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PDF MAX1578 Data sheet ( Hoja de datos )
| Número de pieza | MAX1578 | |
| Descripción | Complete Bias and White LED Power Supplies for Small TFT Displays | |
| Fabricantes | Maxim Integrated Products | |
| Logotipo |  |
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19-3359; Rev 0; 8/04
EVAALVUAAILTAIOBNLEKIT
www.DataSheet4U.com
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 VON and VOFF, 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.
Applications
PDAs, Palmtops
Smart Phones
Internet Appliances
LCD Displays with White LED Backlight
Ordering Information
PART
MAX1578ETG
MAX1579ETG
TEMP RANGE PIN-PACKAGE
-40°C to +85°C
24 Thin QFN 4mm x 4mm
(T2444-4)
-40°C to +85°C
24 Thin QFN 4mm x 4mm
(T2444-4)
Features
♦ Four Regulators in One Package
♦ Bias Power Using Charge Pumps
+5V at 25mA for Source Driver
+15V at 100µA for VON
-10V at 100µA for VOFF
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
Pin Configuration
18 17 16 15 14 13
C1N 19
C2N 20
IN 21
C2P 22
C1P 23
VDD 24
MAX1578
MAX1579
123456
12 GND
11 ONBIAS
10 NEG
9 CD2
8 CD1
7 PMPB
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.
1 page  
www.DataSheet4U.com
Complete Bias and White LED Power Supplies
for Small TFT Displays
Typical Operating Characteristics (continued)
(Circuit of Figure 3, VIN = 3.6V, ILED = 20mA, 4 LEDs, CTRL = IN, TA = +25°C, unless otherwise noted.)
LED CURRENT vs.
DIRECT-PWM DUTY CYCLE
25
BOOST-SWITCHING WAVEFORMS
20 VIN 20mV/div
AC-COUPLED
15
VOUT 200mV/div
AC-COUPLED
10
5
fPWM = 200Hz TO 200kHz
0
0 10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
VLX
400ns/div
10V/div
VCTRL
VIN
AC-COUPLED
BOOST STARTUP AND
SHUTDOWN WAVEFORMS
2V/div
20mV/div
VCTRL
VIN
AC-COUPLED
PWM-DIMMING WAVEFORMS
2V/div
20mV/div
VOUT
IIN
4ms/div
5V/div
0V
100mA/div
VOUT
AC-COUPLED
IIN
40µs/div
50mV/div
20mA/div
LCD BIAS SUPPLY CURRENT
vs. INPUT VOLTAGE
1400
1200
VONBIAS
LCD BIAS STARTUP SEQUENCE
5V/div
1000
VMAIN
5V/div
800
600 VNEG 5V/div
400
200
0
0
VCTRL = 0V
123 4
INPUT VOLTAGE (V)
5
VPOS
10ms/div
10V/div
_______________________________________________________________________________________ 5
5 Page 
www.DataSheet4U.com
Complete Bias and White LED Power Supplies
for Small TFT Displays
Component Selection
Use only ceramic capacitors with an X5R, X7R, or bet-
ter dielectric. See Table 1 for a list of recommended
components.
Capacitor Selection
Use low-ESR ceramic capacitors. Recommended val-
ues for the capacitors are shown in Table 1. To ensure
stability over a wide temperature range, ceramic
capacitors with an X5R or X7R dielectric are recom-
mended. Place these capacitors as close to the IC as
possible.
Inductor Selection
Recommended inductor values range from 10µH to
47µH. A 22µH inductor optimizes the efficiency for most
applications while maintaining low 15mVP-P input ripple.
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:
IPEAK
=
VOUT(MAX) × ILED(MAX)
0.8 × VIN(MIN)
+
VIN(MIN) × 0.8µs
2×L
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-
age must exceed VOUT. The RMS diode current is
calculated as:
IDIODE(RMS) = IOUT × IPEAK
PC Board Layout and Routing
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
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.
The IN bypass capacitor (CIN) should be placed as
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
ground connections of CIN and COUT should be as
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.
Chip Information
TRANSISTOR COUNT: 3801
PROCESS: BiCMOS
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 13 Páginas | |
| PDF Descargar | [ Datasheet MAX1578.PDF ] | |
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