Rainbow Electronics MAX686 - Manual (Page 11)

MAX686
DAC-Controlled Boost/Inverter
LCD Bias Supply with Internal Switch
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Note that for a positive output voltage, V OUT increases
as V DACOUT decreases. V OUT(MAX) corresponds to
V DACOUT = 0V, and V OUT(MIN) corresponds to
V DACOUT = 1.25V.
Setting the Minimum Negative Output Voltage
For a negative output voltage, the FB threshold voltage
(V
FB ) is 0V, and R1 is placed between FB and REF
(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 | V OUT / V REF
|
For example, if V OUT(MIN) = -12.5V:
R2 = 120k
Ω
x | (-12.5) / (1.25) | = 1.2M
Ω
Setting the Maximum Negative Output Voltage
Assume V OUT(MAX) = -25V and V OUT(MIN) = -12.5V,
then determine R3 and V OUT(MID) as follows:
R3 = R2 x (V FB - V DACOUT(MAX) ) / (V OUT(MAX) -
V OUT(MIN) )
= 1.2M
Ω
x (0 - 1.25) / (-25 - -12.5) =120k
Ω
For a negative output voltage,
V OUT = V OUT(MIN) + (V FB - V DACOUT ) x R2 / R3.
At power-up, the DAC resets to mid-scale where V DACOUT
= 0.635V. Therefore, the output voltage after reset is:
V OUT(MID) = -12.5 + (0 - 0.635) x (1.2M) /
(120k) = -18.85V
Note that for a negative output voltage, | V OUT
| increas-
es as V DACOUT increases. | V OUT(MAX)
| corresponds to
V DACOUT = 1.25V, and | V OUT(MIN)
| corresponds to
V DACOUT = 0V.
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 220k
Ω
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
voltage at FB to V
REF . Choose the value of R1 in the
10k
Ω
to 220k
Ω
range and calculate R2 as follows:
R2 = R1 x ( V OUT / V REF -1 )
where V REF = 1.25V.
Setting the Negative Output Voltage
For negative output voltages, configure R1 and R2 as
shown in Figures 2 and 3, connecting POL to V
CC and
omitting R3. Connecting POL to V CC sets the FB thresh-
old voltage to GND for negative output voltages.
Choose R1 in the 10k
Ω
to 220k
Ω
range and calculate
R2 as follows:
R2 = R1 x | V OUT
| / V REF
where V REF = 1.25V.
Figures 2 and 3 demonstrate two possible methods of
generating a negative voltage with the MAX686. In
Figure 3, D2 connects to the input supply (V
IN ). This
connection features the best output ripple perfor-
mance, but
| V OUT
| must be limited to values less than
-27.5V - V IN . If the application requires a larger nega-
tive voltage, use the method of Figure 2, connecting D2
to GND. This method allows a maximum output voltage
of -27.5V, but
| V OUT
| must be greater than V IN .
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
connecting ISET to V
CC . When the load requires lower
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.
Connecting ISET to V
CC selects the 500mA current-limit
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.