from inrush currents and switching does not pose

an overvoltage hazard to the power switches and control-

ler. To dampen input voltage transients, add a small 5μF

to 50μF aluminum electrolytic capacitor with an ESR in

the range of 0.5Ω to 2Ω. High performance through-hole

capacitors may also be used, but an additional ceramic

capacitor in parallel is recommended to reduce the effect

of their lead inductance.

Top MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CBconnectedtotheBOOST

pin supplies the gate drive voltage for the topside MOSFET.

This capacitor is charged through diode DB from INTVCC

when the switch node is low. When the top MOSFET turns

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