uts. 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.
________________________Applications
Notebook Computers
Laptop Computers
Palmtop Computers
Personal Digital Assistants
Communicating Computers
Portable Data-Collection Terminals
____________________________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
_______________________________________________________________
Maxim Integrated Products
1
1
2
3
4
8
7
6
5
CS
DHI
DLOW
GND
FB
CTRL
ADJ
V+
DIP/SO
TOP VIEW
MAX749
__________________Pin Configuration
V+
ADJ
CTRL
FB
CS
DHI
DLOW
GND
V
IN
+5V
-V
OUT
R
SENSE
C
COMP
ON/OFF
DIGITAL
ADJUST
R
FB
1
0.1
µ
F
2
3
4
5
6
7
8
MAX749
__________Typical Operating Circuit
Call toll free 1-800-998-8800 for free samples or literature.
19-0143; Rev 1; 2/95
PART
TEMP. RANGE
PIN-PACKAGE
MAX749CPA
0°C to +70°C
8 Plastic DIP
MAX749CSA
0°C to +70°C
8 SO
MAX749EPA
-40°C to +85°C
8 Plastic DIP
MAX749ESA
-40°C to +85°C
8 SO
MAX749C/D
0°C to +70°C
Dice*
______________Ordering Information
* Contact factory for dice specifications.
EVALUATION KIT MANUAL
FOLLOWS DATA SHEET T
A
= T
MIN
to T
MAX
MIN
MAX
125
300
MAX749
Digitally Adjustable LCD Bias Supply
2
______________________________________________________________________________________
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
ELECTRICAL CHARACTERISTICS
(2V < V+ < 6V, T
A
= T
MIN
to T
MAX
, 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 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
.
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
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
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 V
Logic Low Threshold (Note 3)
V
IL
CTRL, ADJ
0.4
TIMING CHARACTERISTICS
T
A
= +25°C
MIN
TYP
MAX
PARAMETER
SYMBOL
CONDITIONS
UNITS
Minimum Reset Pulse Width
t
R
V+ = 5V
100
Minimum Reset Setup
t
RS
0
0
Minimum Reset Hold
t
RH
Not tested
0
0
t
SH
V+ = 2V
15
85
100
Minimum ADJ High Pulse Width
V+ = 5V
10
85
100
t
SL
V+ = 2V
170
400
500
Minimum ADJ Low Pulse Width
V+ = 5V
60
150
200
Minimum ADJ Low to CTRL Low
t
SD
V+ = 2V
70
200
250
V+ = 5V
20
85
100
ns
V+ = 2V
400
25
85
Not tested
ns
ns
ns
ns
ns
FB Offset Voltage
±15
mV MAX749
Digitally Adjustable LCD Bias Supply
_______________________________________________________________________________________
3
85
65
-24
-20
-12
-4
EFFICIENCY vs.
OUTPUT VOLTAGE
70
80
MAX749-TOC1-A
OUTPUT VOLTAGE (V)
EFFICIENCY (
%
)
-16
-8
75
-40mA
V+ = 3V
R
BASE
= 470 R
SENSE
= 0.25 TRANSISTOR : ZTX750
-22
-18
-14
-10
-6
-20mA
-5mA
85
65
-24
-20
-12
-4
EFFICIENCY vs.
OUTPUT VOLTAGE
70
80
MAX749-TOC1-B
OUTPUT VOLTAGE (V)
EFFICIENCY (
%
)
-16
-8
75
-5mA
-20mA
-40mA
V+ = 5V
R
SENSE
= 0.25 TRANSISTOR : SMD10P05L
-22
-18
-14
-10
-6
80
65
0
EFFICIENCY vs. OUTPUT
CURRENT PNP
MAX749TOC2-A
OUTPUT CURRENT (mA)
EFFICIENCY (%)
70
75
10
20
30
40
50
85
60
-24V
-12V
-5V
V+ = 3V
R
BASE
= 470 R
SENSE
= 0.25 TRANSISTOR: ZTX750
80
64
0
20
60
100
EFFICIENCY vs. OUTPUT
CURRENT PNP
68
76
MAX749TOC2-B
OUTPUT CURRENT (mA)
EFFICIENCY (%)
40
80
72
78
74
70
66
10
30
50
70
90
V+ = 3V
R
BASE
= 160 R
SENSE
= 0.25 TRANSISTOR = ZTX750
-12V
-5V
-24V
0
2
LOAD CURRENT vs. INPUT VOLTAGE
MAX749-TOC3-B
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
4
50
100
150
200
250
300
350
400
450
500
3
5
6
-5V
-12V
-24V
-48V
R
BASE
= 160 R
SENSE
= 0.25 TRANSISTOR = ZTX750
85
65
0
10
30
50
EFFICIENCY vs. OUTPUT
CURRENT MOSFET
70
80
MAX749TOC2-C
OUTPUT CURRENT (mA)
EFFICIENCY (%)
20
40
75
5
15
25
35
45
V+ = 5V
R
SENSE
= 0.25 TRANSISTOR: SMD10P05L
-12V
-5V
-24V
0
LOAD CURRENT vs. INPUT VOLTAGE
MAX749-TOC3-A
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
50
100
150
200
250
300
350
3
4
5
6
2
R
BASE
= 470 R
SENSE
= 0.25 TRANSISTOR = ZTX750
-5V
-12V
-24V
-48V
400
__________________________________________Typical Operating Characteristics
(T
A
= +25°C, L = 47µH, unless otherwise noted.) MAX749
Digitally Adjustable LCD Bias Supply
4
______________________________________________________________________________________
PIN
NAME
FUNCTION
1
V+
+2V to +6V Input Voltage to power the MAX749 and external circuitry. When using an external
P-channel MOSFET, V+ must exceed the MOSFETs gate threshold voltage.
2
ADJ
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.
3
CTRL
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.
4
FB
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.
5
GND
Ground
6
DLOW
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.
7
DHI
Output Driver High. Connect to the gate of the external P-channel transistor, or to the base of the
external PNP transistor.
8
CS
Current-Sense Input. The external transistor is turned off when current through the sense resistor,
R
SENSE
, brings CS below V+ by 140mV (typ).
OUTPUT
VOLTAGE
LOAD
CURRENT
100mV
AC
/div
10mA/div
0mA
LOAD-TRANSIENT RESPONSE
V
OUT
= -15V
TRANSISTOR = ZTX750
50
µ
s/div
V
OUT
= -15V
TRANSISTOR = ZTX750
OUTPUT
VOLTAGE
INPUT
VOLTAGE
100mV
AC
/div
1 V/div
0V
LINE-TRANSIENT RESPONSE
V
OUT
= -15V
I
LOAD
= 5mA
TRANSISTOR = ZTX750
50ms/div
____________________________Typical Operating Characteristics (continued)
(T
A
= +25°C, L = 47µH, unless otherwise noted.)
______________________________________________________________Pin Description _______________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