Ceramic output capacitor applications – Rainbow Electronics MAX1813 User Manual

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MAX1813

Dynamically-Adjustable, Synchronous Step-Down

Controller with Integrated Voltage Positioning

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to V+ on the MAX1813, with the same attenuation factor
as the output divider. The V+ input has a nominal
600k

Ω input impedance, which should be considered

when selecting resistor values.

One-Stage (Battery Input) vs. Two-Stage

(5V Input) Applications

The MAX1813 can be used with a direct battery con-
nection (one stage) or can obtain power from a regulat-
ed 5V supply (two stage). Each approach has
advantages, and careful consideration should go into
the selection of the final design.

The one-stage approach offers smaller total inductor
size and fewer capacitors overall due to the reduced
demands on the 5V supply. The transient response of
the single stage is better due to the ability to ramp the
inductor current faster. The total efficiency of a single
stage is better than the two-stage approach.

The two-stage approach allows flexible placement due
to smaller circuit size and reduced local power dissipa-
tion. The power supply can be placed closer to the
CPU for better regulation and lower I

R losses from PC

board traces. Although the two-stage design has slow-
er transient response than the single stage, this can be
offset by the use of a voltage-positioned converter.

Ceramic Output Capacitor Applications

Ceramic capacitors have advantages and disadvan-
tages. They have ultra-low ESR and are noncom-
bustible, relatively small, and nonpolarized. However,
they are also expensive and brittle, and their ultra-low
ESR characteristic can result in excessively high ESR
zero frequencies. In addition, their relatively low capac-
itance value can cause output overshoot when step-
ping from full-load to no-load conditions, unless a small
inductor value is used (high switching frequency) or
there are some bulk tantalum or electrolytic capacitors
in parallel to absorb the stored inductor energy. In
some cases, there may be no room for electrolytics,
creating a need for a DC-DC design that uses nothing
but ceramics.

The MAX1813 can take advantage of the small size and
low ESR of ceramic output capacitors. To ensure stable
operation, there must be sufficient resistance in series
with the inductor and output capacitor (see “Output
Capacitor Stablility Considerations”).

Output overshoot (V

SOAR

) determines the minimum

output capacitance requirement (see Output Capacitor
Selection). Often the switching frequency is increased
to 1000kHz or 600kHz, and the inductor value is
reduced to minimize the energy transferred from induc-
tor to capacitor during load-step recovery. The efficien-
cy penalty for operating at 1000kHz is about 5% and

MAX1813

VPCS

PGND

GND

REF

OUT

VPCS

LOGIC

ADD0

ADD1

CONTROL
LOGIC

INH

SENSE

REF

MAX4634

MAX4322

Figure 18. Adding a Negative Offset Voltage