TPS65100_07 Datasheet, PDF (19/32 Pages) Texas Instruments – TRIPLE OUTPUT LCD SUPPLY WITH LINEAR REGULATOR AND VCOM BUFFER

www.ti.com
TPS65100, TPS65101
TPS65105
SLVS496C – SEPTEMBER 2003 – REVISED APRIL 2006
The output voltage is set by the external resistor divider and is calculated as:
ƪ ƫ Vout + 1.214
1
)
R5
R6
ƪ ƫ ƪ ƫ R5 + R6
Vout
VFB
*
1
+ R6
Vout
1.214
*
1
VCOM Buffer
The VCOM buffer is typically used to drive the backplane of a TFT panel. The VCOM output voltage is typically
set to half of the main output voltage V_O1 plus a small shift to implement the specific compensation voltage. The
TFT video signal gets coupled through the TFT storage capacitor plus the LCD cell capacitance to the output of
the VCOM buffer. Because of these, short current pulses in the positive and negative direction appear at the
output of the VCOM buffer. To minimize the output voltage ripple caused by the current pulses, a
transconductance amplifier having a current source output and an output capacitor is used. The output capacitor
supports the high frequency part of the current pulses drawn from the LCD panel. The VCOM buffer only needs
to handle the low frequency portion of the current pulses. A 1-µF ceramic output capacitor is sufficient for most
of the applications. When using other output capacitor values it is important to keep in mind that the output
capacitor is part of the VCOM buffer loop stabilization.
The VCOM buffer has an integrated soft start to avoid voltage drops on V_O1 during start-up. The soft start is
implemented as such that the VCOMIN is held low until the VCOM buffer is fully biased and the common mode
range is reached. Then the positive input is released and the VCOM buffer output slowly comes up. Usually a
1-nF capacitor on VCOMIN to GND is used to filter high frequency noise coupled in from V_O1. The size of this
capacitor together with the upper feedback resistor value determines the start-up time. The larger the capacitor
from VCOMIN to GND, the slower the soft start.
Linear Regulator Controller
The TPS6510x series includes a linear regulator controller to generate a 3.3-V rail when the system is powered
from a 5-V supply. Because an external npn transistor is required, the input voltage of the TPS6510x series
applied to VIN needs to be higher than the output voltage of the regulator. To provide a minimum base drive
current of 13.5 mA, a minimum internal voltage drop of 500 mV from V_in to V_base is required. This can be
translated into a minimum input voltage on VIN for a certain output voltage as the following calculation shows:
VIN_min= V_O4 + V_BE + 0.5 V
The base drive current together with the h_FE of the external transistor determines the possible output current.
Using a standard npn transistor like the BCP68 allows an output current of 1 A and using the BCP54 allows a
load current of 337 mA for an input voltage of 5 V. Other transistors can be used as well depending on the
required output current, power dissipation, and PCB space. The device is stable with a 4.7-µF ceramic output
capacitor. Larger output capacitor values can be used to improve the load transient response when higher load
currents are required.
Layout Considerations
For all switching power supplies, the layout is an important step in the design, especially at high-peak currents
and switching frequencies. If the layout is not carefully designed, the regulator might show stability and EMI
problems. Therefore, the traces carrying high-switching currents should be routed first using wide and short
traces. The input filter capacitor should be placed as close as possible to the input pin VIN of the IC. See the
evaluation module (EVM) for a layout example.
Thermal Information
An influential component of thermal performance of a package is board design. To take full advantage of the
heat dissipation abilities of the PowerPAD or QFN package with exposed thermal die, a board that acts similar to
a heat sink and allows the use of an exposed (and solderable) deep downset pad should be used. For further
information, see the Texas Instrumens application notes (SLMA002) PowerPAD Thermally Enhanced Package,
and (SLMA004) PowerPAD Made Easy. For the QFN package, see the application report (SLUA271) QFN/SON
PCB Attachement. Especially for the QFN package it is required to solder down the Thermal Pad to achieve the
required thermal resistance.
Submit Documentation Feedback
19