MAX749ESA Maxim Integrated Products, MAX749ESA Datasheet - Page 8

• Manufacturer Part Number: MAX749ESA
• Description: Display Drivers Digitally Adjustable able LCD Bias Supply
• Manufacturer: Maxim Integrated Products

counter rolls over and sets the DAC to -V
where -V
gle rising edge of ADJ increments the DAC output by
one, and 63 rising edges of ADJ decrement the DAC
output by one.
It is also possible to adjust the output voltage using a
potentiometer instead of the internal DAC (Figure 8). On
power-up (V+ applied), the internal current source is set
to mid-scale, or 13.33µA. Choose R1 and R2 with the fol-
lowing equations:
Where the potentiometer can be varied from 0 (producing
V
is connected to ground, allowing the device to be shut
down.
A positive pulse-width modulated (PWM) logic signal
(e.g., from a microcontroller) can control the MAX749’s
output voltage. Use the PWM signal to pull up the FB
pin through a suitable resistor. An RC network on the
PWM output would also be required. In this configura-
tion, the longer the PWM signal remains high, the more
negative the MAX749’s output will be driven.
Digitally Adjustable LCD Bias Supply
Figure 8. Using a Potentiometer to Adjust the Output Voltage
8
OUT(MIN)
+4.5V to +6V
V
V
OUT(MIN)
OUT(MAX)
INPUT
______________________________________________________________________________________
CTRL
ADJ
OUT(MIN)
) to R2Ω (producing V
= -R1(13.33 A)
= -(R1+R2)(13.33 A)
R1 = -V
R2 = -V
MAX749
GND
V+
DLOW
= R
OUT(MIN)
OUT(MAX)
DHI
CS
0.1 F
FB
x 6.66µA. In other words, a sin-
/13.33µA
/13.33µA - R1.
C
R1
COMP
Potentiometer Adjustment
OUT(MAX)
22 F
47 H
R2
L1
PWM Adjustment
). Notice that ADJ
Q1
SMD10P05L
R
SENSE
D1
1N5819
(NEGATIVE)
OUT(MIN)
22 F
30V
V
OUT
,
The current-sense resistor limits the peak switch cur-
rent to 140mV/R
current-sense resistor, and 140mV is the typical cur-
rent-sense comparator threshold (see V+ to CS Voltage
in the Electrical Characteristics ).
To maximize efficiency and reduce the size and cost of
the external components, minimize the peak current.
However, since the output current is a function of the peak
current (Figures 9a-9e), the limit should not be set too low.
No calculations are required to choose the proper cur-
rent-sense resistor; simply follow this two-step procedure:
1. Determine:
For example, assume that the output voltage must be
adjustable to -24V (V
(I
4.75V to 6V (V
2. In Figures 9a-9e, locate the graph drawn for the
In this example, select the -24V output graph, Figure 9d.
We then want a curve where I
input. The 0.3Ω R
rent with a 4.75V input, so we look next at the 0.25Ω
R
V
vides a current limit in the range 440mA to 720mA.
Alternatively, a 0.2Ω sense resistor can be used. This
gives a current limit in the range 550mA to 900mA, but
enables over 40mA to be generated at -24V with input
voltages down to 4.5V. A 0.2Ω resistor may be easier to
obtain than an 0.25Ω resistor.
The theoretical design curves shown in Figures 9a-9e
assume the minimum (worst-case) value for the current-
limit comparator threshold. Having selected the cur-
rent-sense resistor, the maximum current limit is given
by 180mV/R
ure when choosing the transistor, coil, and diode.
IRC (see Table 2) makes surface-mount resistors with pre-
ferred values including: 0.1Ω, 0.2Ω, 0.3Ω, 0.5Ω, and 1.0Ω.
OUT
OUT(MAX)
SENSE
- the minimum input voltage, V
- the maximum output voltage, V
- the maximum output current, I
appropriate output voltage (which is either the
desired output voltage or, if that is not shown, the
graph for the nearest voltage more negative than the
desired output). On this graph find the curve for the
highest R
put current that is adequate at the lowest input
voltage.
= -24V. Therefore select R
graph. It shows I
= 30mA). The supply voltage ranges from
SENSE
SENSE
IN(MIN)
SENSE
SENSE
. Use the maximum current-limit fig-
(the lowest current limit) with an out-
= 4.75V).
, where R
OUT(MAX)
graph shows 25mA of output cur-
OUT
Current-Sense Resistor
= 30mA for V
OUT
SENSE
SENSE
= -24V) at up to 30mA
IN(MIN),
OUT(MAX)
is ≥30mA with a 4.75V
OUT(MAX)
= 0.25Ω. This pro-
is the value of the
IN
.
, and
= 4.75V and