DAC-Controlled Boost/Inverter

LCD Bias Supply with Internal Switch

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11

Setting the Minimum Negative Output Voltage

For a negative output voltage, the FB threshold voltage

(Figures 2 and 3). Again, choose R1 to be 120k

Ω so

that the current in the divider is about 10µA. Then

determine R2 as follows:

R2 = R1 x

R2 = 120k

Setting the Maximum Negative Output Voltage

= 1.2M

Ω x (0 - 1.25) / (-25 - -12.5) =120kΩ

For a negative output voltage,

= 0.635V. Therefore, the output voltage after reset is:

(120k) = -18.85V

Note that for a negative output voltage,

Setting the Output Voltage

without the DAC

The MAX686 may be used without the DAC to control

the output voltage. For either positive or negative out-

put voltage applications, set the MAX686’s output volt-

age using only two external resistors (R1 and R2) as

shown in Figure 1, 2, or 3. Since the input bias current

at FB has a 50nA maximum value, large resistors can

be used in the feedback loop without a significant loss

of accuracy. Select R1 to be in the 10k

Ω to 220kΩ

range and calculate R2 using the applicable equations

from the following subsections.

Setting the Positive Output Voltage

Use the circuit of Figure 1, connecting POL to GND and

omitting R3. Connecting POL to GND sets the threshold

10k

Ω to 220kΩ range and calculate R2 as follows:

R2 = R1 x

Setting the Negative Output Voltage

For negative output voltages, configure R1 and R2 as

old voltage to GND for negative output voltages.

Choose R1 in the 10k

Ω to 220kΩ range and calculate

R2 as follows:

R2 = R1 x

Figures 2 and 3 demonstrate two possible methods of

generating a negative voltage with the MAX686. In

connection features the best output ripple perfor-

mance, but

tive voltage, use the method of Figure 2, connecting D2

to GND. This method allows a maximum output voltage

of -27.5V, but

Setting the Peak Inductor Current Limit

External current-limit selection provides added control

over the MAX686’s output performance. A higher cur-

rent limit increases the amount of energy stored in the

inductor during each cycle, which provides higher out-

put current capability. For higher output current appli-

cations, choose the 500mA current-limit option by

output current, the 250mA current limit provides several

advantages. First, a smaller inductor saves board area

and cost. Second, smaller energy transfers per cycle

reduce output ripple for a given capacitor. Connecting

ISET to GND selects the 250mA current-limit option.

option. Refer to the

Typical Operating Characteristics

for efficiency and load current graphs at each ISET cur-

rent setting.

Selecting Inductors

The MAX686’s high switching frequency allows for the

use of a small inductor. The 22µH inductor shown in

Figures 1, 2, and 3 is recommended for most applica-

tions, although values between 10µH and 47µH are

acceptable. Use inductors with a ferrite core or equiva-

lent; powder iron cores are not recommended for use

with high switching frequencies. The inductor’s incre-

mental saturation rating must exceed the selected cur-

rent limit. For highest efficiency, use an inductor with a

low DC resistance (under 200m

Ω). See Table 1 for a list

of inductor suppliers.