19-0143; Rev 1; 2/95
NUAL
KIT MA
UATION TA SHEET
EVAL
WS DA
FOLLO
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
MAX749
________________________Applications
Notebook Computers
Laptop Computers
Palmtop Computers
Personal Digital Assistants
Communicating Computers
Portable Data-Collection Terminals
______________Ordering Information
PART
MAX749CPA
MAX749CSA
MAX749C/D
MAX749EPA
MAX749ESA
TEMP. RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
8 Plastic DIP
8 SO
Dice*
8 Plastic DIP
8 SO
* Contact factory for dice specifications.
__________Typical Operating Circuit
V
IN
+5V
__________________Pin Configuration
TOP VIEW
R
SENSE
0.1µF
1
V+
CS
8
V+
ADJ
-V
OUT
1
2
8
7
CS
DHI
DLOW
GND
DIGITAL
ADJUST
ON/OFF
2 ADJ
7
MAX749
DHI
3
4
CTRL
FB
DLOW
GND
6
5
CTRL 3
FB 4
MAX749
6
5
R
FB
DIP/SO
C
COMP
_______________________________________________________________
Maxim Integrated Products
1
Call toll free 1-800-998-8800 for free samples or literature.
Digitally Adjustable LCD Bias Supply
MAX749
ABSOLUTE MAXIMUM RATINGS
V+ ................................................................................-0.3V, +7V
CTRL, ADJ, FB, DLOW, DHI, CS.....................-0.3V, (V+ + 0.3V)
Continuous Power Dissipation (T
A
= +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
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.
ELECTRICAL CHARACTERISTICS
(2V < V+ < 6V, T
A
= T
MIN
to T
MAX
, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
V+ Voltage
2
6
V
FB Source Current
I
FBS
On power-up or reset, V
FB
= 0V (Note 1)
12.80
13.33
13.86
µA
Zero-Count FB Current
V
FB
= 0V
0.45
0.55
I
FBS
Full-Count FB Current
V
FB
= 0V
1.43
1.53
I
FBS
FB Offset Voltage
±15
mV
DAC Step Size (Note 2)
Monotonicity guaranteed, V
FB
= 0V
1.00
1.56
2.12
%I
FBS
DAC Linearity (Note 2)
V
FB
= 0V
±1
%I
FBS
Supply Rejection
V+ = 2V to 6V, full-count current
1.5
%I
FBS
Switching Frequency
100 to 500
kHz
Logic Input Current
0V < V
IN
< V+, CTRL, ADJ
±100
nA
Logic High Threshold (Note 3)
V
IH
CTRL, ADJ
1.6
V
Logic Low Threshold (Note 3)
V
IL
CTRL, ADJ
0.4
V
Quiescent Current
60
µA
Shutdown Current
15
µA
V+ to CS Voltage
Current-limit trip voltage
110
140
180
mV
DHI Source Current
V+ = 2V, V
DHI
= 1V
24
50
mA
DHI Drive Level
No load
V+ - 50mV
V+
V
DLOW On Resistance
V+ = 2V, V
DLOW
= 0.5V
5
10
Ω
Note 1:
The device is in regulation when V
FB
= 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:
V
IH
is guaranteed by design to be 1.8V min for V+ = 2V to 6V for T
A
= T
MIN
to T
MAX
. V
IL
is guaranteed by design from
T
A
= T
MIN
to T
MAX
.
TIMING CHARACTERISTICS
PARAMETER
Minimum Reset Pulse Width
Minimum Reset Setup
Minimum Reset Hold
Minimum ADJ High Pulse Width
Minimum ADJ Low Pulse Width
Minimum ADJ Low to CTRL Low
SYMBOL
t
R
t
RS
t
RH
t
SH
t
SL
t
SD
CONDITIONS
V+ = 2V
V+ = 5V
Not tested
Not tested
V+ = 2V
V+ = 5V
V+ = 2V
V+ = 5V
V+ = 2V
V+ = 5V
MIN
T
A
= +25°C
TYP
MAX
125
25
0
0
15
10
170
60
70
20
85
85
400
150
200
85
300
85
0
0
100
100
500
200
250
100
T
A
= T
MIN
to T
MAX
MIN
MAX
400
100
UNITS
ns
ns
ns
ns
ns
ns
2
______________________________________________________________________________________
Digitally Adjustable LCD Bias Supply
__________________________________________Typical Operating Characteristics
(T
A
= +25°C, L = 47µH, unless otherwise noted.)
EFFICIENCY vs. OUTPUT
CURRENT – PNP
MAX749TOC2-A
MAX749
EFFICIENCY vs. OUTPUT
CURRENT – PNP
MAX749TOC2-B
EFFICIENCY vs. OUTPUT
CURRENT – MOSFET
-24V
-12V
80
EFFICIENCY (%)
MAX749TOC2-C
85
80
78
76
EFFICIENCY (%)
74
72
70
68
66
-24V
V+ = 3V
R
BASE
= 160Ω
R
SENSE
= 0.25Ω
TRANSISTOR = ZTX750
-12V
-5V
85
80
EFFICIENCY (%)
-24V
-12V
-5V
-5V
75
V+ = 5V
R
SENSE
= 0.25Ω
TRANSISTOR: SMD10P05L
75
V+ = 3V
R
BASE
= 470Ω
R
SENSE
= 0.25Ω
TRANSISTOR: ZTX750
70
70
65
0
10
20
30
40
50
60
OUTPUT CURRENT (mA)
64
0
10 20 30 40 50 60 70 80 90 100
OUTPUT CURRENT (mA)
65
0
5 10 15 20 25 30 35 40 45 50
OUTPUT CURRENT (mA)
EFFICIENCY vs.
OUTPUT VOLTAGE
MAX749-TOC1-A
EFFICIENCY vs.
OUTPUT VOLTAGE
-20mA
-5mA
MAX749-TOC1-B
85
-20mA
80
EFFICIENCY (%)
-5mA
75
V+ = 3V
R
BASE
= 470Ω
R
SENSE
= 0.25Ω
TRANSISTOR : ZTX750
-40mA
85
80
EFFICIENCY (%)
-40mA
75
70
70
V+ = 5V
R
SENSE
= 0.25Ω
TRANSISTOR : SMD10P05L
65
-24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4
OUTPUT VOLTAGE (V)
65
-24 -22 -20 -18 -16 -14 -12 -10 -8 -6
OUTPUT VOLTAGE (V)
-4
LOAD CURRENT vs. INPUT VOLTAGE
MAX749-TOC3-A
LOAD CURRENT vs. INPUT VOLTAGE
450
400
LOAD CURRENT (mA)
350
300
250
200
150
100
50
0
-24V
-48V
2
3
4
INPUT VOLTAGE (V)
5
6
-12V
R
BASE
= 160Ω
-5V
R
SENSE
= 0.25Ω
TRANSISTOR = ZTX750
MAX749-TOC3-B
400
350
LOAD CURRENT (mA)
300
250
200
150
100
50
0
2
3
4
INPUT VOLTAGE (V)
5
6
-12V
-24V
-48V
R
BASE
= 470Ω
R
SENSE
= 0.25Ω
TRANSISTOR = ZTX750
-5V
500
_______________________________________________________________________________________
3
Digitally Adjustable LCD Bias Supply
MAX749
____________________________Typical Operating Characteristics (continued)
(T
A
= +25°C, L = 47µH, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
OUTPUT
VOLTAGE
100mV
AC
/div
OUTPUT
VOLTAGE
100mV
AC
/div
LOAD
CURRENT
V
OUT
= -15V
TRANSISTOR = ZTX750
50µs/div
V
OUT
= -15V
TRANSISTOR = ZTX750
10mA/div
0mA
INPUT
VOLTAGE
1 V/div
50ms/div
V
OUT
= -15V
I
LOAD
= 5mA
TRANSISTOR = ZTX750
0V
______________________________________________________________Pin Description
PIN
1
NAME
V+
FUNCTION
+2V to +6V Input Voltage to power the MAX749 and external circuitry. When using an external
P-channel MOSFET, V+ must exceed the MOSFET’s gate threshold voltage.
Logic Input. When CTRL is high, a rising edge on ADJ increments an internal counter. When CTRL is
low, the counter is reset to mid-scale when ADJ is high. When ADJ is low, the counter does not
change (regardless of activity on CTRL) as long as V+ is applied.
Logic Input. When CTRL and ADJ are low, the MAX749 is shut down, but the counter is not reset.
When CTRL is low, the counter is reset to mid-scale when ADJ is high. The device is always on when
CTRL is high.
Feedback Input for output full-scale voltage selection. -V
OUT(MAX)
= (R
FB
) x (20µA) where R
FB
is
connected from FB to -V
OUT
. The device is in regulation when V
FB
= 0V.
Ground
Output Driver Low. Connect to DHI when using an external P-channel MOSFET. When using an
external PNP transistor, connect a resistor R
BASE
from DLOW to the base of the PNP to set the maxi-
mum base-drive current.
Output Driver High. Connect to the gate of the external P-channel transistor, or to the base of the
external PNP transistor.
Current-Sense Input. The external transistor is turned off when current through the sense resistor,
R
SENSE
, brings CS below V+ by 140mV (typ).
2
ADJ
3
CTRL
4
5
6
FB
GND
DLOW
7
8
DHI
CS
4
______________________________________________________________________________________
Digitally Adjustable LCD Bias Supply
MAX749
+2V TO +6V
INPUT
0.1µF
22µF
6.2V
V+
POWER-ON
RESET
RESET
6-BIT
COUNTER
6-BIT
CURRENT-OUTPUT
DAC
REF
LOGIC
INCREMENT
CS
DHI
DLOW
R
BASE
470Ω
Q1
ZTX750
D1
1N5819
L1
47µH
V
OUT
(NEGATIVE)
22µF
30V
6.66µA TO 20µA
R
SENSE
CTRL
ADJ
ON/OFF
SWITCH-
MODE
POWER
SUPPLY
BIAS
R
FB
FB
MAX749
GND
C
COMP
Figure 1. Block Diagram, Showing External Circuitry Using a PNP Transistor
_______________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.
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
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
_______________________________________________________________________________________
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