MAX749 [MAXIM]
Digitally Adjustable LCD Bias Supply; 数字调节LCD偏置电源
| 型号： | MAX749 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Digitally Adjustable LCD Bias Supply |
| 文件： | 总12页 (文件大小：107K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0143; Rev 1; 2/95
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
MAX749
_______________Ge n e ra l De s c rip t io n
____________________________Fe a t u re s
The MAX749 generates negative LCD-bias contrast
voltages from 2V to 6V inputs. Full-scale output voltage
c a n b e s c a le d to -100V or g re a te r, a nd is d ig ita lly
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.
♦ +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
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.
♦ Small Size – 8-Pin SO and Plastic DIP Packages
________________________Ap p lic a t io n s
Notebook Computers
______________Ord e rin g In fo rm a t io n
PART
TEMP. RANGE
0°C to +70°C
PIN-PACKAGE
8 Plastic DIP
8 SO
Laptop Computers
MAX749CPA
MAX749CSA
MAX749C/D
MAX749EPA
MAX749ESA
Palmtop Computers
0°C to +70°C
Personal Digital Assistants
Communicating Computers
Portable Data-Collection Terminals
0°C to +70°C
Dice*
-40°C to +85°C
-40°C to +85°C
8 Plastic DIP
8 SO
* Contact factory for dice specifications.
__________Typ ic a l Op e ra t in g Circ u it
__________________P in Co n fig u ra t io n
V
IN
+5V
TOP VIEW
R
SENSE
0.1µF
1
2
8
7
V+
CS
1
2
3
4
8
7
6
5
V+
ADJ
CS
ADJ
DHI
MAX749
DIGITAL
ADJUST
DHI
-V
OUT
MAX749
6
5
3
4
CTRL
FB
DLOW
GND
DLOW
CTRL
FB
ON/OFF
GND
R
FB
DIP/SO
C
COMP
_______________________________________________________________ Maxim Integrated Products
1
Ca ll t o ll fre e 1 -8 0 0 -9 9 8 -8 8 0 0 fo r fre e s a m p le s o r lit e ra t u re .
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
ABSOLUTE MAXIMUM RATINGS
V+ ................................................................................-0.3V, +7V
CTRL, ADJ, FB, DLOW, DHI, CS.....................-0.3V, (V+ + 0.3V)
Continuous Power Dissipation (T = +70°C)
A
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.
MAX749
ELECTRICAL CHARACTERISTICS
(2V < V+ < 6V, T = T
A
to T , unless otherwise noted.)
MAX
MIN
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
V+ Voltage
2
6
V
FB Source Current
I
On power-up or reset, V = 0V (Note 1)
12.80
0.45
1.43
13.33
13.86
0.55
1.53
±15
2.12
±1
µA
FBS
FB
Zero-Count FB Current
Full-Count FB Current
FB Offset Voltage
V
= 0V
= 0V
I
FBS
FB
V
FB
I
FBS
mV
DAC Step Size (Note 2)
DAC Linearity (Note 2)
Supply Rejection
Monotonicity guaranteed, V = 0V
1.00
1.56
%I
FB
FBS
FBS
FBS
V
FB
= 0V
%I
%I
V+ = 2V to 6V, full-count current
1.5
Switching Frequency
Logic Input Current
100 to 500
kHz
0V < V < V+, CTRL, ADJ
IN
±100
nA
V
Logic High Threshold (Note 3)
Logic Low Threshold (Note 3)
Quiescent Current
V
CTRL, ADJ
CTRL, ADJ
1.6
IH
V
IL
0.4
60
V
µA
µA
mV
mA
V
Shutdown Current
15
V+ to CS Voltage
Current-limit trip voltage
110
24
140
50
V+
5
180
DHI Source Current
DHI Drive Level
V+ = 2V, V
= 1V
DHI
No load
V+ - 50mV
DLOW On Resistance
V+ = 2V, V
= 0.5V
10
Ω
DLOW
Note 1: The device is in regulation when V = 0V (see Figures 3 - 6).
FB
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 is guaranteed by design to be 1.8V min for V+ = 2V to 6V for T = T to T . V is guaranteed by design from
IH
A
MIN
MAX IL
T
A
= T
to T
.
MIN
MAX
TIMING CHARACTERISTICS
T
= +25°C
T
= T
MIN
to T
MIN
MAX
A
A
PARAMETER
SYMBOL
CONDITIONS
V+ = 2V
UNITS
MIN
TYP
MAX
MAX
125
25
300
85
400
100
Minimum Reset Pulse Width
t
ns
R
V+ = 5V
Not tested
Not tested
V+ = 2V
V+ = 5V
V+ = 2V
V+ = 5V
V+ = 2V
V+ = 5V
Minimum Reset Setup
Minimum Reset Hold
t
RS
0
0
0
0
ns
ns
t
RH
15
10
85
85
100
100
500
200
250
100
Minimum ADJ High Pulse Width
Minimum ADJ Low Pulse Width
Minimum ADJ Low to CTRL Low
t
ns
ns
ns
SH
170
60
400
150
200
85
t
SL
70
t
SD
20
2
______________________________________________________________________________________
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
MAX749
__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(T = +25°C, L = 47µH, unless otherwise noted.)
A
EFFICIENCY vs. OUTPUT
EFFICIENCY vs. OUTPUT
CURRENT – PNP
EFFICIENCY vs. OUTPUT
CURRENT – MOSFET
CURRENT – PNP
85
80
78
85
80
75
70
-24V
-12V
-12V
80
76
74
-12V
-24V
-5V
-5V
-5V
-24V
75
72
70
68
V+ = 5V
= 0.25Ω
V+ = 3V
V+ = 3V
= 160Ω
R
SENSE
R
BASE
= 470Ω
= 0.25Ω
SENSE
TRANSISTOR: ZTX750
R
R
BASE
TRANSISTOR: SMD10P05L
R
70
65
= 0.25Ω
SENSE
TRANSISTOR = ZTX750
66
64
65
0
10
20
30
40
50
60
0
10 20 30 40 50 60 70 80 90 100
OUTPUT CURRENT (mA)
0
5
10 15 20 25 30 35 40 45 50
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
EFFICIENCY vs.
OUTPUT VOLTAGE
EFFICIENCY vs.
OUTPUT VOLTAGE
85
80
75
70
85
80
75
-20mA
-5mA
-20mA
-5mA
-40mA
-40mA
V+ = 3V
= 470Ω
= 0.25Ω
SENSE
TRANSISTOR : ZTX750
V+ = 5V
70
65
R
R
BASE
R
SENSE
= 0.25Ω
TRANSISTOR : SMD10P05L
65
-24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4
OUTPUT VOLTAGE (V)
-24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4
OUTPUT VOLTAGE (V)
LOAD CURRENT vs. INPUT VOLTAGE
LOAD CURRENT vs. INPUT VOLTAGE
400
500
450
400
350
300
250
200
350
-5V
R
= 160Ω
= 0.25Ω
R
= 470Ω
= 0.25Ω
BASE
BASE
R
SENSE
-5V
R
SENSE
300
250
200
150
100
50
TRANSISTOR = ZTX750
TRANSISTOR = ZTX750
-12V
-12V
-24V
150
100
-24V
-48V
-48V
50
0
0
2
3
4
5
6
2
3
4
5
6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
_______________________________________________________________________________________
3
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(T = +25°C, L = 47µH, unless otherwise noted.)
A
LINE-TRANSIENT RESPONSE
LOAD-TRANSIENT RESPONSE
MAX749
OUTPUT
VOLTAGE
100mV /div
AC
OUTPUT
VOLTAGE
100mV /div
AC
1 V/div
INPUT
VOLTAGE
LOAD
CURRENT
10mA/div
0mA
0V
50ms/div
50µs/div
V
= -15V
= 5mA
V
= -15V
OUT
OUT
I
TRANSISTOR = ZTX750
LOAD
TRANSISTOR = ZTX750
______________________________________________________________P in De s c rip t io n
PIN
NAME
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.
1
V+
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.
2
3
ADJ
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.
CTRL
Feedback Input for output full-scale voltage selection. -V
= (R ) x (20µA) where R is
FB
FB
OUT(MAX)
4
5
FB
connected from FB to -V . The device is in regulation when V = 0V.
OUT FB
GND
Ground
Output Driver Low. Connect to DHI when using an external P-channel MOSFET. When using an
6
DLOW
external PNP transistor, connect a resistor R from DLOW to the base of the PNP to set the maxi-
BASE
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.
7
8
DHI
CS
Current-Sense Input. The external transistor is turned off when current through the sense resistor,
R
, brings CS below V+ by 140mV (typ).
SENSE
4
______________________________________________________________________________________
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
MAX749
+2V TO +6V
INPUT
0.1µF
22µF
6.2V
V+
RESET
6.66µA TO 20µA
6-BIT
CURRENT-OUTPUT
DAC
R
SENSE
6-BIT
COUNTER
POWER-ON
RESET
REF
CTRL
ADJ
LOGIC
INCREMENT
CS
DHI
ON/OFF
Q1
ZTX750
SWITCH-
MODE
POWER
SUPPLY
DLOW
D1
1N5819
R
BASE
BIAS
470Ω
L1
47µH
R
FB
V
OUT
FB
(NEGATIVE)
MAX749
GND
22µF
30V
C
COMP
Figure 1. Block Diagram, Showing External Circuitry Using a PNP 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.
_______________De t a ile d De s c rip t io n
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’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) c onve rte rs . This c ontrol s c he me a llows 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.
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.
Switching control is accomplished through the combi-
nation of a current limit in the switch plus on- and off-
time limits (Figure 2).
Op e ra t in g P rin c ip le
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
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
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+2V TO +6V
INPUT
0.1µF
22µF
V+
MAX749
140mV
R
SENSE
Q
TRIG
CURRENT
COMPARATOR
MINIMUM
OFF-TIME
ONE-SHOT
FLIP-FLOP
R
DHI
Q1
ZTX750
Q
S
DLOW
MAXIMUM
ON-TIME
ONE-SHOT
R
470Ω
BASE
D1
1N5819
L1
47µH
VOLTAGE
COMPARATOR
TRIG
Q
V
OUT
R
FB
FB
(NEGATIVE)
6-BIT
CURRENT-OUTPUT
DAC
22µF
30V
REF
MAX749
GND
C
COMP
Figure 2. Switch-Mode Power-Supply Section Block Diagram
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).
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.
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.
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.
Ou t p u t Vo lt a g e Co n t ro l
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
Table 1. Input Truth Table
ADJ
CTRL
RESULT
resistor, R . The output voltage is controlled from 33%
Low
Low
Shut down
FB
to 100% of the full-scale output by an internal 64-step
DAC/counter.
Reset counter to mid-range. The
device is not shut down.
High
X
Low
On power-up or after a reset, the counter sets the DAC
output to mid-range. Each rising edge of ADJ incre-
High
High
On
Increment the counter
6
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Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
MAX749
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.
ADJ
CTRL
Figure 7 provides an example of a sequence of opera-
tions: Starting from shutdown, the device is turned on,
t
t
SD
R
incremented, reset to mid-scale without being shut
down, incremented again, and finally shut down.
ON
SHUTDOWN RESET
SHUTDOWN
S h u t d o w n Mo d e
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.
Figure 3. Shutdown-Reset-On-Shutdown Sequence of Operation.
The device is not shut down during reset.
Re s e t Mo d e
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.
ADJ
CTRL
HIGH
t
SH
t
SL
De s ig n P ro c e d u re
_________a n d Co m p o n e n t S e le c t io n
Figure 4. Count-Up Operation
S e t t in g t h e Ou t p u t Vo lt a g e
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
ADJ
resistor R according to the formula:
FB
CTRL
-V
= R x 20µA (Figure 1).
FB
OUT(MAX)
The device is in regulation when V = 0V.
FB
t
RS
t
RH
t
R
DAC Adjustment
On power-up or after a reset, the counter sets the DAC
output to mid-range, and -V = R x 13.33µA. Each
ON
RESET
ON
OUT
FB
rising edge of ADJ increments the counter (and there-
fore the DAC output) in the direction of -V by
Figure 5. Reset Sequence without Shutdown. The device is not
shut down during reset.
OUT(MAX)
one count. When incremented beyond -V
, the
OUT(MAX)
INCREMENT
RESET
INCREMENT
ADJ
ADJ
CTRL
CTRL
t
RH
t
R
SHUTDOWN
ON
SHUTDOWN
SHUTDOWN
RESET
ON
Figure 6. Reset Sequence with Shutdown
Figure 7. Control Sequence Example (see Output Voltage
Control section)
_______________________________________________________________________________________
7
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Cu rre n t -S e n s e Re s is t o r
The current-sense resistor limits the peak switch cur-
rent to 140mV/R , where R 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).
+4.5V to +6V
INPUT
0.1µF
22µF
SENSE
SENSE
R
SENSE
V+
CS
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.
MAX749
Q1
DHI
MAX749
SMD10P05L
CTRL
DLOW
No calculations are required to choose the proper cur-
rent-sense resistor; simply follow this two-step procedure:
L1
47µH
D1
1N5819
ADJ
1. Determine:
V
OUT
- the minimum input voltage, V
IN(MIN),
(NEGATIVE)
R2
R1
- the maximum output voltage, V
, and
OUT(MAX)
GND
- the maximum output current, I
.
OUT(MAX)
22µF
30V
For example, assume that the output voltage must be
adjustable to -24V (V = -24V) at up to 30mA
OUT(MAX)
C
COMP
(I
= 30mA). The supply voltage ranges from
V
= -R1(13.33µA)
= -(R1+R2)(13.33µA)
OUT(MIN)
OUT(MAX)
V
OUT(MAX)
4.75V to 6V (V
= 4.75V).
IN(MIN)
2. In Figures 9a-9e, locate the graph drawn for the
a p p rop ria te outp ut volta g e (whic h is e ithe r 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
Figure 8. Using a Potentiometer to Adjust the Output Voltage
c ounte r rolls ove r a nd s e ts the DAC to -V
,
OUT(MIN)
where -V
= R x 6.66µA. In other words, a sin-
highest R
(the lowest current limit) with an out-
OUT(MIN)
FB
SENSE
gle rising edge of ADJ increments the DAC output by
one, and 63 rising edges of ADJ decrement the DAC
output by one.
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 I
is ≥30mA with a 4.75V
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:
OUT
input. The 0.3Ω R
graph shows 25mA of output cur-
SENSE
rent with a 4.75V input, so we look next at the 0.25Ω
graph. It shows I = 30mA for V = 4.75V and
R
SENSE
OUT
IN
V
= -24V. Therefore select R
= 0.25Ω. This pro-
OUT
SENSE
vides a current limit in the range 440mA to 720mA.
Alternatively, a 0.2Ω sense 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 0.2Ω resistor may be easier to
obtain than an 0.25Ω resistor.
R1 = -V
R2 = -V
/13.33µA
OUT(MIN)
OUT(MAX)
/13.33µA - R1.
Where the potentiometer can be varied from 0 (producing
) to R2Ω (producing V ). Notice that ADJ
is connected to ground, allowing the device to be shut
down.
V
OUT(MIN)
OUT(MAX)
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
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.
by 180mV/R
. Use the maximum current-limit fig-
SENSE
ure when choosing the transistor, coil, and diode.
IRC (see Table 2) makes surface-mount resistors with pre-
ferred values including: 0.1Ω, 0.2Ω, 0.3Ω, 0.5Ω, and 1.0Ω.
8
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Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
MAX749
Ch o o s in g a n In d u c t o r
Practical inductor values range from 22µH to 100µH,
0.2
100
80
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
c urre nt limit (s e e Curre nt-Se ns e Re s is tor s e c tion).
Howe ve r, b e c a us e the c urre nt is limite d b y the
MAX749, the inductor can safely be driven into satura-
tion with only a slight impact on efficiency.
V
= -15V
OUT
L = 47µH
0.25
0.3
60
40
0.5
1.0
20
0
For highest efficiency, use a coil with low resistance,
preferably under 300mΩ. To minimize radiated noise,
use a toroid, pot-core, or shielded inductor.
2
3
4
5
6
INPUT VOLTAGE (V)
Figure 9c. Maximum Output Current vs. Input Voltage,
V
OUT
= -15V
0.2
250
200
150
100
50
60
50
0.2
V
OUT
= -24V
V
= -5V
OUT
L = 47µH
L = 47µH
0.25
0.3
40
30
20
10
0
0.25
0.3
0.5
1.0
0.5
1.0
0
2
3
4
5
6
2
3
4
5
6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 9d. Maximum Output Current vs. Input Voltage,
Figure 9a. Maximum Output Current vs. Input Voltage,
V
OUT
= -24V
V
OUT
= -5V
0.2
140
25
0.2
V
= -12V
OUT
120
100
V
= -48V
OUT
L = 47µH
20
15
10
5
0.25
0.3
L = 47µH
0.25
0.3
80
60
40
20
0
0.5
1.0
0.5
1.0
0
2
3
4
5
6
2
3
4
5
6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 9b. Maximum Output Current vs. Input Voltage,
= -12V
Figure 9e. Maximum Output Current vs. Input Voltage,
= -48V
V
OUT
V
OUT
_______________________________________________________________________________________
9
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
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.
Table 2. Component Suppliers
SUPPLIER
INDUCTORS
Coiltronics
Gowanda
PHONE
FAX
(305) 781-8900
(716) 532-2234
(708) 956-0666
81-3-3607-511
(305) 782-4163
(716) 532-2702
(708) 956-0702
81-3-3607-5428
Dio d e S e le c t io n
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
Sumida USA
Sumida Japan
CAPACITORS
Kemet
MAX749
current, as determined by 180mV/R
and a break-
SENSE
+
down voltage greater than V + -V
.
I
OUTMAXI
(803) 963-6300
(714) 969-2491
(708) 843-7500
(603) 224-1961
(619) 661-6322
81-3-3837-6242
(803) 963-6322
(714) 960-6492
(708) 843-2798
(603) 224-1430
Matsuo
Ex t e rn a l S w it c h in g Tra n s is t o r
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.
Nichicon
Sprague
Sanyo USA
Sanyo Japan
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.
United Chemi-Con (714) 255-9500
(714) 255-9400
DIODES
Motorola
(800) 521-6274
(805) 867-2555
81-3-3494-7411
Nihon USA
Nihon Japan
(805) 867-2698
81-3-3494-7414
POWER TRANSISTORS - MOSFETS
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-
Harris
(407) 724-3739
(213) 772-2000
(408) 988-8000
(407) 724-3937
(213) 772-9028
(408) 727-5414
International
Rectifier
tor between the base and DLOW (R
) (Figure 1). The
BASE
PNP transistor is turned off quickly by the direct pull-up
of DHI, and turned on by the base current provided
Siliconix
POWER TRANSISTORS - PNP TRANSISTORS
through R
. This resistor limits the transistor’s base-
BASE
Zetex USA
Zetex UK
(516) 543-7100
(516) 864-7630
drive current to (V - 140mV - V )/R
, where V is
IN
BE
BASE
IN
the input voltage, 140mV is the drop across R
, V
44 (61) 727 5105
44 (61) 627 5467
SENSE BE
is the transistor’s base-emitter voltage, and R
is the
BASE
CURRENT-SENSE RESISTORS
current-limiting resistor. For maximum efficiency, make
as large as possible, but small enough so that the
IRC
(512) 992-7900
(512) 992-3377
R
BASE
transistor is always driven into saturation.
Ba s e Re s is t o r
in Figure 1, controls the amount of
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
The base resistor, R
BASE
base current in the PNP transistor. A low value for R
BASE
increases base drive, which provides higher output cur-
rents and compensates for lower input voltages, but
speeds of 40ns (t ) and 500ns (t
).
ON
OFF
decreases efficiency. Conversely, a high R
value
BASE
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
The transistor must have a collector-to-emitter (PNP) or
drain-to-source (MOSFET) voltage rating greater than the
input-to-output voltage differential (V - V ). In either
IN
OUT
R
are in the 150Ω to 510Ω range, but will depend on
case the transistor must have a current rating that exceeds
the peak current set by the current-sense resistor.
BASE
the required input voltage range and output current (see
Typical Operating Characteristics). Lower-gain transistors
PNP transistors are generally less expensive than P-
c ha nne l MOSFETs. Ta b le 2 lists some sup p lie rs of
switching transistors suitable for use with the MAX749.
require lower values for R
and are less efficient. Larger
BASE
R
values are suitable if less output power is required.
BASE
10 _____________________________________________________________________________________
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
MAX749
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
Ca p a c it o rs
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.
Surfa c e -mount c a p a c itors a re g e ne ra lly p re fe rre d
because they lack the inductance and resistance of the
leads of their through-hole equivalents.
may be necessary to connect a capacitor, C
, in
COMP
p a ra lle l with R . Althoug h C
is norma lly not
FB
COMP
required, the value of C
depends upon the value
COMP
of R and on the individual circuit layout—typical val-
FB
ues range from 0pF to 220pF.
P C La yo u t a n d Gro u n d in g
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 p in a nd thos e c onne c ting Q1, L1, a nd D1
Input Bypass Capacitor
A 22µF ta nta lum c a p a c itor in p a ra lle l with a 0.1µF
ceramic normally provides sufficient bypassing. Mount
the 0.1µF capacitor very close to the IC. La rg e r
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.
together. Mount the R resistor very close to the IC.
FB
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.
The e ffe c tive s e rie s re s is ta nc e (ESR) of b oth the
bypass and filter capacitors affects efficiency. Best per-
forma nc e is ob ta ine d b y d oub ling up on the filte r
capacitors or using low-ESR types.
___________________Ch ip To p o g ra p h y
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.
0. 070"
(0. 1178mm)
V+ CS
V+
ADJ
CTRL
DHI
0. 808"
(0. 2032mm)
DLOW
FB
GND
TRANSISTOR COUNT: 521;
SUBSTRATE CONNECTED TO GND.
______________________________________________________________________________________ 11
Dig it a lly Ad ju s t a b le LCD Bia s S u p p ly
_______________________________________________________P a c k a g e In fo rm a t io n
INCHES
MILLIMETERS
DIM
E
MIN
MAX
0.200
–
MIN
–
MAX
5.08
–
A
–
E1
D
A1 0.015
A2 0.125
A3 0.055
0.38
3.18
1.40
0.41
1.14
0.20
0.13
7.62
6.10
2.54
7.62
–
0.175
0.080
0.022
0.065
0.012
0.080
0.325
0.310
–
4.45
2.03
0.56
1.65
0.30
2.03
8.26
7.87
–
A3
A2
A1
MAX749
A
L
B
0.016
B1 0.045
0.008
D1 0.005
0.300
E1 0.240
0.100
eA 0.300
C
0° - 15°
E
C
e
e
B1
eA
eB
–
–
B
eB
L
–
0.400
0.150
10.16
3.81
0.115
2.92
D1
INCHES
MILLIMETERS
PKG. DIM
PINS
Plastic DIP
PLASTIC
DUAL-IN-LINE
PACKAGE
(0.300 in.)
MIN
MAX MIN
MAX
8
P
P
P
P
P
N
D
D
D
D
D
D
0.348 0.390 8.84
9.91
14
16
18
20
24
0.735 0.765 18.67 19.43
0.745 0.765 18.92 19.43
0.885 0.915 22.48 23.24
1.015 1.045 25.78 26.54
1.14 1.265 28.96 32.13
21-0043A
INCHES
MILLIMETERS
DIM
MIN
0.053
MAX
0.069
0.010
0.019
0.010
0.157
MIN
1.35
0.10
0.35
0.19
3.80
MAX
1.75
0.25
0.49
0.25
4.00
A
D
A1 0.004
B
C
E
e
0.014
0.007
0.150
0°-8°
A
0.101mm
0.004in.
0.050
1.27
e
H
L
0.228
0.016
0.244
0.050
5.80
0.40
6.20
1.27
A1
C
B
L
INCHES
MILLIMETERS
DIM PINS
Narrow SO
SMALL-OUTLINE
PACKAGE
MIN MAX
MIN
MAX
5.00
8.75
8
0.189 0.197 4.80
D
D
D
E
H
14 0.337 0.344 8.55
16 0.386 0.394 9.80 10.00
21-0041A
(0.150 in.)
12 ______________________________________________________________________________________
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