
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
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 VLCD1 on the charge
pump. The charge pump boosts VLCD1 into VLCD2 =
2*VLCD1 and VLCD3 = 3 * VLCD1. When the charge pump
is not operating, Vlcd3 will be internally tied to VDD. See
the Electrical Specications 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.
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
C2
VLCD2
VLCD1
VLCD3
C1
VDD
VLCDADJ
10
A
Connections for
internal charge
pump, VGEN = 1
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
F*
0.47
F*
0.47
F*
0.47
F*
10k*
5k*
10k*
130k*
100k*
* These values are provided for design guidance only and should be
optimized to the application by the designer.