19-3359; Rev 0; 8/04
KIT
ATION
EVALU
BLE
AVAILA
Complete Bias and White LED Power Supplies
for Small TFT Displays
General Description
The MAX1578/MAX1579 provide four regulated outputs
to meet all the voltage requirements for small active-
matrix TFT-LCD displays in handheld devices where
minimum external components and high efficiency are
required. Each device consists of three advanced
charge pumps for LCD bias power and a step-up con-
verter for driving up to 8 series white LEDs for back-
lighting. The input voltage range is from 2.7V to 5.5V.
The charge pumps provide fixed +5V, +15V, and -10V
for the LCD bias circuits. No external diodes are need-
ed. A high-efficiency, fractional (1.5x/2x) charge pump
followed by a low-dropout linear regulator provides +5V
to power the source driver. Automatic mode changing
achieves the highest conversion efficiency. Two multi-
stage, high-voltage charge pumps generate +15V and
-10V to provide V
ON
and V
OFF
, respectively. Utilizing a
unique clocking scheme and internal drivers, these
charge pumps eliminate parasitic charge-current glitch-
es and reduce maximum input current, resulting in low
electromagnetic emissions. The outputs are sequenced
during startup and shutdown. In shutdown, the outputs
are discharged to zero.
The high-efficiency inductor step-up converter drives
up to 8 white LEDs in series with a constant current to
provide backlighting. The series connection allows the
LED currents to be identical for uniform brightness and
minimizes the number of traces to the LEDs. The
MAX1578 regulates constant LED current over the
entire temperature range. The MAX1579 features a tem-
perature derating function to avoid overdriving the
white LEDs during high ambient temperatures, enabling
higher drive current below +42°C.
The MAX1578/MAX1579 are available in space-saving
24-lead 4mm x 4mm thin QFN packages.
Features
♦
Four Regulators in One Package
♦
Bias Power Using Charge Pumps
+5V at 25mA for Source Driver
+15V at 100µA for V
ON
-10V at 100µA for V
OFF
No External Diodes Required
Output Sequencing
POS, NEG, and MAIN Are Autodischarged
During Shutdown
♦
LED Backlight Power Using Boost Converter
Series LED Connection for Uniform Illumination
Supports Up to 8 LEDs at 25mA (max)
900mW (max) Power
PWM or Analog Dimming Control
Overvoltage Protection
Low Input/Output Ripple
Soft-Start with Zero Inrush Current
Fast 1MHz PWM Operation for Small
Component Size
Temperature Derating Function (MAX1579)
♦
High Efficiency
Bias: 83% (5.0V at 25mA, 15V/-10V at 100µA)
LED: 84% (6 LEDs at 20mA)
♦
Uses Only Ceramic Capacitors and Only One
Inductor
♦
Independent Enable Inputs for LED and Bias
Power
♦
Thermal-Shutdown Protection
♦
1µA Shutdown Current
♦
Tiny 4mm x 4mm Thin QFN Package
MAX1578/MAX1579
MAX1578/MAX1579
Pin Configuration
COMP
13
12
GND
11
ONBIAS
10
NEG
PGND
CTRL
15
OUT
CS
14
LX
17
Applications
PDAs, Palmtops
Smart Phones
Internet Appliances
LCD Displays with White LED Backlight
C1N
19
C2N
20
IN
21
C2P
22
C1P
23
V
DD
24
18
16
MAX1578
MAX1579
9
8
7
CD2
CD1
PMPB
Ordering Information
PART
MAX1578ETG
MAX1579ETG
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
24 Thin QFN 4mm x 4mm
(T2444-4)
24 Thin QFN 4mm x 4mm
(T2444-4)
1
MAIN
2
CU1
3
CU2
4
CU3
5
POS
6
PMP
THIN QFN
4mm x 4mm
See Figure 3 for Typical Application Circuit.
________________________________________________________________
Maxim Integrated Products
1
For pricing delivery, and ordering information please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Complete Bias and White LED Power Supplies
for Small TFT Displays
MAX1578/MAX1579
ABSOLUTE MAXIMUM RATINGS
IN, CS, C1N, C2N, MAIN, ONBIAS, V
DD
to GND.....-0.3V to +6V
CTRL to GND..................-0.3V to the lesser of +6V or (V
IN
+ 2V)
LX, OUT to GND .....................................................-0.3V to +37V
COMP to GND .............................................-0.3V to (V
IN
+ 0.3V)
CU1 to MAIN ............................................................-0.3V to +6V
CU2 to CU1 ..............................................................-0.3V to +6V
CU3 to CU2 ..............................................................-0.3V to +6V
CU3 to POS ............................................................-0.3V to +18V
CU3 to GND ...........................................................-0.3V to +18V
POS to GND ...........................................................-0.3V to +18V
CD1 to MAIN ..........................................................+0.3V to -12V
CD1 to GND .............................................................+0.3V to -6V
CD2 to CD1 ..............................................................+0.3V to -6V
NEG to CD2..............................................................+0.3V to -6V
NEG, CD2 to GND..................................................+0.3V to -12V
C1P, C2P to GND ...........................................-0.3V to (V
IN
+ 6V)
PMP, PMPB to GND ................................-0.3V to (V
MAIN
+ 0.3V)
GND to PGND .......................................................-0.3V to +0.3V
I
LX
...................................................................................1.0A
RMS
Continuous Power Dissipation (T
A
= +70°C)
24-Pin 4mm x 4mm Thin QFN
(derate 20.8mW/°C above +70°C) .............................1667mW
Short-Circuit Duration (MAIN, POS, NEG)..................Continuous
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) ................................ +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
(Circuit of Figure 3, V
IN
= 3V, CTRL = ONBIAS = IN, T
A
= -40°C to +85°C, typical values are at T
A
= +25°C, unless otherwise noted. Note 1)
PARAMETER
IN Operating Supply Range
IN Undervoltage-Lockout (UVLO)
Threshold
IN Quiescent Current
IN Shutdown Current
Thermal Shutdown
Main Pump Efficiency
V
DD
Charge-Pump Open-Loop
Output Impedance
Operating Frequency
V
DD
Output Voltage
V
IN
Falling Switchover to 2.0x Mode
V
IN
Rising Switchover to 1.5x Mode
Quiescent Current
(Charge Pumps Only)
V
MAIN
Regulation Voltage
Discharge Switch Resistance
at V
MAIN
POS, NEG CHARGE PUMPS
Operating Frequency
Duty Cycle
POS Pump Efficiency
POS Output Voltage
POS Discharge Switch Resistance
NEG Pump Efficiency
I
LOAD
= 100µA
I
LOAD
= 0 to 100µA
V
ONBIAS
= 0V
I
LOAD
= -100µA
13.9
12.0
15.6
50
97
14.7
3
97
15.3
6
19.5
kHz
%
%
V
kΩ
%
V
CTRL
= 0V, ONBIAS = IN
0.1mA < I
LOAD
< 25mA
V
ONBIAS
= 0V
4.9
Charge-pump pause threshold
Rising edge, 30mV hysteresis
Switching
V
CTRL
= V
ONBIAS
= 0V
Rising temperature, 20°C hysteresis (typ)
I
LOAD
= 25mA, V
IN
= 3.9V
V
IN
≥
3.8V in 1.5x mode
V
IN
≥
3.0V in 2.0x mode
200
5.2
3.75
3.8
T
A
= +25°C
T
A
= +85°C
CONDITIONS
MIN
2.7
2.1
2.35
3
0.4
1
+160
83
9
7.5
250
5.5
3.85
3.9
0.87
5.0
1
20
20
300
5.7
3.95
4.0
1.30
5.1
3
TYP
MAX
5.5
2.6
5
1
UNITS
V
V
mA
µA
°C
%
Ω
kHz
V
V
V
mA
V
kΩ
MAIN CHARGE PUMP WITH LINEAR REGULATOR
2
_______________________________________________________________________________________
Complete Bias and White LED Power Supplies
for Small TFT Displays
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, V
IN
= 3V, CTRL = ONBIAS = IN, T
A
= -40°C to +85°C, typical values are at T
A
= +25°C, unless otherwise noted. Note 1)
PARAMETER
NEG Output Voltage
NEG Discharge Switch Resistance
LOGIC INPUT (ONBIAS)
Logic Input Low Voltage
Logic Input High Voltage
Input Current
LED BACKLIGHTING
Efficiency
OUT Voltage Range
Overvoltage-Lockout (OVLO)
Threshold
OUT Input Bias Current
ERROR AMPLIFIER
CTRL to CS Regulation
CS Input Bias Current
V
CTRL
= 1.5V, V
IN
= 2.7V to
5.5V
V
CS
= V
CTRL
/ 5
T
A
= +25°C
T
A
= -40°C to +85°C
T
A
= +25°C
T
A
= +85°C
MAX1578
CTRL Input Resistance
CTRL Dual Mode™ Threshold
CTRL Shutdown Delay
CS to COMP Transconductance
CS Regulation Derating Function
Start Temperature
CS Regulation Derating Function
Slope
CS Maximum Brightness Clamp
Voltage
CS Maximum Brightness Voltage
at CTRL
OSCILLATOR
Operating Frequency
Minimum Duty Cycle
Maximum Duty Cycle
f
BOOST
PWM mode
Pulse skipping
CTRL = IN, CS = GND
92
0.8
1.0
12
0
95
1.2
MHz
%
%
V
CTRL
< 1.0V
5mV hysteresis
(Note 3)
V
COMP
= 1.0V
V
CTRL
= 3V, MAX1579 only
V
CTRL
= 3V, T
A
= +65°C, MAX1579 only
MAX1578, V
CTRL
= 3V
MAX1579, V
CTRL
= 3V, T
A
= +25°C
MAX1578
MAX1579
310
322
MAX1579
T
A
= +25°C
T
A
= +85°C
100
6.5
32
250
250
0.295
0.292
0.300
0.300
0.01
0.03
500
500
185
170
8.2
60
+42
-6
327
340
1.635
1.70
345
358
780
780
240
10.5
90
kΩ
mV
ms
µS
°C
mV/°C
mV
V
0.305
0.308
1
V
µA
LOAD = 6 LEDs in series at 20mA
(Note 2)
V
OUT
rising, 2V hysteresis
V
OUT
= 32V, V
CTRL
> 0.24V
V
OUT
= V
IN
, V
CTRL
= 0
T
A
= +25°C
T
A
= +85°C
(V
IN
-
V
D1
)
32
10
34
20
0.01
0.1
84
32
36
32
1
µA
%
V
V
T
A
= +25°C
T
A
= +85°C
1.6
0.01
1
1
0.72
V
V
µA
I
LOAD
= 0 to -100µA
V
ONBIAS
= 0V
CONDITIONS
MIN
-10.2
TYP
-9.8
1.5
MAX
-9.3
3
UNITS
V
kΩ
MAX1578/MAX1579
Dual Mode is a trademark of Maxim Integrated Products, Inc.
_______________________________________________________________________________________
3
Complete Bias and White LED Power Supplies
for Small TFT Displays
MAX1578/MAX1579
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, V
IN
= 3V, CTRL = ONBIAS = IN, T
A
= -40°C to +85°C, typical values are at T
A
= +25°C, unless otherwise noted. Note 1)
PARAMETER
N-CHANNEL SWITCH
LX On-Resistance
LX Leakage Current
LX Current Limit
I
LX
= 190mA
V
LX
= 28V, CTRL = GND
Duty cycle = 90%
T
A
= +25°C
T
A
= +85°C
500
0.82
0.01
1
700
900
1.5
5
Ω
µA
mA
CONDITIONS
MIN
TYP
MAX
UNITS
Note 1:
All devices are 100% production tested at T
A
= +25°C. Limits over the operating temperature range are guaranteed by design.
Note 2:
V
D1
is the forward-voltage drop of diode D1 in Figure 3.
Note 3:
Time from CTRL going below the Dual-Mode threshold to IC shutdown.
Typical Operating Characteristics
(Circuit of Figure 3, V
IN
= 3.6V, I
LED
= 20mA, 4 LEDs, CTRL = IN, T
A
= +25°C, unless otherwise noted.)
LED EFFICIENCY vs. I
LED
MAX1578toc01
LED EFFICIENCY vs. V
IN
I
LED
= 20mA
85
80
EFFICIENCY (%)
MAX1578toc02
90
85
80
EFFICIENCY (%)
75
70
65
60
55
50
0
5
10
15
20
2 LEDs
8 LEDs
6 LEDs
4 LEDs
90
75
70
65
60
55
50
I
LED
= 10mA
I
LED
= 2mA
25
2.5
3.0
3.5
4.0
4.5
5.0
5.5
LED CURRENT (mA)
INPUT VOLTAGE (V)
LED CURRENT vs. TEMPERATURE
MAX1578toc03
LED CURRENT vs. V
CTRL
MAX1578toc04
30
25
20
I
LED
(mA)
15
10
5
0
-40
-15
10
35
60
MAX1578
R
CS
= 22Ω
25
MAX1579
R
CS
= 13Ω
20
I
LED
(mA)
85
15
10
5
0
0
0.5
1.0
1.5
2.0
2.5
CTRL VOLTAGE (V)
TEMPERATURE (°C)
4
_______________________________________________________________________________________
Complete Bias and White LED Power Supplies
for Small TFT Displays
MAX1578/MAX1579
Typical Operating Characteristics (continued)
(Circuit of Figure 3, V
IN
= 3.6V, I
LED
= 20mA, 4 LEDs, CTRL = IN, T
A
= +25°C, unless otherwise noted.)
LED CURRENT vs.
DIRECT-PWM DUTY CYCLE
MAX1578toc05
BOOST-SWITCHING WAVEFORMS
MAX1578toc06
25
20
V
IN
AC-COUPLED
20mV/div
I
LED
(mA)
15
V
OUT
AC-COUPLED
10
200mV/div
5
f
PWM
= 200Hz TO 200kHz
0
0
10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
V
LX
10V/div
400ns/div
BOOST STARTUP AND
SHUTDOWN WAVEFORMS
MAX1578toc07
PWM-DIMMING WAVEFORMS
MAX1578toc08
V
CTRL
2V/div
V
CTRL
V
IN
AC-COUPLED
2V/div
V
IN
AC-COUPLED
20mV/div
20mV/div
5V/div
V
OUT
I
IN
4ms/div
0V
100mA/div
V
OUT
AC-COUPLED
50mV/div
I
IN
40µs/div
20mA/div
LCD BIAS SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1578toc09
LCD BIAS STARTUP SEQUENCE
MAX1578toc10
1400
1200
1000
I
IN
(µA)
800
600
V
ONBIAS
5V/div
V
MAIN
5V/div
V
NEG
400
200
V
CTRL
= 0V
0
0
1
2
3
4
5
10ms/div
INPUT VOLTAGE (V)
V
POS
5V/div
10V/div
_______________________________________________________________________________________
5
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