參數(shù)資料
型號(hào)： MAX1620-MAX1621
廠商： Maxim Integrated Products, Inc.
英文描述： Digitally Adjustable LCD Bias Supplies
中文描述： 數(shù)字式可調(diào)LCD偏置電源
文件頁(yè)數(shù)： 16/20頁(yè)
文件大小： 253K
代理商： MAX1620-MAX1621
M
Digitally Adjustable LCD Bias S upplies
16
______________________________________________________________________________________
Setting the Maximum Output Voltage
(DAC Adjustment)
The DAC is adjustable from 0V to 1.5V in 32 steps, and
1LSB = 1.5V / 31. DAC adjustment of V
OUT
is provided
by adding R3 to the divider circuit (Figure 4). Be sure
that V
OUT,MAX
does not exceed the LCD panel rating.
For V
OUT,MAX
= 25V and V
OUT,MIN
= 12.5V, R3 is deter-
mined as follows:
R3 = R5 x (V
FB
) / (V
OUT,MAX
- V
OUT,MIN
)
= 2.2M
x (1.5) / (25 - 12.5) = 264k
The general form for V
OUT
as a function of the DAC out-
put (V
DOUT
) is:
V
OUT
= V
OUT,MIN
+ (V
FB
- V
DOUT
) x R5 / R3
At power-up the DAC resets to mid-scale (10000), which
corresponds to V
DOUT
= 0.774V; therefore, the output
voltage after reset is as follows:
V
OUT,RESET
= V
OUT,MIN
+ (1.5 - 0.774) x R5 / R3
Note that for a positive output voltage, V
OUT
increases
as V
DOUT
decreases. V
OUT,MAX
corresponds to V
DOUT
= 0V, and V
OUT,MIN
corresponds to V
DOUT
= 1.5V.
For a negative output voltage, V
OUT
= V
OUT,MIN
+
(V
FB
- V
DOUT
) x R5 / R3. Assume V
OUT,MAX
= -25V and
V
OUT,MIN
= -12.5V; then determine R3 and V
OUT,RESET
as follows:
R3 = R5 x (V
FB
- V
DOUT,MAX
) / (V
OUT,MAX
- V
OUT,MIN
)
= 2.5M
x (0 - 1.5) / (-25 - -12.5) = 300k
V
OUT,RESET
= -12.5 + (0 - 0.774) x (2.5M) /
(300k) = -18.95V
Note that for a negative output voltage,
V
OUT
increases
as V
DOUT
increases.
V
OUT,MAX
corresponds to V
DOUT
= 1.5V, and
V
OUT,MIN
corresponds to V
DOUT
= 0V.
Potentiometer Adjustment
The output can be adjusted with a potentiometer instead
of the DAC. Choose R
POT
= 100k
, and connect it
between REF and GND. Connect R3 to the potentiome-
ter’s wiper, instead of to DOUT. The same design equa-
tions as above apply.
Controlling the LCD Using
POK and
LCDON
When voltage at POK is greater than 1V, the open-drain
LCDON
output pulls low.
LCDON
withstands 27V; there-
fore, it can drive a PFET or PNP transistor to switch on
the MAX1620/MAX1621’s positive output. The following
represent three cases for using this feature:
1) As an off switch, to ensure that a positive boosted
output goes to 0V during shutdown. In this case,
connect POK to
SHDN
. Without this switch, the posi-
tive output falls to one diode-drop below the input
voltage (V
BATT
) in shutdown.
LCDON
is not needed
for negative outputs, which will fall to 0V in shut-
down anyway.
2) As an output sensing cutoff for positive outputs.
Connect POK to the feedback voltage divider to
sense the output voltage. The output is switched on
only when it reaches a set percentage of the set
voltage.
3) As an input sensing output cutoff for positive out-
puts. Connect POK to a voltage divider to sense the
input voltage. The output is switched on only when
the input reaches the set level (Figure 4).
To control the open-drain output
LCDON
by sensing
the input voltage, connect a resistor-divider (R1-R2,
Figure 4) from V
BATT
to POK. Choose R2 = 100k. For
example, if the minimum battery voltage is 5.3V, deter-
mine R1 as follows:
R1 = R2 x [(V
BATT
/ V
POK
) - 1]
= 100k x [(5.3 / 0.992) - 1] = 434k
LCDON
can also be controlled via software (MAX1621,
Table 4).
Table 4. MAX1621
LCDON
Output
Truth Table
POK Pin
LCDON
Output
LCDON Bit
<1V
0
Floating
<1V
1
Floating
>1V
1
ON, pulls low
>1V
0
Floating
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