RT6908
The RT6908 is a programmable multi-functional power
solution for TFT LCDpanels. 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 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
AVDDpin larger than ILIMGDI2C register 0Ch [3:0] setting.
The IC will shut down if this difference voltage remains
above setting value after 200μs. Besides, IC will also shut
down if input voltage below UVLO threshold atAVDDshort
circuit period. Only input voltage below 5V (typ.) then re-
power on and remove fault condition, IC can return to normal
operation.
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 LCDpanel. 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 Under Voltage Fault Protection
The main boost converter has a fault protection. This
function disables the boost converter ifAVDDis 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 :
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 I2C register
0Bh [3:0].
VOUT ×IOUT(MAX)
I
=
IN(MAX)
η × V
IN
Boost Over Voltage Protection
IRIPPLE = 0.4×I
IN(MAX)
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.
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 :
Boost Over Current Protection
IPEAK = IRIPPLE + IIN(MAX) = 1.2 IIN(MAX)
The RT6908 senses the inductor current that is flowing
into the LX pin. The internalN-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 :
Note that the saturated current of inductor must be greater
than IPEAK. The inductance can eventually be determined
according to the following equation :
η × V 2 × V
− V
IN
(
)
(
)
IN
OUT
L =
0.4× V
2 ×IOUT(MAX)×fOSC
(
)
OUT
V ×(V
− V )
T
IN
⎛
⎝
⎞
V
V
OUT
1
2
IN
OUT
IN
S
I
= η ×
× I
−
×
×
where fOSC is the switching frequency. For better system
performance, a shielded inductor is preferred to avoid EMI
problems.
OUT(LIM)
LIM
⎜
⎟
⎠
V
OUT
L
where η is the efficiency of the boost converter, ILIM is the
value of the current limit and TS is the switching period.
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
16
DS6908-01 March 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
