MAX749EPA ,Digitally Adjustable LCD Bias SupplyMAX74919-0143; Rev 1; 2/95Digitally Adjustable LCD Bias Supply_______________
MAX749ESA ,Digitally Adjustable LCD Bias SupplyApplications______________Ordering InformationNotebook ComputersPART TEMP. RANGE PIN-PACKAGELaptop ..
MAX749ESA ,Digitally Adjustable LCD Bias SupplyGeneral Description ________
MAX749ESA ,Digitally Adjustable LCD Bias SupplyMAX74919-0143; Rev 1; 2/95Digitally Adjustable LCD Bias Supply_______________
MAX749ESA+ ,Digitally Adjustable LCD Bias SupplyFeaturesThe MAX749 generates negative LCD-bias contrast♦ +2.0V to +6.0V Input Voltage Rangevoltages ..
MAX749ESA+T ,Digitally Adjustable LCD Bias SupplyMAX74919-0143; Rev 1; 2/95Digitally Adjustable LCD Bias Supply_______________
MB6M ,MINIATURE GLASS PASSIVATED SINGLE-PHASE BRIDGE RECTIFIERThermal Characteristics (TA = 25°C unless otherwise noted)Parameter Symbol MB2M MB4M MB6M UnitDevic ..
MB6S ,Bridge RectifiersThermal Characteristics (T = 25°C unless otherwise noted)AParameter Symbol MB2S MB4S MB6S UnitDevic ..
MB7117E , Schottky TTL 2048-Bit Bipolar Programmable Read-Only Memory
MB71A38-25 , PROGRAMMABLE SCHOTTKY 16384-BIT READ ONLY MEMORY
MAX749CPA-MAX749CSA-MAX749EPA-MAX749ESA
Digitally Adjustable LCD Bias Supply
Digitally Adjustable LCD Bias Supply
_______________General DescriptionThe 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.
________________________ApplicationsNotebook Computers
Laptop Computers
Palmtop Computers
Personal Digital Assistants
Communicating Computers
Portable Data-Collection Terminals
____________________________Features+2.0V to +6.0V Input Voltage RangeFlexible Control of Output Voltage:
Digital Control
Potentiometer Adjustment
PWM ControlOutput Voltage Range Set by One ResistorLow, 60µA Max Quiescent Current15µA Max Shutdown ModeSmall Size – 8-Pin SO and Plastic DIP Packages
MAX749
_______________________________________________________________Maxim Integrated Products 1
__________________Pin Configuration
__________Typical Operating Circuit
Call toll free 1-800-998-8800 for free samples or literature.
______________Ordering Information* Contact factory for dice specifications.
MAX749
Digitally Adjustable LCD Bias Supply______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGSV+................................................................................-0.3V, +7V
CTRL, ADJ, FB, DLOW, DHI, CS.....................-0.3V, (V++ 0.3V)
Continuous Power Dissipation (TA= +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C)............727mW
SO (derate 5.88mW/°C above +70°C).........................471mW
Operating Temperature Ranges:
MAX749C_A........................................................0°C to +70°C
MAX749E_A.....................................................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
ELECTRICAL CHARACTERISTICS(2V < V+ < 6V, TA= TMINto TMAX, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Note 1:The device is in regulation when VFB= 0V (see Figures 3 - 6).
Note 2:These tests performed at V+ = 3.3V. Operation over supply range is guaranteed by supply rejection test of full-count current.
Note 3:VIHis guaranteed by design to be 1.8V min for V+ = 2Vto 6V for TA= TMINto TMAX. VILis guaranteed by design from = TMINto TMAX.
TIMING CHARACTERISTICS
MAX749
Digitally Adjustable LCD Bias Supply
_______________________________________________________________________________________ 3EFFICIENCY vs.
OUTPUT VOLTAGE
MAX749-TOC1-A
OUTPUT VOLTAGE (V)
EFFICIENCY (
EFFICIENCY vs.
OUTPUT VOLTAGE
MAX749-TOC1-B
OUTPUT VOLTAGE (V)
EFFICIENCY (
EFFICIENCY vs. OUTPUT
CURRENT – PNP
MAX749TOC2-A
OUTPUT CURRENT (mA)
EFFICIENCY (%)203040502060100
EFFICIENCY vs. OUTPUT
CURRENT – PNPMAX749TOC2-B
OUTPUT CURRENT (mA)
EFFICIENCY (%)8030507090
LOAD CURRENT vs. INPUT VOLTAGEMAX749-TOC3-B
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
EFFICIENCY vs. OUTPUT
CURRENT – MOSFET
MAX749TOC2-C
OUTPUT CURRENT (mA)
EFFICIENCY (%)4015253545
LOAD CURRENT vs. INPUT VOLTAGEMAX749-TOC3-A
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
__________________________________________Typical Operating Characteristics
(TA = +25°C, L = 47µH, unless otherwise noted.)
MAX749
Digitally Adjustable LCD Bias Supply______________________________________________________________________________________
____________________________Typical Operating Characteristics (continued)(TA = +25°C, L = 47µH, unless otherwise noted.)
______________________________________________________________Pin Description
_______________Detailed DescriptionThe 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 PrincipleThe MAX749 and the external components shown in the
Typical Operating Circuitform 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).
MAX749
Digitally Adjustable LCD Bias Supply
_______________________________________________________________________________________ 5
MAX749Once turned off, a one-shot holds the switch off for a
minimum of 1µs, and the switch either stays off (if the
output is in regulation), or turns on again (if the output
is out of regulation).
With light loads, the transistor switches for one or more
cycles and then turns off, much like a traditional PFM
converter. With heavy loads, the transistor stays on until
the switch current reaches the current limit; it then
shuts off for 1µs, and immediately turns on again until
the next time the switch current reaches its limit. This
cycle repeats until the output is in regulation.
Output Voltage ControlThe output voltage is set using a single external resistor
and the internal current-output DAC (Figure 1). The full-
scale output voltage is set by selecting the feedback
resistor, RFB. The output voltage is controlled from 33%
to 100% of the full-scale output by an internal 64-step
DAC/counter.
On power-up or after a reset, the counter sets the DAC
output to mid-range. Each rising edge of ADJ incre-
ments the DAC output. When incremented beyond full
scale, the counter rolls over and sets the DAC to the
minimum value. In this way, a single pulse applied to
ADJ increases the DAC set point by one step, and 63
pulses decrease the set point by one step.
Table 1 is the logic table for the CTRL and ADJ inputs,
which control the internal DAC and counter. Figures 3-7
show various timing specifications and different ways of
incrementing and resetting the DAC, and of placing it in
the low-power standby mode. As long as the timing
specifications for ADJ and CTRL are observed, any
sequence of operations can be implemented.
Table 1. Input Truth Table
Digitally Adjustable LCD Bias Supply______________________________________________________________________________________Figure 2.Switch-Mode Power-Supply Section Block Diagram
