BeagleBone LCD7 Cape
System Reference Manual
Revision A3
Page 23 of 27
pin as an input voltage for positive and negative charge pumps and bias supply for
VCOM buffer. The input of the VCOM buffer is VCOMIN pin. If VCOM is not used in a
circuit, the buffer can be shut down and the quiescent current can be reduced by
grounding the VCOMIN pin. In this case, VCOMIN is connected to a 4V resistive
divider of AVDD_10V, which results in VCOM output to be 4V. TPS65105 is also
integrated with adjustable positive and negative charge pumps. The input voltage to these
charge pumps is the aforementioned AVDD_10V output that is fed to SUP input. These
positive and negative outputs are 15Vand -7V, which provide the positive and negative
gate drive voltages VGH and VGL of the LCD panel.
TPS65105 also provides a linear regulator controller to generate a 3.3V output from a 5V
source. This linear regulator controller is independent from the system since it has its
own enable input. In the BeagleBone LCD7 Cape, this enable pin is connected to the
VDD_3V3 power rail to make sure the controller is always active. An external npn
transistor is used, and its base is driven by the Base output from the PMIC. A feedback
signal is fed back to TPS65105 to ensure the transistor’s output is fixed at 3.3V. This
voltage is connected to power rail VDD_LCD and also powers the Power LED.
The main boost converter of TPS65105 as well as positive and negative charge pumps,
linear regulator, and VCOM buffer have an internal soft start. The purpose of soft start is
to avoid heavy voltage drop at the input or output of the boost converter due to high
inrush current at start-up. The output voltages have a power on sequence as follow:
AVDD_10V -> VGL_-7V -> VGH_15V -> VCOM
This sequence matches the power on sequence of the LCD panel.
5.3.2
Backlight Power Supply
The backlight of TFC-S9700RTWV35TR-01 LCD panel is composed of 27 LEDs
divided into 9 parallel series. Each LED has a maximum current rating of 20mA;
therefore, a maximum current of 180mA is required to power the LCD backlight.
The backlight of the LCD panel is powered by a High Voltage DC/DC Boost Converter
TPS61080. This boost converter can operate from input supply of 5V and provide output
voltage up to 27V. This IC features an input/output isolation which prevents any output
leakage under shutdown, avoid inrush current during start up or unlimited short current.
The enable (EN) pin is controlled by signal EHRPWM1A which is exposed at pin 14 of
expansion connector J2. When the EN pin is pulled high, the device is active. TPS61080
uses a current mode control with constant PWM. The switching frequency can be
configured to be 600kHz or 1.2MHz by connecting FSW pin to a logic low or high
