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PDF MAX749 Data sheet ( Hoja de datos )
| Número de pieza | MAX749 | |
| Descripción | Digitally Adjustable LCD Bias Supply | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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19-0143; Rev 1; 2/95
EVFAOLLULAOTWIOSNDKAITTAMSAHNEUEATL
Digitally Adjustable LCD Bias Supply
_______________General Description
The MAX749 generates negative LCD-bias contrast
voltages from 2V to 6V inputs. Full-scale output voltage
can be scaled to -100V or greater, and is digitally
adjustable in 64 equal steps by an internal digital-to-
analog converter (DAC). Only seven small surface-
mount components are required to build a complete
supply. The output voltage can also be adjusted using
a PWM signal or a potentiometer.
A unique current-limited control scheme reduces supply
current and maximizes efficiency, while a high switching
frequency (up to 500kHz) minimizes the size of external
components. Quiescent current is only 60µA max and is
reduced to under 15µA in shutdown mode. While shut
down, the MAX749 retains the voltage set point, simpli-
fying software control. The MAX749 drives either an
external P-channel MOSFET or a PNP transistor.
____________________________Features
o +2.0V to +6.0V Input Voltage Range
o Flexible Control of Output Voltage:
Digital Control
Potentiometer Adjustment
PWM Control
o Output Voltage Range Set by One Resistor
o Low, 60µA Max Quiescent Current
o 15µA Max Shutdown Mode
o Small Size – 8-Pin SO and Plastic DIP Packages
________________________Applications
Notebook Computers
Laptop Computers
Palmtop Computers
Personal Digital Assistants
Communicating Computers
Portable Data-Collection Terminals
______________Ordering Information
PART
TEMP. RANGE
MAX749CPA
0°C to +70°C
MAX749CSA
0°C to +70°C
MAX749C/D
0°C to +70°C
MAX749EPA
-40°C to +85°C
MAX749ESA
-40°C to +85°C
* Contact factory for dice specifications.
PIN-PACKAGE
8 Plastic DIP
8 SO
Dice*
8 Plastic DIP
8 SO
__________Typical Operating Circuit __________________Pin Configuration
VIN +5V
TOP VIEW
0.1µF
1 V+
CS 8
DIGITAL
ADJUST
ON/OFF
2 ADJ MAX749 DHI 7
3 CTRL
4 FB
6
DLOW
GND 5
RFB
RSENSE
-VOUT
V+ 1
ADJ 2
CTRL 3
FB 4
MAX749
DIP/SO
8 CS
7 DHI
6 DLOW
5 GND
CCOMP
_______________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
1 page  
Digitally Adjustable LCD Bias Supply
+2V TO +6V
INPUT
0.1µF
22µF
6.2V
POWER-ON
RESET
RESET
6-BIT
COUNTER
V+
6-BIT
CURRENT-OUTPUT
DAC
6.66µA TO 20µA
CTRL
ADJ
LOGIC
INCREMENT
REF
CS
ON/OFF
BIAS
SWITCH-
MODE
POWER
SUPPLY
DHI
DLOW
RBASE
470Ω
MAX749
GND
Figure 1. Block Diagram, Showing External Circuitry Using a PNP Transistor
RFB
FB
CCOMP
RSENSE
Q1
ZTX750
L1
47µH
D1
1N5819
VOUT
(NEGATIVE)
22µF
30V
_______________Detailed Description
The MAX749 is a negative-output inverting power con-
troller that can drive an external PNP transistor or P-
channel MOSFET. An external resistor and an internal
DAC control the output voltage (Figure 1).
The MAX749 is designed to operate from 2V to 6V inputs,
ideal for operation from low-voltage batteries. In systems
with higher-voltage batteries, such as notebook comput-
ers, the MAX749 may also be operated from the regulat-
ed +5V supply. A high-efficiency +5V regulator, such as
the MAX782, is an ideal source for the MAX749. In this
example, the MAX749 efficiency (80%) is compounded
with the MAX782 efficiency (95%): 80% x 95% = 76%,
which is still high.
Operating Principle
The MAX749 and the external components shown in the
Typical Operating Circuit form a flyback converter.
When the external transistor is on, current flows through
the current-sense resistor, the transistor, and the coil.
Energy is stored in the core of the coil during this phase,
and the diode does not conduct. When the transistor
turns off, current flows from the output through the diode
and the coil, driving the output negative. Feedback con-
trol adjusts the external transistor’s timing to provide a
regulated negative output voltage.
The MAX749’s unique control scheme combines the
ultra-low supply current of pulse-skipping, pulse-fre-
quency modulation (PFM) converters with the high full-
load efficiency characteristic of pulse-width modulation
(PWM) converters. This control scheme allows the
device to achieve high efficiency over a wide range of
loads. The current-sense function and high operating
frequency allow the use of tiny external components.
Switching control is accomplished through the combi-
nation of a current limit in the switch plus on- and off-
time limits (Figure 2).
Once turned on, the transistor stays on until either:
- the maximum on-time one-shot turns it off
(8µs later), or
- the switch current reaches its limit (as determined
by the current-sense resistor and the current
comparator).
_______________________________________________________________________________________ 5
5 Page 
Digitally Adjustable LCD Bias Supply
Capacitors
Output Filter Capacitor
A 22µF, 30V surface-mount (SMT) tantalum output filter
capacitor typically maintains 100mVp-p output ripple
when generating -24V at 40mA from a 5V input. Smaller
capacitors, down to 10µF, may be used for light loads
in applications that can tolerate higher output ripple.
Surface-mount capacitors are generally preferred
because they lack the inductance and resistance of the
leads of their through-hole equivalents.
Input Bypass Capacitor
A 22µF tantalum capacitor in parallel with a 0.1µF
ceramic normally provides sufficient bypassing. Mount
the 0.1µF capacitor very close to the IC. Larger
capacitors may be needed if the incoming supply has
high impedance. Less bypass capacitance is accept-
able if the circuit is run off a low-impedance supply.
Begin prototyping with a large bypass capacitor; when
the circuit is working, reduce the bypass to the smallest
value that gives good results. Although bench power
supplies have low impedance at DC, they often have
high impedance at the frequencies used by switching
DC-DC converters.
The effective series resistance (ESR) of both the
bypass and filter capacitors affects efficiency. Best per-
formance is obtained by doubling up on the filter
capacitors or using low-ESR types.
The smallest low-ESR SMT capacitors currently avail-
able are Sprague 595D series, which are about half the
size of competing products. Sanyo OS-CON organic
semiconductor through-hole capacitors also exhibit low
ESR, and are especially useful when operation below
0°C is required. Table 2 lists the phone numbers of
these and other manufacturers.
Compensation Capacitor
The high value of the feedback resistor makes the feed-
back loop susceptible to phase lag if parasitic capaci-
tance is present at the FB pin. To compensate for this, it
may be necessary to connect a capacitor, CCOMP, in
parallel with RFB. Although CCOMP is normally not
required, the value of CCOMP depends upon the value
of RFB and on the individual circuit layout—typical val-
ues range from 0pF to 220pF.
PC Layout and Grounding
Due to high current levels and fast switching wave-
forms, proper PC board layout is essential. In particular,
keep all leads short, especially the lead connected to
the FB pin and those connecting Q1, L1, and D1
together. Mount the RFB resistor very close to the IC.
Use a star ground configuration: Connect the ground
lead of the input bypass capacitor, the output capaci-
tor, and the inductor at a common point next to the
GND pin of the MAX749. Additionally, connect the posi-
tive lead of the input bypass capacitor as close as pos-
sible to the V+ pin of the IC.
___________________Chip Topography
0.070"
(0.1178mm)
V+ V+ CS
ADJ
CTRL
DHI 0.808"
(0.2032mm)
DLOW
FB GND
TRANSISTOR COUNT: 521;
SUBSTRATE CONNECTED TO GND.
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet MAX749.PDF ] | |
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