MAX1578 [MAXIM]
Complete Bias and White LED Power Supplies for Small TFT Displays; 完备偏置的白光LED电源，用于TFT显示器
| 型号： | MAX1578 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Complete Bias and White LED Power Supplies for Small TFT Displays |
| 文件： | 总13页 (文件大小：222K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3359; Rev 0; 8/04
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
Ge n e ra l De s c rip t io n
Fe a t u re s
♦ Four Regulators in One Package
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
re q uire d . Ea c h d e vic e c ons is ts of thre e a d va nc e d
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.
♦ 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
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
♦ 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
-10V to provide V
and V , respectively. Utilizing a
OFF
ON
Soft-Start with Zero Inrush Current
Fast 1MHz PWM Operation for Small
Component Size
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.
Temperature Derating Function (MAX1579)
♦ High Efficiency
Bias: 83% (5.0V at 25mA, 15V/-10V at 100µA)
LED: 84% (6 LEDs at 20mA)
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
minimize s the numb e r of tra c e s to the LEDs . The
MAX1578 re g ula te s c ons ta nt LED c urre nt ove r the
entire temperature range. The MAX1579 features a tem-
p e ra ture d e ra ting func tion to a void ove rd riving the
white LEDs during high ambient temperatures, enabling
higher drive current below +42°C.
♦ 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
P in Co n fig u ra t io n
The MAX1578/MAX1579 are available in space-saving
24-lead 4mm x 4mm thin QFN packages.
18 17 16 15 14 13
Ap p lic a t io n s
PDAs, Palmtops
12
11
10
9
C1N 19
GND
ONBIAS
NEG
Smart Phones
20
21
22
C2N
IN
Internet Appliances
CD2
MAX1578
MAX1579
C2P
LCD Displays with White LED Backlight
CD1
8
C1P 23
24
Ord e rin g In fo rm a t io n
7
PMPB
V
DD
PART
TEMP RANGE PIN-PACKAGE
1
2
3
4
5
6
24 Thin QFN 4mm x 4mm
MAX1578ETG
-40°C to +85°C
-40°C to +85°C
(T2444-4)
24 Thin QFN 4mm x 4mm
(T2444-4)
THIN QFN
4mm x 4mm
MAX1579ETG
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.
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
ABSOLUTE MAXIMUM RATINGS
IN, CS, C1N, C2N, MAIN, ONBIAS, V to GND.....-0.3V to +6V
NEG, CD2 to GND..................................................+0.3V to -12V
C1P, C2P to GND ...........................................-0.3V to (V + 6V)
DD
CTRL to GND..................-0.3V to the lesser of +6V or (V + 2V)
IN
IN
LX, OUT to GND.....................................................-0.3V to +37V
PMP, PMPB to GND ................................-0.3V to (V
+ 0.3V)
MAIN
COMP to GND .............................................-0.3V to (V + 0.3V)
GND to PGND .......................................................-0.3V to +0.3V
I ...................................................................................1.0A
LX
IN
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
RMS
Continuous Power Dissipation (T = +70°C)
A
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 = 3V, CTRL = ONBIAS = IN, T = -40°C to +85°C, typical values are at T = +25°C, unless otherwise noted. Note 1)
IN
A
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
IN Operating Supply Range
2.7
5.5
V
IN Undervoltage-Lockout (UVLO)
Threshold
Rising edge, 30mV hysteresis
Switching
2.1
2.35
2.6
V
IN Quiescent Current
IN Shutdown Current
Thermal Shutdown
3
0.4
1
5
1
mA
µA
°C
T
= +25°C
= +85°C
A
V
CTRL
= V
= 0V
ONBIAS
T
A
Rising temperature, 20°C hysteresis (typ)
+160
MAIN CHARGE PUMP WITH LINEAR REGULATOR
Main Pump Efficiency
I
= 25mA, V = 3.9V
IN
83
9
%
LOAD
V
≥ 3.8V in 1.5x mode
≥ 3.0V in 2.0x mode
20
20
IN
V
DD
Charge-Pump Open-Loop
Ω
Output Impedance
V
7.5
250
5.5
3.85
3.9
IN
Operating Frequency
200
5.2
300
5.7
3.95
4.0
kHz
V
V
DD
Output Voltage
Charge-pump pause threshold
V
Falling Switchover to 2.0x Mode
Rising Switchover to 1.5x Mode
3.75
3.8
V
IN
V
IN
V
Quiescent Current
(Charge Pumps Only)
V
= 0V, ONBIAS = IN
0.87
5.0
1
1.30
5.1
3
mA
V
CTRL
V
MAIN
Regulation Voltage
0.1mA < I
< 25mA
4.9
LOAD
Discharge Switch Resistance
at V
V
= 0V
kΩ
ONBIAS
MAIN
POS, NEG CHARGE PUMPS
Operating Frequency
Duty Cycle
12.0
13.9
15.6
50
19.5
kHz
%
POS Pump Efficiency
POS Output Voltage
I
= 100µA
97
%
LOAD
I
= 0 to 100µA
= 0V
14.7
3
15.3
6
V
LOAD
POS Discharge Switch Resistance
NEG Pump Efficiency
V
kΩ
%
ONBIAS
I
= -100µA
97
LOAD
2
_______________________________________________________________________________________
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, V = 3V, CTRL = ONBIAS = IN, T = -40°C to +85°C, typical values are at T = +25°C, unless otherwise noted. Note 1)
IN
A
A
PARAMETER
CONDITIONS
MIN
TYP
-9.8
1.5
MAX
-9.3
3
UNITS
V
NEG Output Voltage
I
= 0 to -100µA
= 0V
-10.2
LOAD
NEG Discharge Switch Resistance
LOGIC INPUT (ONBIAS)
Logic Input Low Voltage
V
kΩ
ONBIAS
0.72
1
V
V
Logic Input High Voltage
1.6
T
= +25°C
0.01
1
A
Input Current
µA
T
= +85°C
A
LED BACKLIGHTING
Efficiency
LOAD = 6 LEDs in series at 20mA
(Note 2)
84
%
V
(V
V
D1
-
)
IN
OUT Voltage Range
32
36
Overvoltage-Lockout (OVLO)
Threshold
V
OUT
rising, 2V hysteresis
32
10
34
V
V
= 32V, V
> 0.24V
= 0
20
0.01
0.1
32
1
OUT
CTRL
OUT Input Bias Current
µA
T
= +25°C
A
V
OUT
= V , V
IN CTRL
T
A
= +85°C
ERROR AMPLIFIER
T
= +25°C
0.295
0.292
0.300
0.300
0.01
0.03
500
500
185
170
8.2
0.305
0.308
1
A
V
5.5V
= 1.5V, V = 2.7V to
IN
CTRL
CTRL to CS Regulation
V
T
A
= -40°C to +85°C
= +25°C
T
A
CS Input Bias Current
CTRL Input Resistance
V
CS
= V / 5
CTRL
µA
T
A
= +85°C
MAX1578
250
250
780
780
V
CTRL
< 1.0V
kΩ
T
= +25°C
= +85°C
A
MAX1579
T
A
CTRL Dual Mode™ Threshold
CTRL Shutdown Delay
5mV hysteresis
(Note 3)
100
6.5
32
240
10.5
90
mV
ms
µS
CS to COMP Transconductance
V
COMP
= 1.0V
60
CS Regulation Derating Function
Start Temperature
V
= 3V, MAX1579 only
+42
-6
°C
mV/°C
mV
CTRL
CS Regulation Derating Function
Slope
V
CTRL
= 3V, T = +65°C, MAX1579 only
A
MAX1578, V
= 3V
310
322
327
340
345
358
CTRL
CS Maximum Brightness Clamp
Voltage
MAX1579, V
= 3V, T = +25°C
A
CTRL
MAX1578
MAX1579
1.635
1.70
CS Maximum Brightness Voltage
at CTRL
V
OSCILLATOR
Operating Frequency
f
0.8
92
1.0
12
0
1.2
MHz
%
BOOST
PWM mode
Minimum Duty Cycle
Maximum Duty Cycle
Pulse skipping
CTRL = IN, CS = GND
95
%
Dual Mode is a trademark of Maxim Integrated Products, Inc.
_______________________________________________________________________________________
3
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, V = 3V, CTRL = ONBIAS = IN, T = -40°C to +85°C, typical values are at T = +25°C, unless otherwise noted. Note 1)
IN
A
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
N-CHANNEL SWITCH
LX On-Resistance
I
= 190mA
0.82
0.01
1
1.5
5
Ω
LX
T
= +25°C
= +85°C
A
LX Leakage Current
LX Current Limit
V
= 28V, CTRL = GND
µA
mA
LX
T
A
Duty cycle = 90%
500
700
900
Note 1: All devices are 100% production tested at T = +25°C. Limits over the operating temperature range are guaranteed by design.
A
Note 2: V is the forward-voltage drop of diode D1 in Figure 3.
D1
Note 3: Time from CTRL going below the Dual-Mode threshold to IC shutdown.
Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
= 20mA, 4 LEDs, CTRL = IN, T = +25°C, unless otherwise noted.)
A
(Circuit of Figure 3, V = 3.6V, I
IN
LED
LED EFFICIENCY vs. I
LED EFFICIENCY vs. V
LED
IN
90
85
80
75
70
65
60
55
50
90
85
80
75
70
65
60
55
50
I
= 20mA
4 LEDs
6 LEDs
LED
I
= 10mA
LED
8 LEDs
2 LEDs
I
= 2mA
LED
0
5
10
15
20
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
LED CURRENT vs. V
CTRL
30
25
20
15
10
5
25
MAX1579
20
15
10
5
R
CS
= 13Ω
MAX1578
= 22Ω
R
CS
0
0
-40
-15
10
35
60
85
0
0.5
1.0
1.5
2.0
2.5
TEMPERATURE (°C)
CTRL VOLTAGE (V)
4
_______________________________________________________________________________________
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
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 )
= 20mA, 4 LEDs, CTRL = IN, T = +25°C, unless otherwise noted.)
A
(Circuit of Figure 3, V = 3.6V, I
IN
LED
LED CURRENT vs.
DIRECT-PWM DUTY CYCLE
BOOST-SWITCHING WAVEFORMS
25
20
15
10
5
V
IN
20mV/div
AC-COUPLED
V
200mV/div
OUT
AC-COUPLED
V
LX
10V/div
f
= 200Hz TO 200kHz
PWM
0
0
10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
400ns/div
BOOST STARTUP AND
SHUTDOWN WAVEFORMS
PWM-DIMMING WAVEFORMS
2V/div
2V/div
V
CTRL
V
CTRL
V
20mV/div
IN
V
IN
20mV/div
AC-COUPLED
AC-COUPLED
V
50mV/div
20mA/div
OUT
5V/div
AC-COUPLED
V
OUT
I
IN
0V
100mA/div
I
IN
4ms/div
40µs/div
LCD BIAS SUPPLY CURRENT
vs. INPUT VOLTAGE
LCD BIAS STARTUP SEQUENCE
1400
1200
1000
800
600
400
200
0
5V/div
5V/div
V
ONBIAS
V
MAIN
5V/div
V
NEG
10V/div
V
POS
V
CTRL
= 0V
0
1
2
3
4
5
10ms/div
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
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 )
= 20mA, 4 LEDs, CTRL = IN, T = +25°C, unless otherwise noted.)
A
(Circuit of Figure 3, V = 3.6V, I
IN
LED
MAIN OUTPUT LOAD REGULATION
POS AND NEG LOAD REGULATION
0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
POS
NEG
0
5
10
15
20
25
0
20
40
60
80
100
LOAD CURRENT (mA)
LOAD CURRENT (µA)
P in De s c rip t io n
PIN
NAME
FUNCTION
LDO Output and the POS and NEG Charge-Pump Inputs. V
with a 1µF capacitor. Output is internally discharged with a 1kΩ resistor when V
is regulated to 5V. Bypass to GND
MAIN
1
MAIN
= 0V.
ONBIAS
2
3
4
CU1
CU2
CU3
POS Charge-Pump Capacitor Connection 1. Connect a 1µF capacitor between CU1 and PMP.
POS Charge-Pump Capacitor Connection 2. Connect a 1µF capacitor between CU2 and PMPB.
POS Charge-Pump Capacitor Connection 3. Connect a 1µF capacitor between CU3 and GND.
Output of Positive (3x) Charge Pump. Bypass POS to GND with a 1µF capacitor. POS is internally
5
6
POS
PMP
PMPB
CD1
discharged with a 3kΩ resistor when V
= 0V.
ONBIAS
Charge-Pump Capacitor Connection. Connect a 1µF capacitor between PMP and CU1 and another
1µF capacitor between PMP and CD1.
Charge-Pump Capacitor Connection. Connect a 1µF capacitor between PMPB and CU2 and another
1µF capacitor between PMPB and CD2. PMPB is 180° out of phase with PMP.
7
NEG Charge-Pump Capacitor Connection 1. Connect a 1µF capacitor and a 200Ω ±5% resistor in
series between CD1 and PMP.
8
NEG Charge-Pump Capacitor Connection 2. Connect a 1µF capacitor and a 200Ω ±5% resistor in
series between CD2 and PMPB.
9
CD2
Output of Inverting (-2x) Charge Pump. Bypass NEG to GND with a 1µF capacitor. Output is internally
10
NEG
discharged with a 1.5kΩ resistor when V
= 0V.
ONBIAS
6
_______________________________________________________________________________________
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
P in De s c rip t io n (c o n t in u e d )
PIN
11
NAME
ONBIAS
GND
FUNCTION
Logic Input to Enable V , MAIN, POS, and NEG Charge Pumps. Drive ONBIAS high to enable all the
DD
charge pumps. Connect to GND to disable the charge pumps.
12
Ground. Connect to PGND and the exposed pad directly under the IC.
LED Driver Compensation. Connect a 0.1µF from COMP to GND. C
sets the soft-start time.
stabilizes the driver and
COMP
13
COMP
Current-Sense Feedback Input. Connect a resistor from CS to GND to set the LED current. For the
MAX1578, CS regulates to V / 5 or 0.327V, whichever is lower. For the MAX1579, CS regulates to
14
CS
CTRL
V
CTRL
/ 5 or 0.340V, whichever is lower.
LED Brightness Control Input. Connect CTRL to a 0.24V to 1.65V input to set the brightness of the
external LEDs. Hold CTRL below 100mV for more than 10.5ms, to shut down the LED driver. Drive CTRL
with a 200Hz to 200kHz unfiltered PWM dimming signal for DC LED current that is proportional to the
signal’s duty cycle.
15
16
CTRL
OUT
Overvoltage Sense Input. The MAX1578/MAX1579 turn off the n-channel MOSFET when V
exceeds 34V.
OUT
Once V
drops below 32V, the IC re-enters soft-start. Bypass OUT to GND with a 0.1µF capacitor.
OUT
Inductor Connection. Connect to the switched side of the external inductor as well as the anode of the
external diode. LX is high impedance during shutdown.
17
18
19
LX
PGND
C1N
Power Ground. Connect to GND and the exposed pad directly under the IC.
Main Charge-Pump Transfer Capacitor Negative Connection 1. Connect a 2.2µF capacitor between
C1N and C1P.
Main Charge-Pump Transfer Capacitor Negative Connection 2. Connect a 2.2µF capacitor between
C2N and C2P.
20
21
22
C2N
IN
Power-Supply Input. Connect to a 2.7V to 5.5V input supply. Bypass IN to GND with a 4.7µF capacitor.
Main Charge-Pump Transfer Capacitor Positive Connection 2. Connect a 2.2µF capacitor between
C2P and C2N.
C2P
Main Charge-Pump Transfer Capacitor Positive Connection 1. Connect a 2.2µF capacitor between
C1P and C1N.
23
C1P
Regulated Main Charge-Pump Output. V is regulated to 5.5V. Bypass V to GND with a 4.7µF
DD
DD
24
—
V
DD
capacitor. V is connected to IN when ONBIAS is pulled low.
DD
EP
Exposed Paddle. Connect directly to a ground plane, GND, and PGND directly under the IC.
_______________________________________________________________________________________
7
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
Blo c k Dia g ra m
1MHz
OSCILLATOR
LX
IN
BIAS
REF
PWM
CONTROL
N
GND
PGND
OUT
OSC
OVERVOLTAGE
PROTECTION
COMP
CS
CTRL
TIMER
170mV
SHUTDOWN
SEQUENCING
CHARGE-PUMP
CONTROL
ONBIAS
C1P
C1N
C2P
C2N
MULTIMODE
CHARGE PUMP
1.5X/2X
OSC
DIVIDE BY FOUR
OSC
V
DD
PMP
DIVIDE BY 64
5V LDO
MAIN
PMPB
SHUTDOWN
N
CU1
CU2
CU3
CD2
CD1
3X POSITIVE
CHARGE PUMP
-2X INVERTING
CHARGE PUMP
NEG
POS
MAX1578
MAX1579
SHUTDOWN
SHUTDOWN
N
N
8
_______________________________________________________________________________________
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
MAIN Ch a rg e P u m p
The MAX1578/MAX1579 include a charge pump that
De t a ile d De s c rip t io n
Bia s P o w e r a n d UVLO
The MAX1578/MAX1579 contain an LED driver boost
converter and three charge pumps for LCD bias. The
undervoltage-lockout (UVLO) feature disables the LED
boost converter and the charge pumps when the input
uses two external capacitors to provide +5.5V output
(V ) that is used to power the regulated LDO +5V out-
DD
put (MAIN). The control logic configures the pump to
switch automatically between 1.5x and 2x modes to
maximize efficiency. If V
exceeds 5.5V, the charge
DD
voltage is below 2.35V (typ). Once V rises above
IN
pump stops switching. When ONBIAS is driven low,
is connected to IN.
2.35V, and V
and V
are high, the boost
CTRL
ONBIAS
V
DD
converter and charge pumps are enabled, respectively.
A low-dropout linear regulator regulates the output of
the main charge pump to +5V at MAIN. The MAIN out-
put is capable of sourcing as much as 25mA to an
e xte rna l loa d a nd a ls o s up p lie s the POS a nd NEG
charge pumps. Drive ONBIAS low to disable the MAIN,
POS, and NEG outputs. During shutdown, MAIN is dis-
charged to GND with an internal 1kΩ resistor.
Ch a rg e -P u m p Ou t p u t S e q u e n c in g
The outputs of the MAX1578/MAX1579 charge pumps
are sequenced to turn on and off in a predictable fash-
ion. The turn-on sequence is as follows (Figure 1):
1) When ONBIAS is high, the MAIN regulator (5V) is
enabled.
P OS /NEG Ch a rg e P u m p s
The MAX1578/MAX1579 include a positive and nega-
tive charge pump for LCD bias. The POS and NEG
2) When V
exceeds 4.6V, the NEG charge pump
MAIN
(-10V) is enabled.
3) When V
reaches -8V, the POS charge pump
NEG
charge pumps are powered from V . The POS and
MAIN
(+15V) is enabled.
NEG charge pumps operate at 15.6kHz with a 50%
duty cycle.
The turn-off sequence is as follows (Figure 2):
1) When ONBIAS is driven low, the NEG charge pump
(-10V) is disabled.
NEG Ch a rg e P u m p (-1 0 V S u p p ly)
The NEG charge pump uses capacitors at CD1 and
2) Once V
is discharged to -0.87V, the POS charge
NEG
CD2 to generate -10V (-2 x V ). Connect 1µF ceram-
MAIN
pump (+15V) is disabled.
ic capacitors and 200Ω ±5% resistors in series between
CD1 and PMP and between CD2 and PMPB. Drive
ONBIAS high to enable MAIN, NEG, and POS. During
shutdown, the NEG output is discharged to GND with an
internal 1.5kΩ resistor.
3) Once V
falls to 0.87V, the MAIN regulator (+5V)
POS
is disabled and discharged.
P OS Ch a rg e P u m p (+1 5 V S u p p ly)
The POS charge pump uses capacitors at CU1, CU2,
ONBIAS
and CU3 to generate +15V (3 x V ). Connect 1µF
MAIN
ceramic capacitors between CU1 and PMP, between
CU2 and PMPB, and between CU3 and GND. Drive
ONBIAS high to enable MAIN, NEG, and POS. During
shutdown, POS is discharged to GND with an internal
3kΩ resistor.
V
(5V)
MAIN
V
POS
(+15V)
(-10V)
V
NEG
Figure 1. Charge-Pump Turn-On Sequence
LED Ba c k lig h t in g P o w e r
LED power is supplied by an internal MOSFET, 1MHz
boost converter. The boost converter is capable of dri-
ving up to 8 series LEDs at 25mA.
ONBIAS
The output of the boost converter is regulated to main-
tain a constant voltage at CS, and therefore a constant
V
(5V)
MAIN
V
POS
(+15V)
current through the LEDs. Once V is increased above
IN
the UVLO voltage (2.35V) and V
is above 0.17V,
CTRL
the boost converter enters soft-start and charges the
output to its regulation voltage. An overvoltage-protec-
V
NEG
(-10V)
tion circuit shuts down the boost converter if V
exceeds 34V.
OUT
Figure 2. Charge-Pump Turn-Off Sequence
_______________________________________________________________________________________
9
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
Drive ONBIAS low to shut down the internal POS and
NEG charge pumps and disable the MAIN LDO output.
On-chip pulldown resistors discharge these outputs
during shutdown. Drive ONBIAS high for normal opera-
S o ft -S t a rt
The LED boost converter utilizes a soft-start function to
eliminate inrush current during startup. Once the boost
converter is enabled, LX begins switching at the mini-
tion. V
is connected to IN when ONBIAS is low. The
mum duty cycle until C
is charged to 1.25V. Once
DD
COMP
quiescent current is reduced to 430µA when the charge
p ump s a re s hut d own a nd the b oos t c onve rte r is
enabled.
this occurs, the duty cycle increases to further charge
the output until V reaches 20% of V . The soft-
start time is adjustable using the capacitor from COMP
to GND. Calculate the required COMP capacitor as:
CS
CTRL
Ap p lic a t io n s In fo rm a t io n
12µA× t
SS
C
=
Ad ju s t in g LED Cu rre n t
Set the maximum LED current using a resistor from CS
to GND. Calculate the resistance as follows:
COMP
1.25V
where t is the desired soft-start time in seconds.
SS
330mV
Ove rvo lt a g e P ro t e c t io n
The output of the LED boost converter is protected from
overvoltage conditions by internal overvoltage circuitry.
R
R
=
=
for the MAX1578
for the MAX1579
CS
CS
I
LED
340mV
If V
exceeds 34V, the LX switching terminates.
OUT
I
LED
Once V
falls below 32V, LX switches normally and
OUT
soft-start is re-initiated.
where I
is the desired maximum current through the
LED
LEDs in Amps when V
is 1.65V.
CTRL
Am b ie n t Te m p e ra t u re De ra t in g Fu n c t io n
(MAX1 5 7 9 )
The MAX1579 limits the maximum LED current depend-
LED Dim m in g Co n t ro l Us in g a DAC
controls the LED drive current. The voltage at CS
V
CTRL
ing on the die temperature. V is limited to 340mV up
CS
re g ula te s to 20% of V
to c ontrol the c urre nt
CTRL
to +42°C. Once the temperature reaches +42°C, the
through the LEDs and, therefore, the brightness. Drive
CTRL using a DAC with an output voltage between
0.24V and 1.65V to control the brightness of the LEDs.
maximum V declines by 6mV/°C until the minimum
CS
40mV threshold is reached at +100°C. Due to the pack-
age’s exposed paddle, the die temperature is always
very close to the PC board temperature.
Increasing V
beyond 1.65V results in no further
CTRL
b rig htne s s inc re a s e . Hold CTRL b e low 100mV for
longer than 10.5ms to shut down the boost converter.
The temperature derating function allows the LED cur-
rent to be safely set higher at normal operating temper-
atures, thereby allowing either a brighter display or
fewer LEDs to be used for normal display brightness.
LED Dim m in g Us in g Dire c t P WM in t o CTRL
Another useful technique for LED dimming control is the
application of a logic-level PWM signal applied directly
to CTRL. LED current may be varied from zero to full
scale. The frequency range of the PWM signal is from
200Hz to 200kHz, while 0% duty cycle corresponds to
zero current and 100% duty cycle corresponds to full
current. The error amplifier and compensation capaci-
tor form a lowpass filter so PWM dimming results in DC
current to the LEDs without the need for any additional
RC filters. See the Typical Operating Characteristics.
S h u t d o w n
The MAX1578/MAX1579 include a low-quiescent-current
shutdown mode. To enter shutdown, drive CTRL below
0.1V for longer than 10.5ms and drive ONBIAS low. The
quiescent current is reduced to less than 1µA when the
boost converter and charge pumps are disabled.
To disable the LED boost converter, drive CTRL below
0.1V for longer than 10.5ms. During shutdown, the
internal boost switch from LX to PGND is high imped-
a nc e ; howe ve r, a DC p a th e xis ts from IN to OUT
through the external inductor and Schottky diode. Drive
CTRL with an analog voltage between 0.24V and 1.65V
or a 200Hz to 200kHz digital PWM dimming signal for
normal operation. The quiescent current is reduced to
870µA when the boost converter is shut down and the
charge pumps are enabled.
In p u t /Ou t p u t Rip p le
For LED drivers, input and output ripple may be impor-
tant. Input ripple depends on the source supply’s output
impedance. Adding a lowpass filter to the input further
reduces input ripple. Alternately, increasing C to 10µF
cuts input ripple in half. Likewise, an output filter or high-
er output capacitance value reduces output ripple.
IN
10 ______________________________________________________________________________________
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
inductor, the diode, the input capacitor, and the output
capacitor. Keep traces short, direct, and wide. Keep
noisy traces, such as the LX node trace, away from CS.
Co m p o n e n t S e le c t io n
Use only ceramic capacitors with an X5R, X7R, or bet-
ter dielectric. See Table 1 for a list of recommended
components.
The IN bypass capacitor (C ) should be placed as
IN
close to the IC as possible. The transfer capacitors for
the charge pumps should be located as close as possi-
ble to the IC. PGND and GND should be connected
directly to the exposed paddle underneath the IC. The
Capacitor Selection
Use low-ESR ceramic capacitors. Recommended val-
ues for the capacitors are shown in Table 1. To ensure
s ta b ility ove r a wid e te mp e ra ture ra ng e , c e ra mic
capacitors with an X5R or X7R dielectric are recom-
mended. Place these capacitors as close to the IC as
possible.
ground connections of C and C
should be as
IN
OUT
close together as possible. The traces from IN to the
inductor and from the Schottky diode to the LEDs may
be longer. The MAX1579 evaluation kit contains a sam-
ple layout to speed designs.
Inductor Selection
Recommended inductor values range from 10µH to
47µH. A 22µH inductor optimizes the efficiency for most
Ch ip In fo rm a t io n
TRANSISTOR COUNT: 3801
applications while maintaining low 15mV
input ripple.
P-P
PROCESS: BiCMOS
With input voltages near 5V, a larger value of inductance
can be more efficient. To prevent core saturation, ensure
that the inductor-saturation current rating exceeds the
peak inductor current for the application. Calculate the
peak inductor current with the following formula:
V
×I
V
× 0.8µs
OUT(MAX) LED(MAX)
IN(MIN)
I
=
+
PEAK
0.8 × V
2 ×L
IN(MIN)
Schottky Diode Selection
The MAX1578/MAX1579 require a high-speed rectifica-
tion diode (D1) for optimum performance. A Schottky
diode is recommended due to its fast recovery time
and low forward-voltage drop. Ensure that the diode’s
average and peak current ratings exceed the average
output current and the peak inductor current, respec-
tively. In addition, the diode’s reverse breakdown volt-
a g e mus t e xc e e d V . The RMS d iod e c urre nt is
OUT
calculated as:
I
= I
×I
DIODE(RMS)
OUT PEAK
P C Bo a rd La yo u t a n d Ro u t in g
Due to fast switching waveforms, careful PC board lay-
out is required. An evaluation kit (MAX1578EVKIT) is
available to speed design. When laying out a board,
minimize trace lengths between the IC and R1, the
______________________________________________________________________________________ 11
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
Table 1. Recommended Components for
the Typical Application Circuit
L1
22µH
DESIGNATION
DESCRIPTION
INPUT
2.7V TO 5.5V
4.7µF, 6.3V X5R ceramic capacitors (0603)
Murata GRM188R60J475KE19
C1
4.7µF
OUTPUT UP TO
8 LEDS IN SERIES
C1, C8
D1
IN
LX
PWM OR
ANALOG
DIMMING
C3
OUT
0.1µF, 6.3V X5R ceramic capacitor (0402)
TDK C1005X5R1A104K
CTRL
COMP
0.1µF
C2
C3
PGND
CS
C2
D2–D7
0.1µF
0.1µF, 50V X7R ceramic capacitor (0603)
TDK C1608X7R1H104K
MAX1578
MAX1579
ON
C10
1µF
OFF
ONBIAS
C1P
CU1
PMP
CD1
1µF, 16V X7R ceramic capacitors (0805)
TDK C2012X7R1C105K
C4, C5, C12
C6, C7
C6
2.2µF
R1
22.1Ω
C1N
C2P
C13
1µF
R2
200Ω
2.2µF, 6.3V X5R ceramic capacitors (0603)
Taiyo Yuden JMK107BJ225KA
C7
C11
1µF
2.2µF
C2N
CU2
PMPB
CD2
V
DD
C9, C10, C11, 1µF, 6.3V X5R ceramic capacitors (0402)
C8
C13, C14
Murata GRM155R60J105KE19
4.7µF
C14
1µF
R3
200Ω
POSITIVE OUTPUT
+15V, 100µA
40V, 0.5A Schottky diode
International Rectifier MBRX0540
MAIN OUTPUT
+5V, 25mA
D1
POS
CU3
NEG
MAIN
GND
C4
1µF
C9
1µF
White LEDs
Nichia NSCW215T
C12
1µF
D2–D7
L1
NEGATIVE OUTPUT
-10V, 100µA
C5
1µF
22µH, 250mA inductor (1210)
Murata LQH32CN220K53
R1
22.1Ω ±1% resistor (0402)
200Ω ±5% resistors (0402)
R2, R3
Figure 3. Typical Application Circuit
12 ______________________________________________________________________________________
Co m p le t e Bia s a n d Wh it e LED P o w e r S u p p lie s
fo r S m a ll TFT Dis p la ys
P a c k a g e In fo rm a t io n
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm
1
C
21-0139
2
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm
2
C
21-0139
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Ma x im In t e g ra t e d P ro d u c t s , 1 2 0 S a n Ga b rie l Drive , S u n n yva le , CA 9 4 0 8 6 4 0 8 -7 3 7 -7 6 0 0 ____________________ 13
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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