Epson S1D13706 - VEEY - LCD Panel Drive Voltage for Vertical Power Supplies - Brightness Reference

Epson Research and Development
Page 9
Vancouver Design Center
Connecting to the Epson D-TFD Panels S1D13706
Issue Date: 01/02/23 X31B-G-012-03
2.2 VEEY - LCD Panel Drive Voltage for Vertical Power Supplies - Brightness
Reference
A negative voltage potential (VEEY) must be provided as a brightness reference and a
temperature compensator to the vertical logic and vertical liquid crystal driving power
supplies. The recommended voltage is -32.0V with a minimum allowable value of -37.0V.
Figure 2-2: “VEE Switching Power Supply” shows a standard topology buck-boost
switching power supply controlled by the S1D13706 output signal GPIO5 (DD_P1).
Figure 2-2: VEE Switching Power Supply
The circuit in Figure 2-2: “VEE Switching Power Supply” uses GPIO5 (DD_P1), a
S1D13706 output that has a 200KHz - 96% duty cycle signal, as the switching control of
the switching power supply. The duty cycle of the input to the gate of Q1 is varied by the
feedback of VEE through D1. This diode feedback causes an overshoot on the rising edge
of GPIO5 (DD_P1) that is proportional to the output level of VEE. This overshoot settles
to a steady level after a variable time depending on how high the overshoot was. This
variable time causes the high speed CMOS inverter (U1) to trigger at different times,
thereby varying the duty cycle of the control input to Q1.
When Q1 turns on, the inductor L1 builds up its magnetic field using current from VDDH,
and D2 is reversed biased. When Q2 turns off, current flows from L1 causing the voltage
across it to reverse polarity and forward bias D2. The output capacitor C3 is charged and
holds the output voltage with an acceptable output ripple when the cycle repeats (Q1 turns
on). The output voltage is regulated by the feedback controlling the on/off times of Q1. The
longer Q1 is turned on, the more current is stored in L1 and the resulting polarity change
when Q1 is turned off is greater.
VCC
VDDH
VEE
Q1
2SJ285
1
3
2
U1A
7W14F
1 7
C2
4.7u 16V
D2
1SS388
L1
560u
C3
2.2u 50V
R2
150K
D1
PTZTE2536A
R1
1K
C1
220p 50V
DD_P1