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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 activematrix 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 converter for driving up to 8 series white LEDs for backlighting. 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 needed. 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 multistage, 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 glitches 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 temperature derating function to avoid overdriving the white LEDs during high ambient temperatures, enabling higher drive current below +42C. 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 100A for VON -10V at 100A 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 100A) 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 1A 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 VDD 24
18
16
MAX1578 MAX1579
9 8 7
CD2 CD1 PMPB
Ordering Information
PART MAX1578ETG MAX1579ETG TEMP RANGE -40C to +85C -40C to +85C 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
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Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579
ABSOLUTE MAXIMUM RATINGS
IN, CS, C1N, C2N, MAIN, ONBIAS, VDD to GND.....-0.3V to +6V CTRL to GND..................-0.3V to the lesser of +6V or (VIN + 2V) LX, OUT to GND .....................................................-0.3V to +37V COMP to GND .............................................-0.3V to (VIN + 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 (VIN + 6V) PMP, PMPB to GND ................................-0.3V to (VMAIN + 0.3V) GND to PGND .......................................................-0.3V to +0.3V ILX ...................................................................................1.0ARMS Continuous Power Dissipation (TA = +70C) 24-Pin 4mm x 4mm Thin QFN (derate 20.8mW/C above +70C) .............................1667mW Short-Circuit Duration (MAIN, POS, NEG)..................Continuous Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) ................................ +300C
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, VIN = 3V, CTRL = ONBIAS = IN, TA= -40C to +85C, typical values are at TA= +25C, 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 VDD Charge-Pump Open-Loop Output Impedance Operating Frequency VDD Output Voltage VIN Falling Switchover to 2.0x Mode VIN Rising Switchover to 1.5x Mode Quiescent Current (Charge Pumps Only) VMAIN Regulation Voltage Discharge Switch Resistance at VMAIN POS, NEG CHARGE PUMPS Operating Frequency Duty Cycle POS Pump Efficiency POS Output Voltage POS Discharge Switch Resistance NEG Pump Efficiency ILOAD = 100A ILOAD = 0 to 100A VONBIAS = 0V ILOAD = -100A 13.9 12.0 15.6 50 97 14.7 3 97 15.3 6 19.5 kHz % % V k % VCTRL = 0V, ONBIAS = IN 0.1mA < ILOAD < 25mA VONBIAS = 0V 4.9 Charge-pump pause threshold Rising edge, 30mV hysteresis Switching VCTRL = VONBIAS = 0V Rising temperature, 20C hysteresis (typ) ILOAD = 25mA, VIN = 3.9V VIN 3.8V in 1.5x mode VIN 3.0V in 2.0x mode 200 5.2 3.75 3.8 TA = +25C TA = +85C 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 A C % kHz V V V mA V k
MAIN CHARGE PUMP WITH LINEAR REGULATOR
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Complete Bias and White LED Power Supplies for Small TFT Displays
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, VIN = 3V, CTRL = ONBIAS = IN, TA= -40C to +85C, typical values are at TA= +25C, 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 VCTRL = 1.5V, VIN = 2.7V to 5.5V VCS = VCTRL / 5 TA = +25C TA = -40C to +85C TA = +25C TA = +85C MAX1578 CTRL Input Resistance CTRL Dual ModeTM 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 fBOOST PWM mode Pulse skipping CTRL = IN, CS = GND 92 0.8 1.0 12 0 95 1.2 MHz % % VCTRL < 1.0V 5mV hysteresis (Note 3) VCOMP = 1.0V VCTRL = 3V, MAX1579 only VCTRL = 3V, TA = +65C, MAX1579 only MAX1578, VCTRL = 3V MAX1579, VCTRL = 3V, TA = +25C MAX1578 MAX1579 310 322 MAX1579 TA = +25C TA = +85C 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 k mV ms S C mV/C mV V 0.305 0.308 1 V A LOAD = 6 LEDs in series at 20mA (Note 2) VOUT rising, 2V hysteresis VOUT = 32V, VCTRL > 0.24V VOUT = VIN, VCTRL = 0 TA = +25C TA = +85C (VIN VD1) 32 10 34 20 0.01 0.1 84 32 36 32 1 A % V V TA = +25C TA = +85C 1.6 0.01 1 1 0.72 V V A ILOAD = 0 to -100A VONBIAS = 0V CONDITIONS MIN -10.2 TYP -9.8 1.5 MAX -9.3 3 UNITS V k
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, VIN = 3V, CTRL = ONBIAS = IN, TA= -40C to +85C, typical values are at TA= +25C, unless otherwise noted. Note 1)
PARAMETER N-CHANNEL SWITCH LX On-Resistance LX Leakage Current LX Current Limit ILX = 190mA VLX = 28V, CTRL = GND Duty cycle = 90% TA = +25C TA = +85C 500 0.82 0.01 1 700 900 1.5 5 A mA CONDITIONS MIN TYP MAX UNITS
Note 1: All devices are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed by design. Note 2: VD1 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, VIN = 3.6V, ILED = 20mA, 4 LEDs, CTRL = IN, TA = +25C, unless otherwise noted.)
LED EFFICIENCY vs. ILED
MAX1578toc01
LED EFFICIENCY vs. VIN
ILED = 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
ILED = 10mA
ILED = 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. VCTRL
MAX1578toc04
30 25 20 ILED (mA) 15 10 5 0 -40 -15 10 35 60 MAX1578 RCS = 22
25
MAX1579 RCS = 13
20
ILED (mA) 85
15
10
5
0 0 0.5 1.0 1.5 2.0 2.5 CTRL VOLTAGE (V)
TEMPERATURE (C)
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Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579
Typical Operating Characteristics (continued)
(Circuit of Figure 3, VIN = 3.6V, ILED = 20mA, 4 LEDs, CTRL = IN, TA = +25C, unless otherwise noted.)
LED CURRENT vs. DIRECT-PWM DUTY CYCLE
MAX1578toc05
BOOST-SWITCHING WAVEFORMS
MAX1578toc06
25
20
VIN AC-COUPLED
20mV/div
ILED (mA)
15 VOUT AC-COUPLED 10 200mV/div
5 fPWM = 200Hz TO 200kHz 0 0 10 20 30 40 50 60 70 80 90 100 PWM DUTY CYCLE (%)
VLX
10V/div
400ns/div
BOOST STARTUP AND SHUTDOWN WAVEFORMS
MAX1578toc07
PWM-DIMMING WAVEFORMS
MAX1578toc08
VCTRL
2V/div
VCTRL VIN AC-COUPLED
2V/div
VIN AC-COUPLED
20mV/div
20mV/div
5V/div VOUT IIN 4ms/div 0V 100mA/div
VOUT AC-COUPLED
50mV/div
IIN 40s/div
20mA/div
LCD BIAS SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1578toc09
LCD BIAS STARTUP SEQUENCE
MAX1578toc10
1400 1200 1000 IIN (A) 800 600
VONBIAS
5V/div
VMAIN
5V/div
VNEG 400 200 VCTRL = 0V 0 0 1 2 3 4 5 10ms/div INPUT VOLTAGE (V) VPOS
5V/div
10V/div
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Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579
Typical Operating Characteristics (continued)
(Circuit of Figure 3, VIN = 3.6V, ILED = 20mA, 4 LEDs, CTRL = IN, TA = +25C, unless otherwise noted.)
MAIN OUTPUT LOAD REGULATION
MAX1578toc11
POS AND NEG LOAD REGULATION
-0.5 -1.0 VOLTAGE DROOP (%) -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 NEG POS
MAX1578toc12
0 -0.1 VOLTAGE DROOP (%) -0.2 -0.3 -0.4 -0.5 -0.6 0 5 10 15 20
0
-5.0 25 0 20 40 60 80 100 LOAD CURRENT (mA) LOAD CURRENT (A)
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 NAME MAIN CU1 CU2 CU3 POS PMP PMPB CD1 CD2 NEG FUNCTION LDO Output and the POS and NEG Charge-Pump Inputs. VMAIN is regulated to 5V. Bypass to GND with a 1F capacitor. Output is internally discharged with a 1k resistor when VONBIAS = 0V. POS Charge-Pump Capacitor Connection 1. Connect a 1F capacitor between CU1 and PMP. POS Charge-Pump Capacitor Connection 2. Connect a 1F capacitor between CU2 and PMPB. POS Charge-Pump Capacitor Connection 3. Connect a 1F capacitor between CU3 and GND. Output of Positive (3x) Charge Pump. Bypass POS to GND with a 1F capacitor. POS is internally discharged with a 3k resistor when VONBIAS = 0V. Charge-Pump Capacitor Connection. Connect a 1F capacitor between PMP and CU1 and another 1F capacitor between PMP and CD1. Charge-Pump Capacitor Connection. Connect a 1F capacitor between PMPB and CU2 and another 1F capacitor between PMPB and CD2. PMPB is 180 out of phase with PMP. NEG Charge-Pump Capacitor Connection 1. Connect a 1F capacitor and a 200 5% resistor in series between CD1 and PMP. NEG Charge-Pump Capacitor Connection 2. Connect a 1F capacitor and a 200 5% resistor in series between CD2 and PMPB. Output of Inverting (-2x) Charge Pump. Bypass NEG to GND with a 1F capacitor. Output is internally discharged with a 1.5k resistor when VONBIAS = 0V.
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Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579
Pin Description (continued)
PIN 11 12 13 NAME ONBIAS GND COMP FUNCTION Logic Input to Enable VDD, MAIN, POS, and NEG Charge Pumps. Drive ONBIAS high to enable all the charge pumps. Connect to GND to disable the charge pumps. Ground. Connect to PGND and the exposed pad directly under the IC. LED Driver Compensation. Connect a 0.1F from COMP to GND. CCOMP stabilizes the driver and sets the soft-start time. Current-Sense Feedback Input. Connect a resistor from CS to GND to set the LED current. For the MAX1578, CS regulates to VCTRL / 5 or 0.327V, whichever is lower. For the MAX1579, CS regulates to VCTRL / 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. Overvoltage Sense Input. The MAX1578/MAX1579 turn off the n-channel MOSFET when VOUT exceeds 34V. Once VOUT drops below 32V, the IC re-enters soft-start. Bypass OUT to GND with a 0.1F capacitor. 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. Power Ground. Connect to GND and the exposed pad directly under the IC. Main Charge-Pump Transfer Capacitor Negative Connection 1. Connect a 2.2F capacitor between C1N and C1P. Main Charge-Pump Transfer Capacitor Negative Connection 2. Connect a 2.2F capacitor between C2N and C2P. Power-Supply Input. Connect to a 2.7V to 5.5V input supply. Bypass IN to GND with a 4.7F capacitor. Main Charge-Pump Transfer Capacitor Positive Connection 2. Connect a 2.2F capacitor between C2P and C2N. Main Charge-Pump Transfer Capacitor Positive Connection 1. Connect a 2.2F capacitor between C1P and C1N. Regulated Main Charge-Pump Output. VDD is regulated to 5.5V. Bypass VDD to GND with a 4.7F capacitor. VDD is connected to IN when ONBIAS is pulled low. Exposed Paddle. Connect directly to a ground plane, GND, and PGND directly under the IC.
14
CS
15
CTRL
16 17 18 19 20 21 22 23 24 --
OUT LX PGND C1N C2N IN C2P C1P VDD EP
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Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579
Block Diagram
1MHz OSCILLATOR IN GND REF OSC OVERVOLTAGE PROTECTION BIAS PWM CONTROL N
LX
PGND
OUT
COMP
CS CTRL TIMER 170mV ONBIAS CHARGE-PUMP CONTROL SHUTDOWN SEQUENCING C1P OSC DIVIDE BY FOUR MULTIMODE CHARGE PUMP 1.5X/2X C1N C2P C2N OSC VDD PMP PMPB DIVIDE BY 64 5V LDO SHUTDOWN N MAIN
CD2 CD1
CU1 -2X INVERTING CHARGE PUMP 3X POSITIVE CHARGE PUMP CU2 CU3
NEG N SHUTDOWN
MAX1578 MAX1579
POS SHUTDOWN N
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Complete Bias and White LED Power Supplies for Small TFT Displays
Detailed Description
Bias Power and 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 voltage is below 2.35V (typ). Once VIN rises above 2.35V, and V CTRL and V ONBIAS are high, the boost converter and charge pumps are enabled, respectively.
MAIN Charge Pump
The MAX1578/MAX1579 include a charge pump that uses two external capacitors to provide +5.5V output (VDD) that is used to power the regulated LDO +5V output (MAIN). The control logic configures the pump to switch automatically between 1.5x and 2x modes to maximize efficiency. If VDD exceeds 5.5V, the charge pump stops switching. When ONBIAS is driven low, VDD is connected to IN. A low-dropout linear regulator regulates the output of the main charge pump to +5V at MAIN. The MAIN output is capable of sourcing as much as 25mA to an external load and also supplies the POS and NEG charge pumps. Drive ONBIAS low to disable the MAIN, POS, and NEG outputs. During shutdown, MAIN is discharged to GND with an internal 1k resistor.
MAX1578/MAX1579
Charge-Pump Output Sequencing
The outputs of the MAX1578/MAX1579 charge pumps are sequenced to turn on and off in a predictable fashion. The turn-on sequence is as follows (Figure 1): 1) When ONBIAS is high, the MAIN regulator (5V) is enabled. 2) When VMAIN exceeds 4.6V, the NEG charge pump (-10V) is enabled. 3) When VNEG reaches -8V, the POS charge pump (+15V) is enabled. The turn-off sequence is as follows (Figure 2): 1) When ONBIAS is driven low, the NEG charge pump (-10V) is disabled. 2) Once VNEG is discharged to -0.87V, the POS charge pump (+15V) is disabled. 3) Once VPOS falls to 0.87V, the MAIN regulator (+5V) is disabled and discharged.
POS/NEG Charge Pumps
The MAX1578/MAX1579 include a positive and negative charge pump for LCD bias. The POS and NEG charge pumps are powered from VMAIN. The POS and NEG charge pumps operate at 15.6kHz with a 50% duty cycle.
NEG Charge Pump (-10V Supply)
The NEG charge pump uses capacitors at CD1 and CD2 to generate -10V (-2 x VMAIN). Connect 1F ceramic capacitors and 200 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 1.5k resistor.
POS Charge Pump (+15V Supply)
ONBIAS
VMAIN (5V) VPOS (+15V) VNEG (-10V)
The POS charge pump uses capacitors at CU1, CU2, and CU3 to generate +15V (3 x VMAIN). Connect 1F 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 3k resistor.
Figure 1. Charge-Pump Turn-On Sequence
LED Backlighting Power
LED power is supplied by an internal MOSFET, 1MHz boost converter. The boost converter is capable of driving up to 8 series LEDs at 25mA. The output of the boost converter is regulated to maintain a constant voltage at CS, and therefore a constant current through the LEDs. Once VIN is increased above the UVLO voltage (2.35V) and VCTRL is above 0.17V, the boost converter enters soft-start and charges the output to its regulation voltage. An overvoltage-protection circuit shuts down the boost converter if V OUT exceeds 34V.
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ONBIAS VMAIN (5V) VPOS (+15V)
VNEG (-10V)
Figure 2. Charge-Pump Turn-Off Sequence
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Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579
Soft-Start
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 minimum duty cycle until CCOMP is charged to 1.25V. Once this occurs, the duty cycle increases to further charge the output until VCS reaches 20% of VCTRL. The softstart time is adjustable using the capacitor from COMP to GND. Calculate the required COMP capacitor as: 12A x t SS CCOMP = 1.25V where tSS is the desired soft-start time in seconds. 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 operation. VDD is connected to IN when ONBIAS is low. The quiescent current is reduced to 430A when the charge pumps are shut down and the boost converter is enabled.
Applications Information
Adjusting LED Current
Set the maximum LED current using a resistor from CS to GND. Calculate the resistance as follows: 330mV for the MAX1578 ILED 340mV RCS = for the MAX1579 ILED RCS = where ILED is the desired maximum current through the LEDs in Amps when VCTRL is 1.65V.
Overvoltage Protection
The output of the LED boost converter is protected from overvoltage conditions by internal overvoltage circuitry. If V OUT exceeds 34V, the LX switching terminates. Once VOUT falls below 32V, LX switches normally and soft-start is re-initiated.
Ambient Temperature Derating Function (MAX1579)
The MAX1579 limits the maximum LED current depending on the die temperature. VCS is limited to 340mV up to +42C. Once the temperature reaches +42C, the maximum VCS declines by 6mV/C until the minimum 40mV threshold is reached at +100C. Due to the package's exposed paddle, the die temperature is always very close to the PC board temperature. The temperature derating function allows the LED current to be safely set higher at normal operating temperatures, thereby allowing either a brighter display or fewer LEDs to be used for normal display brightness.
LED Dimming Control Using a DAC
VCTRL controls the LED drive current. The voltage at CS regulates to 20% of V CTRL to control the current 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. Increasing VCTRL beyond 1.65V results in no further brightness increase. Hold CTRL below 100mV for longer than 10.5ms to shut down the boost converter.
LED Dimming Using Direct PWM into 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 capacitor 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.
Shutdown
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 1A 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 impedance; however, a DC path exists 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 870A when the boost converter is shut down and the charge pumps are enabled.
Input/Output Ripple
For LED drivers, input and output ripple may be important. Input ripple depends on the source supply's output impedance. Adding a lowpass filter to the input further reduces input ripple. Alternately, increasing CIN to 10F cuts input ripple in half. Likewise, an output filter or higher output capacitance value reduces output ripple.
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Complete Bias and White LED Power Supplies for Small TFT Displays
Component Selection
Use only ceramic capacitors with an X5R, X7R, or better dielectric. See Table 1 for a list of recommended components. Capacitor Selection Use low-ESR ceramic capacitors. Recommended values 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 recommended. Place these capacitors as close to the IC as possible. Inductor Selection Recommended inductor values range from 10H to 47H. A 22H 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) x ILED(MAX) 0.8 x VIN(MIN) + VIN(MIN) x 0.8s 2 xL 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 possible to the IC. PGND and GND should be connected directly to the exposed paddle underneath the IC. The ground connections of C IN 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 sample layout to speed designs.
MAX1578/MAX1579
Chip Information
TRANSISTOR COUNT: 3801 PROCESS: BiCMOS
Schottky Diode Selection The MAX1578/MAX1579 require a high-speed rectification 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, respectively. In addition, the diode's reverse breakdown voltage must exceed V OUT . The RMS diode current is calculated as: IDIODE(RMS) = IOUT x IPEAK
PC Board Layout and Routing
Due to fast switching waveforms, careful PC board layout 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
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Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579
Table 1. Recommended Components for the Typical Application Circuit
DESIGNATION C1, C8 C2 C3 C4, C5, C12 C6, C7 C9, C10, C11, C13, C14 D1 D2-D7 L1 R1 R2, R3 DESCRIPTION 4.7F, 6.3V X5R ceramic capacitors (0603) Murata GRM188R60J475KE19 0.1F, 6.3V X5R ceramic capacitor (0402) TDK C1005X5R1A104K 0.1F, 50V X7R ceramic capacitor (0603) TDK C1608X7R1H104K 1F, 16V X7R ceramic capacitors (0805) TDK C2012X7R1C105K 2.2F, 6.3V X5R ceramic capacitors (0603) Taiyo Yuden JMK107BJ225KA 1F, 6.3V X5R ceramic capacitors (0402) Murata GRM155R60J105KE19 40V, 0.5A Schottky diode International Rectifier MBRX0540 White LEDs Nichia NSCW215T 22H, 250mA inductor (1210) Murata LQH32CN220K53 22.1 1% resistor (0402) 200 5% resistors (0402)
INPUT 2.7V TO 5.5V C1 4.7F IN PWM OR ANALOG DIMMING C2 0.1F ON OFF C6 2.2F ONBIAS CU1 C1P C1N C2P C7 2.2F C2N VDD C8 4.7F MAIN OUTPUT +5V, 25mA C9 1F GND PMP CD1 C13 1F C11 1F R2 200 R1 22.1 MAX1578 MAX1579 CTRL COMP LX OUT PGND D2-D7 CS C10 1F C3 0.1F L1 22H D1 OUTPUT UP TO 8 LEDS IN SERIES
CU2 PMPB CD2
C14 1F
R3 200
MAIN
POS CU3 C12 1F NEG C5 1F
POSITIVE OUTPUT +15V, 100A C4 1F NEGATIVE OUTPUT -10V, 100A
Figure 3. Typical Application Circuit
12
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Complete Bias and White LED Power Supplies for Small TFT Displays
Package Information
(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.)
24L QFN THIN.EPS
MAX1578/MAX1579
PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm
21-0139
C
1 2
PACKAGE OUTLINE 12, 16, 20, 24L THIN QFN, 4x4x0.8mm
21-0139
C
2 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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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