Rainbow-electronics MAX1729 Manual do Utilizador descarregar pdf (Página 8)

MAX1729

To use a DC control signal to adjust the output voltage,

use the circuit shown in Figure 4. In this configuration,

OUT

is governed by the following equation:

The impedance looking into COMP is nominally 33kΩ.

A source output impedance of less than 500Ω is rec-

ommended. Also, ensure V

OUT

≤ 18V by keeping

COMP

above 0.6V.

Designing for External Feedback Mode

To solve for V

OUT

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 V

OUT

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 (V

MAX

choose the maximum feedback current and solve for

R1 (10µA to 30µA is recommended for maximum feed-

back current) as follows:

R1 =

V - V

MAX FB

V = R1

OUT FB

COMP TC













−













−













V 24.67V - 22.71V

OUT FB COMP

≈

ECB and LCD Display Bias Supply with Accurate

Output Voltage and Temperature Compensation

8 _______________________________________________________________________________________

OUT

COMP

220µH

CTLIN

DIGITAL

PWM

CONTROLLER

GND

0.1µF

0.068µF

1µF

0.1µF

1000pF

OUT

MAX1729

REF

ADC

Figure 2. Internal Feedback Mode

OUT

220µH

COMP

CTLIN

PWM

GENERATOR

GND

R4 R3

0.1µF

0.068µF

1µF

R5 R2

1µF

0.1µF

OUT

MAX1729

1000pF

REF

Figure 3. External Feedback Mode

OUT

COMP

CTLIN

GND

220µH

0.1µF

0.068µF

1µF

COMP

DC CONTROL

INPUT

0.1µF

1000pF

OUT

MAX1729

REF

Figure 4. Using a DC Control Signal

1 2 3 4 5 6 7 8 9 10 11 12

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