TC7129_13 Datasheet, PDF (10/28 Pages) Microchip Technology – 4-1/2 Digit Analog-to-Digital Converters with On-Chip LCD Drivers

TC7129
4.2.3 +5V Power Supply
Measurements are made with respect to power supply
ground. COMMON (pin 28) is connected to REF LO
(pin 35). A voltage doubler is needed, since the supply
voltage is less than the 6V minimum needed by the
TC7129. DGND (pin 36) must be isolated from power
supply ground (see Figure 4-3).
+5V
0.1 µF
24
V+
34
TC7129
35
36 _DGND
28
0.1 µF
33
+
8
32
V_IN
V+
2
V–
–
+
23
TC7660 ₄ ^{10 µF}
GND
3
5
10 µF
+
Figure 4-3:
Powering the TC7129 From
a +5V Power Supply.
4.3 Connecting to External Logic
External logic can be directly referenced to DGND
(pin 36), provided that the supply current of the external
logic does not exceed the sink current of DGND
(Figure 4-4). A safe value for DGND sink current is
1.2 mA. If the sink current is expected to exceed this
value, a buffer is recommended (see Figure 4-5).
V+
External
Logic
I_LOGIC
24
TC7129
36 _DGND
23
V-
Figure 4-4:
External Logic Referenced
Directly to DGND.
V+
External
Logic
24
TC7129
I_LOGIC
36 _DGND
23
V–
Figure 4-5:
External Logic Referenced
to DGND with Buffer.
4.4 Temperature Compensation
For most applications, V_DISP (pin 19) can be connected
directly to DGND (pin 36). For applications with a wide
temperature range, some LCDs require that the drive
levels vary with temperature to maintain good viewing
angle and display contrast. Figure 4-6 shows two
circuits that can be adjusted to give temperature com-
pensation of about 10 mV/°C between V+ (pin 24) and
V_DISP. The diode between DGND and V_DISP should
have a low turn-on voltage because V_DISP cannot
exceed 0.3V below DGND.
DS21459E-page 10
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