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.
________________________Applications
Notebook Computers
Laptop Computers
Palmtop Computers
Personal Digital Assistants
Communicating Computers
Portable Data-Collection Terminals
____________________________Features
♦+2.0V to +6.0V Input Voltage Range
♦Flexible Control of Output Voltage:
Digital Control
Potentiometer Adjustment
PWM Control
♦Output Voltage Range Set by One Resistor
♦Low, 60µA Max Quiescent Current
♦15µA Max Shutdown Mode
♦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
VIN +5V
-VOUT
RSENSE
CCOMP
ON/OFF
DIGITAL
ADJUST
RFB
1
0.1µF
2
3
45
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
TA= TMIN to TMAX
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 (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= TMIN to 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: VIH is guaranteed by design to be 1.8V min for V+ = 2V to 6V for TA= TMIN to TMAX. VIL is guaranteed by design from
TA= TMIN to TMAX.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
V+ Voltage 26V
FB Source Current IFBS On power-up or reset, V FB = 0V (Note 1) 12.80 13.33 13.86 µA
Zero-Count FB Current VFB = 0V 0.45 0.55 IFBS
Full-Count FB Current VFB = 0V 1.43 1.53 IFBS
DAC Step Size (Note 2) Monotonicity guaranteed, VFB = 0V 1.00 1.56 2.12 %IFBS
DAC Linearity (Note 2) VFB = 0V ±1 %IFBS
Supply Rejection V+ = 2V to 6V, full-count current 1.5 %IFBS
Switching Frequency 100 to 500 kHz
Logic Input Current 0V < VIN < V+, CTRL, ADJ ±100 nA
Logic High Threshold (Note 3) VIH 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, VDHI = 1V 24 50 mA
DHI Drive Level No load V+ - 50mV V+ V
DLOW On Resistance V+ = 2V, VDLOW = 0.5V 510 Ω
VLogic Low Threshold (Note 3) VIL CTRL, ADJ 0.4
TIMING CHARACTERISTICS
TA= +25°C
MIN TYP MAX
PARAMETER SYMBOL CONDITIONS UNITS
Minimum Reset Pulse Width tRV+ = 5V 100
Minimum Reset Setup tRS 0 0
Minimum Reset Hold tRH
Not tested 0 0
tSH V+ = 2V 15 85 100
Minimum ADJ High Pulse Width V+ = 5V 10 85 100
tSL V+ = 2V 170 400 500
Minimum ADJ Low Pulse Width V+ = 5V 60 150 200
Minimum ADJ Low to CTRL Low tSD 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
RBASE = 470Ω
RSENSE = 0.25Ω
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
RSENSE = 0.25Ω
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
RBASE = 470Ω
RSENSE = 0.25Ω
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
RBASE = 160Ω
RSENSE = 0.25Ω
TRANSISTOR = ZTX750
-12V
-5V
-24V
02
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
35
6
-5V
-12V
-24V
-48V
RBASE = 160Ω
RSENSE = 0.25Ω
TRANSISTOR = ZTX750
85
65 010 30 50
EFFICIENCY vs. OUTPUT
CURRENT – MOSFET
70
80
MAX749TOC2-C
OUTPUT CURRENT (mA)
EFFICIENCY (%)
20 40
75
5 15253545
V+ = 5V
RSENSE = 0.25Ω
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
3456
2
R
BASE = 470Ω
RSENSE = 0.25Ω
TRANSISTOR = ZTX750
-5V
-12V
-24V
-48V
400
__________________________________________Typical Operating Characteristics
(TA = +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 MOSFET’s 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. -VOUT(MAX) = (RFB) x (20µA) where RFB is
connected from FB to -VOUT. The device is in regulation when VFB = 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 RBASE 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,
RSENSE, brings CS below V+ by 140mV (typ).
OUTPUT
VOLTAGE
LOAD
CURRENT
100mVAC/div
10mA/div
0mA
LOAD-TRANSIENT RESPONSE
VOUT = -15V
TRANSISTOR = ZTX750
50µs/div
VOUT = -15V
TRANSISTOR = ZTX750
OUTPUT
VOLTAGE
INPUT
VOLTAGE
100mVAC/div
1 V/div
0V
LINE-TRANSIENT RESPONSE
VOUT = -15V
ILOAD = 5mA
TRANSISTOR = ZTX750
50ms/div
____________________________Typical Operating Characteristics (continued)
(TA = +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 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
L1
47µH
DLOW
SWITCH-
MODE
POWER
SUPPLY
6-BIT
CURRENT-OUTPUT
DAC
REF
LOGIC
BIAS
POWER-ON
RESET 6-BIT
COUNTER
MAX749
VOUT
(NEGATIVE)
CTRL
ADJ
RESET
INCREMENT
ON/OFF
+2V TO +6V
INPUT 22µF
6.2V
6.66µA TO 20µA
CS
DHI
FB
GND
RSENSE
Q1
ZTX750
D1
1N5819
RBASE
470Ω
22µF
30V
CCOMP
RFB
V+
0.1µF
Figure 1. Block Diagram, Showing External Circuitry Using a PNP Transistor
MAX749
Once 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 Control
The 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
6 ______________________________________________________________________________________
L1
47µH
DLOW
6-BIT
CURRENT-OUTPUT
DAC
REF
MAXIMUM
ON-TIME
ONE-SHOT
MAX749
VOUT
(NEGATIVE)
22µF
0.1µF
V+
DHI
FB
GND
RSENSE
Q1
ZTX750
D1
1N5819
RBASE
470Ω
22µF
30V
CCOMP
RFB
TRIG
MINIMUM
OFF-TIME
ONE-SHOT
TRIG Q
Q
Q
S
R
FLIP-FLOP
VOLTAGE
COMPARATOR
140mV
+2V TO +6V 
INPUT
CURRENT
COMPARATOR
Figure 2. Switch-Mode Power-Supply Section Block Diagram
ADJ CTRL RESULT
Low Low Shut down
High Low Reset counter to mid-range. The
device is not shut down.
X High On
High Increment the counter
Figure 4. Count-Up Operation
Figure 5. Reset Sequence without Shutdown. The device is not
shut down during reset.
Figure 6. Reset Sequence with Shutdown
In Figure 3, the MAX749 is reset when it is taken out of
shutdown, which sets the output at mid-scale. Figure 4
shows how to increment the counter. Figure 5 illustrates
a reset without shutting the device down.
Figure 7 provides an example of a sequence of opera-
tions: Starting from shutdown, the device is turned on,
incremented, reset to mid-scale without being shut
down, incremented again, and finally shut down.
Shutdown Mode
When CTRL and ADJ are both low, the MAX749 is shut
down (Table 1): The internal reference and biasing cir-
cuitry turn off, the output voltage drops to zero, and the
supply current drops to 15µA. The MAX749 retains its
DAC setting, simplifying software control.
Reset Mode
If ADJ is high when CTRL is low, the DAC set point is
reset to mid-scale and the MAX749 is not shut down.
Mid-scale is 32 steps from the minimum, 31 steps from
the maximum.
Design Procedure
_________and Component Selection
Setting the Output Voltage
The MAX749’s output voltage is set using an external
resistor and the internal current-output DAC. The full-
scale output voltage is set by selecting the feedback
resistor RFB according to the formula:
-VOUT(MAX) = RFB x 20µA (Figure 1).
The device is in regulation when VFB = 0V.
DAC Adjustment
On power-up or after a reset, the counter sets the DAC
output to mid-range, and -VOUT = RFB x 13.33µA. Each
rising edge of ADJ increments the counter (and there-
fore the DAC output) in the direction of -VOUT(MAX) by
one count. When incremented beyond -VOUT(MAX), the
MAX749
Digitally Adjustable LCD Bias Supply
_______________________________________________________________________________________ 7
Figure 3. Shutdown-Reset-On-Shutdown Sequence of Operation.
The device is not shut down during reset.
Figure 7. Control Sequence Example (see Output Voltage
Control section)
ADJ
CTRL
SHUTDOWN RESET ON SHUTDOWN
tRtSD
ADJ
CTRL HIGH
tSH tSL
ADJ
CTRL
ON RESET ON
tRS tRH
tR
ADJ
CTRL
SHUTDOWN
RESET
ON SHUTDOWN
INCREMENT INCREMENT
ADJ
CTRL
SHUTDOWN RESET ON
tRH
tR
MAX749
counter rolls over and sets the DAC to -VOUT(MIN),
where -VOUT(MIN) = RFB x 6.66µA. In other words, a sin-
gle rising edge of ADJ increments the DAC output by
one, and 63 rising edges of ADJ decrement the DAC
output by one.
Potentiometer Adjustment
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:
R1 = -VOUT(MIN)/13.33µA
R2 = -VOUT(MAX)/13.33µA - R1.
Where the potentiometer can be varied from 0 (producing
VOUT(MIN)) to R2Ω(producing VOUT(MAX)). Notice that ADJ
is connected to ground, allowing the device to be shut
down.
PWM Adjustment
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.
Current-Sense Resistor
The current-sense resistor limits the peak switch cur-
rent to 140mV/RSENSE, where RSENSE is the value of the
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:
- the minimum input voltage, VIN(MIN),
- the maximum output voltage, VOUT(MAX), and
- the maximum output current, IOUT(MAX).
For example, assume that the output voltage must be
adjustable to -24V (VOUT(MAX) = -24V) at up to 30mA
(IOUT(MAX) = 30mA). The supply voltage ranges from
4.75V to 6V (V IN(MIN) = 4.75V).
2. In Figures 9a-9e, locate the graph drawn for the
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 RSENSE (the lowest current limit) with an out-
put current that is adequate at the lowest input
voltage.
In this example, select the -24V output graph, Figure 9d.
We then want a curve where IOUT is ≥30mA with a 4.75V
input. The 0.3ΩRSENSE graph shows 25mA of output cur-
rent with a 4.75V input, so we look next at the 0.25Ω
RSENSE graph. It shows IOUT = 30mA for VIN = 4.75V and
VOUT = -24V. Therefore select RSENSE = 0.25Ω. This pro-
vides a current limit in the range 440mA to 720mA.
Alternatively, a 0.2Ωsense 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 0.2Ωresistor may be easier to
obtain than an 0.25Ωresistor.
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/RSENSE. Use the maximum current-limit fig-
ure when choosing the transistor, coil, and diode.
IRC (see Table 2) makes surface-mount resistors with pre-
ferred values including: 0.1 Ω, 0.2Ω, 0.3 Ω, 0.5Ω, and 1.0 Ω.
Digitally Adjustable LCD Bias Supply
8 ______________________________________________________________________________________
L1
47µH
DLOW
VOUT
(NEGATIVE)
+4.5V to +6V
INPUT 22µF
0.1µF
CTRL
CS
DHI
RSENSE
Q1
SMD10P05L
D1
1N5819
22µF
30V
CCOMP
R1 R2
ADJ
MAX749
V+
GND
VOUT(MIN) = -R1(13.33µA)
VOUT(MAX) = -(R1+R2)(13.33µA)
Figure 8. Using a Potentiometer to Adjust the Output Voltage
Figure 9b. Maximum Output Current vs. Input Voltage,
V
OUT
= -12V
Figure 9c. Maximum Output Current vs. Input Voltage,
V
OUT
= -15V
Figure 9d. Maximum Output Current vs. Input Voltage,
V
OUT
= -24V
Figure 9e. Maximum Output Current vs. Input Voltage,
V
OUT
= -48V
Choosing an Inductor
Practical inductor values range from 22µH to 100µH,
and 47µH is normally a good choice. Inductors with a
ferrite core or equivalent are recommended. The induc-
tor’s saturation current rating – the current at which the
core begins to saturate and the inductance falls to 80%
or 90% of its nominal value – should ideally equal the
current limit (see
Current-Sense Resistor
section).
However, because the current is limited by the
MAX749, the inductor can safely be driven into satura-
tion with only a slight impact on efficiency.
For highest efficiency, use a coil with low resistance,
preferably under 300mΩ. To minimize radiated noise,
use a toroid, pot-core, or shielded inductor.
MAX749
Digitally Adjustable LCD Bias Supply
_______________________________________________________________________________________ 9
Figure 9a. Maximum Output Current vs. Input Voltage,
V
OUT
= -5V
0
MAX749-Fig 13
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
RSENSE (Ω)
5
10
15
20
25
3245
6
1.0
0.5
0.3
0.25
0.2
VOUT = -48V
L = 47µH
80
0
26
20
60
MAX-749-Fig10
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
4
40
100
120
140
35
V
OUT
= -12V
L = 47µH
RSENSE (Ω)
0.2
0.25
0.3
0.5
1.0
60
0
MAX749-Fig 11
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
6
20
24
40
80
100
35
R
SENSE (Ω)
V
OUT
= -15V
L = 47µH
0.2
0.25
0.3
0.5
1.0
60
0
10
50
MAX749-Fig 12
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
6
30
20
24
40
35
R
SENSE (Ω)
1.0
0.5
0.3
0.25
0.2
VOUT = -24V
L = 47µH
0
MAX749-Fig 9
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
RSENSE (Ω)
50
100
150
200
250
3245
6
1.0
0.5
0.3
0.25
0.2
VOUT = -5V
L = 47µH
The Sumida CD54-470N (47µH, 720mA, 370mΩ) is suit-
able for a wide range of applications, and the larger
CD105-470N (47µH, 1.17A, 170mΩ) permits higher cur-
rent levels and efficiencies.
Diode Selection
The MAX749’s high switching frequency demands a high-
speed rectifier. Schottky diodes such as the 1N5817-
1N5822 family are recommended. Choose a diode with an
average current rating approximately equal to the peak
current, as determined by 180mV/RSENSE and a break-
down voltage greater than V ++ I-VOUTMAXI.
External Switching Transistor
The MAX749 can drive a PNP transistor or a P-channel
logic-level MOSFET. The choice of a power switch is
dictated by the input voltage range, cost, and efficiency.
MOSFETs provide the highest efficiency because they
do not draw any DC gate-drive current (see
Typical
Operating Characteristics
graphs). However, a gate-
source voltage of several volts is needed to turn on a
MOSFET, so a 5V or greater input supply is required
(although this restriction may change as lower-thresh-
old P-channel MOSFETs become available). PNP tran-
sistors, meanwhile, may be used over the entire 2V to
6V operating voltage range of the MAX749.
When using a MOSFET, connect DHI and DLOW to its
gate (see
Typical Operating Circuit
). When using a PNP
transistor, connect DHI to its base, and connect a resis-
tor between the base and DLOW (RBASE) (Figure 1). The
PNP transistor is turned off quickly by the direct pull-up
of DHI, and turned on by the base current provided
through RBASE. This resistor limits the transistor’s base-
drive current to (VIN - 140mV - VBE)/RBASE, where VIN is
the input voltage, 140mV is the drop across RSENSE, VBE
is the transistor’s base-emitter voltage, and RBASE is the
current-limiting resistor. For maximum efficiency, make
RBASE as large as possible, but small enough so that the
transistor is always driven into saturation.
Highest efficiency with a PNP transistor comes from
using a device with a low collector-emitter saturation
voltage and a high current gain. Use a fast-switching
type. For example the Zetex ZTX792A has switching
speeds of 40ns (tON) and 500ns (tOFF).
The transistor must have a collector-to-emitter (PNP) or
drain-to-source (MOSFET) voltage rating greater than the
input-to-output voltage differential (VIN - VOUT). In either
case the transistor must have a current rating that exceeds
the peak current set by the current-sense resistor.
PNP transistors are generally less expensive than P-
channel MOSFETs. Table 2 lists some suppliers of
switching transistors suitable for use with the MAX749.
Table 2. Component Suppliers
Base Resistor
The base resistor, R BASE in Figure 1, controls the amount of
base current in the PNP transistor. A low value for RBASE
increases base drive, which provides higher output cur-
rents and compensates for lower input voltages, but
decreases efficiency. Conversely, a high RBASE value
increases efficiency but reduces the output capability,
especially at low voltages. When using high-gain transis-
tors, e.g. the Zetex ZTX750 or ZTX792, typical values for
RBASE are in the 150Ωto 510Ωrange, but will depend on
the required input voltage range and output current (see
Typical Operating Characteristics
). Lower-gain transistors
require lower values for RBASE and are less efficient. Larger
RBASE values are suitable if less output power is required.
MAX749
Digitally Adjustable LCD Bias Supply
10 _____________________________________________________________________________________
SUPPLIER
INDUCTORS
Coiltronics (305) 781-8900 (305) 782-4163
Gowanda (716) 532-2234 (716) 532-2702
Sumida USA (708) 956-0666 (708) 956-0702
Sumida Japan 81-3-3607-511 81-3-3607-5428
CAPACITORS
Kemet (803) 963-6300 (803) 963-6322
Matsuo (714) 969-2491 (714) 960-6492
Nichicon (708) 843-7500 (708) 843-2798
Sprague (603) 224-1961 (603) 224-1430
Sanyo USA (619) 661-6322
Sanyo Japan 81-3-3837-6242
United Chemi-Con (714) 255-9500 (714) 255-9400
DIODES
Motorola (800) 521-6274
Nihon USA (805) 867-2555 (805) 867-2698
Nihon Japan 81-3-3494-7411 81-3-3494-7414
POWER TRANSISTORS - MOSFETS
Harris (407) 724-3739 (407) 724-3937
International
Rectifier (213) 772-2000 (213) 772-9028
Siliconix (408) 988-8000 (408) 727-5414
POWER TRANSISTORS - PNP TRANSISTORS
Zetex USA (516) 543-7100 (516) 864-7630
Zetex UK 44 (61) 727 5105 44 (61) 627 5467
CURRENT-SENSE RESISTORS
IRC (512) 992-7900 (512) 992-3377
FAXPHONE
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.
MAX749
Digitally Adjustable LCD Bias Supply
______________________________________________________________________________________ 11
___________________Chip Topography
TRANSISTOR COUNT: 521;
SUBSTRATE CONNECTED TO GND.
ADJ
CTRL
0.070"
(0.1178mm)
0.808"
(0.2032mm)
FB GND
V+ V+ CS
DLOW
DHI
MAX749
Digitally Adjustable LCD Bias Supply
DIM
A
A1
A2
A3
B
B1
C
D1
E
E1
e
eA
eB
L
MIN
–
0.015
0.125
0.055
0.016
0.045
0.008
0.005
0.300
0.240
0.100
0.300
–
0.115
MAX
0.200
–
0.175
0.080
0.022
0.065
0.012
0.080
0.325
0.310
–
–
0.400
0.150
MIN
–
0.38
3.18
1.40
0.41
1.14
0.20
0.13
7.62
6.10
2.54
7.62
–
2.92
MAX
5.08
–
4.45
2.03
0.56
1.65
0.30
2.03
8.26
7.87
–
–
10.16
3.81
INCHES MILLIMETERS
Plastic DIP
PLASTIC
DUAL-IN-LINE
PACKAGE
(0.300 in.)
DIM
D
D
D
D
D
D
PKG.
P
P
P
P
P
N
MIN
0.348
0.735
0.745
0.885
1.015
1.14
MAX
0.390
0.765
0.765
0.915
1.045
1.265
MIN
8.84
18.67
18.92
22.48
25.78
28.96
MAX
9.91
19.43
19.43
23.24
26.54
32.13
INCHES MILLIMETERS
PINS
8
14
16
18
20
24
C
AA2
E1
D
E
eA
eB
A3
B1
B
0° - 15°
A1
L
D1
e
21-0043A
DIM
A
A1
B
C
E
e
H
L
MIN
0.053
0.004
0.014
0.007
0.150
0.228
0.016
MAX
0.069
0.010
0.019
0.010
0.157
0.244
0.050
MIN
1.35
0.10
0.35
0.19
3.80
5.80
0.40
MAX
1.75
0.25
0.49
0.25
4.00
6.20
1.27
INCHES MILLIMETERS
21-0041A
Narrow SO
SMALL-OUTLINE
PACKAGE
(0.150 in.)
DIM
D
D
D
MIN
0.189
0.337
0.386
MAX
0.197
0.344
0.394
MIN
4.80
8.55
9.80
MAX
5.00
8.75
10.00
INCHES MILLIMETERS
PINS
8
14
16
1.270.050
L
0°-8°
HE
D
e
A
A1 C
0.101mm
0.004in.
B
_______________________________________________________Package Information
12 ______________________________________________________________________________________
Mouser Electronics
Authorized Distributor
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