Datasheet

 1997 Microchip Technology Inc. DS30444E - page 101
PIC16C9XX
13.5 Voltage Generation
There are two methods for LCD voltage generation,
internal charge pump, or external resistor ladder.
13.5.1 CHARGE PUMP
The LCD charge pump is shown in Figure 13-13. The
1.0V - 2.3V regulator will establish a stable base volt-
age from the varying battery voltage. This regulator is
adjustable through the range by connecting a variable
external resistor from VLCDADJ to ground. The poten-
tiometer provides contrast adjustment for the LCD. This
base voltage is connected to V
LCD1 on the charge
pump. The charge pump boosts V
LCD1 into VLCD2 =
2*V
LCD1 and VLCD3 = 3 * VLCD1. When the charge pump
is not operating, Vlcd3 will be internally tied to V
DD. See
the Electrical Specifications section for charge pump
capacitor and potentiometer values.
13.5.2 EXTERNAL R-LADDER
The LCD module can also use an external resistor lad-
der (R-Ladder) to generate the LCD voltages.
Figure 13-13 shows external connections for static and
1/3 bias. The VGEN (LCDCON<4>) bit must be cleared
to use an external R-Ladder.
FIGURE 13-13:CHARGE PUMP AND RESISTOR LADDER
C2VLCD2 VLCD1 VLCD3 C1
VDD
VLCDADJ
10 µA
Connections for
internal charge
pump, VGEN = 1
VDD
VDD
Connections for
external R-ladder,
1/3 Bias,
VGEN = 0
Connections for
external R-ladder,
Static Bias,
VGEN = 0
nominal
Charge Pump
LCDEN
SLPEN
0.47 µF* 0.47 µF* 0.47 µF*
0.47 µF*
10k* 10k* 10k*
5k*
5k*
10k*
130k*
100k*
* These values are provided for design guidance only and should be
optimized to the application by the designer.