RT6908 datasheet(16/22 Pages) RICHTEK

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RT6908

DS6908-01

March 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

The RT6908 is a programmable multi-functional power

solution for TFT LCD panels. The RT6908 contains a step-

up converter for main power, a synchronous buck

converter and a synchronous buck controller to provide

the logic voltages for timing controller, voltage detector

for the system. Moreover, a positive charge pump regulator

provides the adjustable gate-high voltage and a negative

charge pump regulator provides the gate low voltage.

Boost Converter

The boost converter is high efficiency PWM architecture.

It performs fast transient responses to generate source

driver supplies for TFT LCD display. The high operation

frequency allows use of smaller components to minimize

the thickness of the LCD panel. The output voltage can be

achieved by setting the I2C register 00h [5:0]. The Boost

minimum gain ratio depends on minimum on time. It

suggested that AVDD higher than 1.14XVIN for better

performance.

Boost Soft-Start

The main boost converter has an internal soft-start function

to reduce the input inrush current. The soft-start time can

be achieved from 0ms to 15ms by setting the I

2C register

0Bh [3:0].

Boost Over Voltage Protection

The main boost converter has an over voltage protection

to protect the main switch at the LXI pin. When the LXI

pin voltage rises above 20V, the boost converter turns the

switch off. As soon as the output voltage falls below the

over voltage threshold, the converter will resume operation.

Boost Over Current Protection

The RT6908 senses the inductor current that is flowing

into the LX pin. The internal N-MOSFET will be turned off

if the peak inductor current reaches 5.6A (typ.). Thus, the

output current at the current limit boundary, denoted as

IOUT(LIM), can be calculated according to the following

equation :

×−

⎛⎞

××

− ×

⎜⎟

⎝⎠

OUT

OUT(LIM)

LIM

OUT

V(V

V )

where

η is the efficiency of the boost converter, ILIM is the

value of the current limit and TS is the switching period.

Boost Short Circuit Protection

The main boost converter has a short circuit protection.

This function disables the boost converter and isolation

P-MOSFET if the difference voltage between the LXI and

AVDD pin larger than ILIMGD I2C register 0Ch [3:0] setting.

The IC will shut down if this difference voltage remains

above setting value after 200

μs. Besides, IC will also shut

down if input voltage below UVLO threshold at AVDD short

circuit period. Only input voltage below 5V (typ.) then re-

power on and remove fault condition, IC can return to normal

operation.

Boost Under Voltage Fault Protection

The main boost converter has a fault protection. This

function disables the boost converter if AVDD is detected

to be below 80%. The IC will shut down if AVDD remains

below 80% after 50ms.

Boost Inductor Selection

The inductor value depends on the maximum input current.

As a general rule the inductor ripple current is 20% to

40% of maximum input current. If 40% is selected as an

example, the inductor ripple current can be calculated

according to the following equation :

OUT

OUT(MAX)

IN(MAX)

RIPPLE

IN(MAX)

= 0.4 I

where

η is the efficiency of the boost converter, IIN(MAX) is

the maximum input current and IRIPPLE is the inductor ripple

current. The input peak current can be obtained by adding

the maximum input current with half of the inductor ripple

current as shown in the following equation :

IPEAK = IRIPPLE + IIN(MAX) = 1.2 IIN(MAX)

Note that the saturated current of inductor must be greater

than IPEAK. The inductance can eventually be determined

according to the following equation :

() (

)

()

η ××

−

××

OUT

OUT(MAX)

OSC

L =

0.4

where fOSC is the switching frequency. For better system

performance, a shielded inductor is preferred to avoid EMI

problems.

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