RT8288A

10

DS8288A-03

June 2012

www.richtek.com

©

Copyright

2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Figure 3. Enable Control Circuit for Logic Control with Low Voltage

Figure 4. The Resistors can be Selected to Set IC Lockout Threshold

Under Output Voltage Protection-Hiccup Mode

For the IC, Hiccup Mode of Under Voltage Protection (UVP)

is provided. When the FB voltage drops below half of the

triggered and the IC will shut down for a period of time and

then recover automatically. The Hiccup Mode of UVP can

reduce input current in short circuit conditions.

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

with higher inductance.

OUT

OUT

L

IN

VV

I =

1

fL

V

⎡⎤ ⎡

⎤

Δ× −

⎢⎥ ⎢

⎥

×

⎣⎦ ⎣

⎦

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output voltage

ripple. Highest efficiency operation is achieved by reducing

ripple current at low frequency, but it requires a large

inductor to attain this goal.

For the ripple current selection, the value of

ripple current stays below a specified maximum, the

inductor value should be chosen according to the following

equation :

OUT

OUT

L(MAX)

IN(MAX)

VV

L =

1

fI

V

⎡⎤ ⎡

⎤

×−

⎢⎥ ⎢

⎥

×Δ

⎣⎦ ⎣

⎦

The inductor's current rating (caused a 40

°C temperature

°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 and it is

highly recommended to keep inductor value as close as

possible to the recommended inductor values for each

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

Table 2. Suggested Inductors for Typical

Application Circuit

Input and Output Capacitors Selection

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 :

VIN

VCC

BOOT

FB

SW

6

1

2, 3

4

L

R1

R2

RT8288A

7

GND

8, 9 (Exposed Pad)

EN

5

Q1

100k

Chip Enable

VIN

VCC

BOOT

FB

SW

6

1

2, 3

4

L

R1

R2

RT8288A

7

GND

8, 9 (Exposed Pad)

EN

5

100k

OUT

IN

RMS

OUT(MAX)

IN

OUT

V

V

I

= I

1

VV

−