MAX1729 Maxim, MAX1729 Datasheet - Page 8

• Manufacturer Part Number: MAX1729
• Description: ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation
• Manufacturer: Maxim

ECB and LCD Display Bias Supply with Accurate
Output Voltage and Temperature Compensation
To use a DC control signal to adjust the output voltage,
use the circuit shown in Figure 4. In this configuration,
V
The impedance looking into COMP is nominally 33k .
A source output impedance of less than 500Ω is rec-
ommended. Also, ensure V
V
To solve for V
the current into the FB pin is zero and the voltage at FB
is 1.228V. Then take the sum of the currents into FB
and solve for V
Using the following formulas, calculate the external
component values required for MAX1729 operation in
external feedback mode, as shown in Figure 3. An
example follows the formulas.
Figure 2. Internal Feedback Mode
8
OUT
COMP
_______________________________________________________________________________________
Designing for External Feedback Mode
is governed by the following equation:
CONTROLLER
V
above 0.6V.
DIGITAL
OUT
PWM
ADC
R
V
3
OUT
R1
= R1
OUT
OUT
V
R
1000pF
IN
0.1 F
0.1 F
4
C3
C5
C1
:
in external feedback mode, assume
V
24.67V
R1
COMP
1
GND
IN
REF
CTLIN
TC
R2
1
FB
MAX1729
220 H
- 22.71V
OUT
L1
R
R5
R1
3
1
COMP
R
OUT
V
PS
LX
FB
4
18V by keeping
TC
COMP
R5
1
C2
0.068 F
C4
1 F
C6
1 F
V
FB
V
OUT
1) Given the maximum output voltage needed (V
choose the maximum feedback current and solve for
R1 (10µA to 30µA is recommended for maximum feed-
back current) as follows:
Figure 3. External Feedback Mode
Figure 4. Using a DC Control Signal
GENERATOR
V
PWM
IN
1000pF
0.1 F
V
0.1 F
IN
0.1 F
0.1 F
C3
C5
C1
C3
C1
External Component Value Formulas
R1 =
GND
IN
REF
CTLIN
TC
GND
IN
REF
CTLIN
COMP
R4
V
MAX1729
MAX
MAX1729
220 H
220 H
L1
L1
I
FB
- V
COMP
C6
1 F
OUT
R3
PS
OUT
LX
FB
FB
LX
PS
FB
TC
C2
0.068 F
C4
1 F
R1
R5
C5
1000pF
C2
0.068 F
DC CONTROL
V
V
COMP
OUT
INPUT
R2
C4
1 F
V
OUT
MAX
),