Rainbow Electronics MAX1621 - Manual (Page 16)

MAX1620/MAX1621
Digitally Adjustable LCD Bias Supplies
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
Ω
x (1.5) / (25 - 12.5) = 264k
Ω
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
Ω
x (0 - 1.5) / (-25 - -12.5) = 300k
Ω
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
Ω
, 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
Ω
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