RT8299 datasheet(10/15 Pages) RICHTEK

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RT8299

DS8299-01 May 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

OUT

I =

⎡⎤ ⎡

⎤

Δ× −

⎢⎥ ⎢

⎥

⎣⎦ ⎣

⎦

Under Voltage Protection

Hiccup Mode

For the RT8299, it provides Hiccup Mode Under Voltage

Protection (UVP). When the FB voltage drops below half

of the feedback reference voltage, VFB, the UVP function

will be triggered and the RT8299 will shut down for a period

of time and then recover automatically. The Hiccup Mode

UVP can reduce input current in short-circuit conditions.

Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current

ΔIL increases with higher VIN

and decreases with higher inductance.

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output voltage

ripple. High frequency with small ripple current can achieve

highest efficiency operation. However, it requires a large

inductor to achieve this goal.

For the ripple current selection, the value of

ΔIL= 0.24(IMAX)

will be a reasonable starting point. The largest ripple

current occurs at the highest VIN. To guarantee that the

ripple current stays below the specified maximum, the

inductor value should be chosen according to the following

equation :

OUT

RMS

OUT(MAX)

OUT

= I

−

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 :

OUT

L(MAX)

IN(MAX)

L =

⎡⎤ ⎡

⎤

×−

⎢⎥ ⎢

⎥

×Δ

⎣⎦ ⎣

⎦

The inductor's current rating (caused a 40

°C temperature

rising from 25

°C ambient) should be greater than the

maximum load current and its saturation current should

be greater than the short circuit peak current limit. Please

see Table 2 for the inductor selection reference.

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, two 10

μF low ESR ceramic

capacitors are recommended.

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 :

OUT

ESR

8fC

⎡⎤

Δ≤ Δ

⎢⎥

⎣⎦

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

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

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