MAX1729 View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX1729
Maxim Integrated
MAX1729 Datasheet PDF : 12 Pages
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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,
VOUT is governed by the following equation:
VOUT ≈ 24.67VFB - 22.71VCOMP
The impedance looking into COMP is nominally 33kΩ.
A source output impedance of less than 500Ω is rec-
ommended. Also, ensure VOUT ≤ 18V by keeping
VCOMP above 0.6V.
Designing for External Feedback Mode
To solve for VOUT in external feedback mode, assume
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 VOUT:
VOUT
=
R1
1
R1
+
1
R2
+
1
R3 + R4
+
1
R5
VFB
−
R1
R3 + R4
VCOMP
−
R1
R5
VTC
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.
External Component Value Formulas
1) Given the maximum output voltage needed (VMAX),
choose the maximum feedback current and solve for
R1 (10µA to 30µA is recommended for maximum feed-
back current) as follows:
R1 = VMAX - VFB
IFB
VIN
C1
0.1µF
C3
0.1µF
PWM
GENERATOR
L1
220µH
IN
LX
PS
GND
MAX1729
REF
OUT
CTLIN
FB
COMP
R4
TC
R3
C6
1µF
Figure 3. External Feedback Mode
C2
0.068µF
VOUT
C4
R1 1µF
R5 R2
C5
1000pF
VIN
C1
0.1µF
DIGITAL
PWM
CONTROLLER
C3
0.1µF
ADC
C5
1000pF
L1
220µH
IN
LX
PS
GND
MAX1729
REF
OUT
CTLIN
TC
FB
COMP
C2
0.068µF
VOUT
C4
1µF
C6
1µF
L1
VIN
220µH
IN
LX
C1
0.1µF
GND
PS
C3
0.1µF
MAX1729
REF
OUT
CTLIN
FB
TC
C5
1000pF
COMP
C2
0.068µF
VOUT
C4
1µF
DC CONTROL
INPUT
VCOMP
Figure 2. Internal Feedback Mode
Figure 4. Using a DC Control Signal
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