RT2808A

11

DS2808A-00 November 2012

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Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

OUT

IN

RMS

OUT(MAX)

IN

OUT

V

V

I

= I

1

VV

−

OUT

L

OUT

1

VI ESR

8fC

⎡⎤

Δ≤ Δ

+

⎢⎥

⎣⎦

The output ripple will be highest at the maximum input

voltage since

capacitors placed in parallel may be needed to meet the

ESR and RMS current handling requirement. Dry tantalum,

special polymer, aluminum electrolytic and ceramic

capacitors are all available in surface mount packages.

Special polymer capacitors offer very low ESR value.

However, it provides lower capacitance density than other

types. Although Tantalum capacitors have the highest

capacitance density, it is important to only use types that

pass the surge test for use in switching power supplies.

Aluminum electrolytic capacitors have significantly higher

ESR. However, it can be used in cost-sensitive applications

for ripple current rating and long term reliability

considerations. Ceramic capacitors have excellent low

ESR characteristics 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.

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 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

enough to damage the part.

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

equal to

ringing that would indicate a stability problem.

Diode Selection

When the power switch turns off, the path for the current

is through the diode connected between the switch output

and ground. This forward biased diode must have a

minimum voltage drop and recovery times. Schottky diode

is recommended and it should be able to handle those

current. The reverse voltage rating of the diode should be

greater than the maximum input voltage, and current rating

should be greater than the maximum load current. For

trapezoidal current at the source of the high side MOSFET.

To prevent large ripple current, a low ESR input capacitor

sized for the maximum RMS current should be used. The

RMS current is given by :

Table 2. Suggested Inductors for Typical

Application Circuit

Component

Supplier

Series

Dimensions

(mm)

TDK

VLF10045

10 x 9.7 x 4.5

TDK

SLF12565

12.5 x 12.5 x 6.5

TAIYO YUDEN

NR8040

8 x 8 x 4

The output ripple,

commonly used for design because even significant

deviations do not offer much relief.

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 the input capacitor, two 4.7

μF low ESR ceramic

capacitors are recommended. For the recommended

capacitor, please refer to Table 3 for more detail.

to minimize voltage ripple.

Moreover, the amount of bulk capacitance is also a key

Loop stability can be checked by viewing the load transient

response as described in a later section.