RT8265 datasheet(10/13 Pages) RICHTEK

10 / 13 page

RT8265

DS8265-02 March 2011

www.richtek.com

Ferrite core material saturates

“hard”, which means that

inductance collapses abruptly when the peak design current

is exceeded. The previous situation results in an abrupt

increase in inductor ripple current and consequent output

voltage ripple.

Do not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy materials

are small and do not radiate energy. However, they are

usually more expensive than the similar powdered iron

inductors. The rule for inductor choice mainly depends on

the price vs. size requirement and any radiated field/EMI

requirements.

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

more detail please refer to Table 3.

CIN and COUT Selection

The input capacitance, CIN,is needed to filter the 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 :

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.

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, a 10

μF low ESR ceramic capacitor

is recommended. For the recommended capacitor, please

refer to table 2 for more detail. The input capacitor has to

connect another 10

μF ceramic capacitor between the input

and ground when the input voltage is lower than 6.5V.

The selection of COUT is determined by the required ESR

to minimize voltage ripple.

Moreover, the amount of bulk capacitance is also a key for

COUT selection to ensure that the control loop is stable.

Loop stability can be checked by viewing the load transient

response as described in a later section.

The output ripple,

ΔVOUT , is determined by :

The output ripple will be highest at the maximum input

voltage since

ΔIL increases with input voltage. Multiple

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 input,

VIN. At best, this ringing can couple to the output and be

mistaken as loop instability. At worst, a sudden inrush of

current through the long wires can potentially cause a

voltage spike at VIN large enough to damage the part.

OUT

RMS

OUT(MAX)

OUT

= I

−

OUT

ESR

8fC

⎡⎤

Δ≤ Δ

⎢⎥

⎣⎦

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