RT8294 datasheet(10/15 Pages) RICHTEK

10 / 15 page

RT8294

DS8294-02

March 2011

www.richtek.com

OUT

L(MAX)

IN(MAX)

L =1



 

×−



 

×∆



 

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.

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

OUT

RMS

OUT(MAX)

OUT

= I1

−

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, one 10

µF low ESR ceramic

capacitors are recommended. For the recommended

capacitor, please refer to table 3 for more detail.

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,

∆VOUT , is determined by :

OUTL

OUT

ESR

8fC



∆

≤ ∆+





The output ripple will be highest at the maximum input

voltage since

∆IL 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 ar e all av ai labl e in surf ace m ount



 

∆

×−



 



 

OUT

I =1

f LV

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

∆IL= 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 :

Hiccup Mode

For the RT8294, Hiccup Mode Under Voltage Protection

(UVP) is provided. When the FB voltage, VFB, drops below

0.5V, the UVP function will be triggered and the RT8294

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

∆IL increases with higher VIN

and decreases with higher inductance.

Figure 4. The Resistors can be Selected to Set IC

Lockout Threshold

VIN

GND

BOOT

VOUT

VIN

RT8294

CSS

COMP

9 (Exposed Pad)

CBOOT

COUT

CIN

100k

12V

REN2

REN1

µF

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