KIT ATION EVALU E L B AVAILA 19-3359; Rev 0; 8/04 Complete Bias and White LED Power Supplies for Small TFT Displays Features 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 +42°C. The MAX1578/MAX1579 are available in space-saving 24-lead 4mm x 4mm thin QFN packages. ♦ 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 LX OUT CTRL CS COMP Applications PGND Pin Configuration 18 17 16 15 14 13 PDAs, Palmtops C1N 19 12 GND Smart Phones C2N 20 11 ONBIAS Internet Appliances 10 NEG IN 21 -40°C to +85°C 24 Thin QFN 4mm x 4mm (T2444-4) MAX1579ETG -40°C to +85°C 24 Thin QFN 4mm x 4mm (T2444-4) 1 2 3 4 5 6 PMP MAX1578ETG POS PIN-PACKAGE CU3 TEMP RANGE VDD 24 CU2 PART C1P 23 CU1 Ordering Information MAIN LCD Displays with White LED Backlight MAX1578 MAX1579 C2P 22 9 CD2 8 CD1 7 PMPB THIN QFN 4mm x 4mm See Figure 3 for Typical Application Circuit. ________________________________________________________________ Maxim Integrated Products 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 MAX1578/MAX1579 General Description MAX1578/MAX1579 Complete Bias and White LED Power Supplies for Small TFT Displays 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 = +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, VIN = 3V, CTRL = ONBIAS = IN, TA= -40°C to +85°C, typical values are at TA= +25°C, unless otherwise noted. Note 1) PARAMETER CONDITIONS MIN MAX UNITS 5.5 V 2.35 2.6 V 3 5 mA TA = +25°C 0.4 1 TA = +85°C 1 IN Operating Supply Range TYP 2.7 IN Undervoltage-Lockout (UVLO) Threshold Rising edge, 30mV hysteresis IN Quiescent Current Switching IN Shutdown Current VCTRL = VONBIAS = 0V Thermal Shutdown Rising temperature, 20°C hysteresis (typ) 2.1 +160 µA °C MAIN CHARGE PUMP WITH LINEAR REGULATOR Main Pump Efficiency ILOAD = 25mA, VIN = 3.9V 83 VDD Charge-Pump Open-Loop Output Impedance VIN ≥ 3.8V in 1.5x mode 9 20 VIN ≥ 3.0V in 2.0x mode 7.5 20 200 250 300 kHz 5.2 5.5 5.7 V VIN Falling Switchover to 2.0x Mode 3.75 3.85 3.95 V VIN Rising Switchover to 1.5x Mode 3.8 3.9 4.0 V 0.87 1.30 mA 5.0 5.1 V 1 3 kΩ 15.6 19.5 kHz Operating Frequency VDD Output Voltage Charge-pump pause threshold Quiescent Current (Charge Pumps Only) VCTRL = 0V, ONBIAS = IN VMAIN Regulation Voltage 0.1mA < ILOAD < 25mA Discharge Switch Resistance at VMAIN VONBIAS = 0V 4.9 % Ω POS, NEG CHARGE PUMPS Operating Frequency 12.0 Duty Cycle POS Pump Efficiency ILOAD = 100µA % 97 % POS Output Voltage ILOAD = 0 to 100µA 14.7 15.3 V POS Discharge Switch Resistance VONBIAS = 0V 3 6 kΩ NEG Pump Efficiency ILOAD = -100µA 97 2 13.9 50 _______________________________________________________________________________________ % Complete Bias and White LED Power Supplies for Small TFT Displays (Circuit of Figure 3, VIN = 3V, CTRL = ONBIAS = IN, TA= -40°C to +85°C, typical values are at TA= +25°C, unless otherwise noted. Note 1) PARAMETER CONDITIONS NEG Output Voltage ILOAD = 0 to -100µA NEG Discharge Switch Resistance VONBIAS = 0V MIN TYP MAX UNITS -10.2 -9.8 -9.3 V 1.5 3 kΩ 0.72 V LOGIC INPUT (ONBIAS) Logic Input Low Voltage Logic Input High Voltage 1.6 V TA = +25°C 0.01 TA = +85°C 1 Efficiency LOAD = 6 LEDs in series at 20mA 84 OUT Voltage Range (Note 2) Overvoltage-Lockout (OVLO) Threshold VOUT rising, 2V hysteresis 32 VOUT = 32V, VCTRL > 0.24V 10 Input Current 1 µA LED BACKLIGHTING OUT Input Bias Current (VIN VD1) VOUT = VIN, VCTRL = 0 % 32 V 34 36 V 20 32 TA = +25°C 0.01 1 TA = +85°C 0.1 µA ERROR AMPLIFIER CTRL to CS Regulation VCTRL = 1.5V, VIN = 2.7V to 5.5V CS Input Bias Current VCS = VCTRL / 5 CTRL Input Resistance VCTRL < 1.0V CTRL Dual Mode™ Threshold CTRL Shutdown Delay TA = +25°C 0.295 0.300 0.305 TA = -40°C to +85°C 0.292 0.300 0.308 TA = +25°C 0.01 1 TA = +85°C 0.03 MAX1578 V µA 250 500 780 250 500 780 kΩ 5mV hysteresis (Note 3) 100 6.5 185 170 8.2 240 10.5 mV ms CS to COMP Transconductance VCOMP = 1.0V 32 60 90 µS CS Regulation Derating Function Start Temperature VCTRL = 3V, MAX1579 only CS Regulation Derating Function Slope VCTRL = 3V, TA = +65°C, MAX1579 only MAX1579 TA = +25°C TA = +85°C +42 °C -6 mV/°C CS Maximum Brightness Clamp Voltage MAX1578, VCTRL = 3V 310 327 345 MAX1579, VCTRL = 3V, TA = +25°C 322 340 358 CS Maximum Brightness Voltage at CTRL MAX1578 MAX1579 1.635 1.70 mV V OSCILLATOR Operating Frequency Minimum Duty Cycle Maximum Duty Cycle fBOOST 0.8 1.0 PWM mode 12 Pulse skipping 0 CTRL = IN, CS = GND 92 95 1.2 MHz % % Dual Mode is a trademark of Maxim Integrated Products, Inc. _______________________________________________________________________________________ 3 MAX1578/MAX1579 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (Circuit of Figure 3, VIN = 3V, CTRL = ONBIAS = IN, TA= -40°C to +85°C, typical values are at TA= +25°C, unless otherwise noted. Note 1) PARAMETER CONDITIONS MIN TYP MAX UNITS Ω N-CHANNEL SWITCH LX On-Resistance ILX = 190mA LX Leakage Current VLX = 28V, CTRL = GND LX Current Limit Duty cycle = 90% 0.82 1.5 TA = +25°C 0.01 5 TA = +85°C 1 500 700 900 µA mA Note 1: All devices are 100% production tested at TA = +25°C. 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 = +25°C, unless otherwise noted.) MAX1578toc01 4 LEDs 6 LEDs 85 90 ILED = 20mA 85 80 75 EFFICIENCY (%) 80 EFFICIENCY (%) MAX1578toc02 LED EFFICIENCY vs. VIN LED EFFICIENCY vs. ILED 90 8 LEDs 70 2 LEDs 65 ILED = 10mA 75 70 65 60 60 55 55 ILED = 2mA 50 50 0 5 10 15 20 2.5 25 3.0 3.5 LED CURRENT vs. TEMPERATURE 4.5 5.0 5.5 LED CURRENT vs. VCTRL MAX1579 RCS = 13Ω MAX1578toc04 25 MAX1578toc03 30 25 4.0 INPUT VOLTAGE (V) LED CURRENT (mA) 20 ILED (mA) 20 ILED (mA) MAX1578/MAX1579 Complete Bias and White LED Power Supplies for Small TFT Displays 15 MAX1578 RCS = 22Ω 10 10 5 5 0 -40 -15 10 35 TEMPERATURE (°C) 4 15 60 85 0 0 0.5 1.0 1.5 2.0 CTRL VOLTAGE (V) _______________________________________________________________________________________ 2.5 Complete Bias and White LED Power Supplies for Small TFT Displays LED CURRENT vs. DIRECT-PWM DUTY CYCLE MAX1578toc05 20 MAX1578toc06 BOOST-SWITCHING WAVEFORMS 25 VIN AC-COUPLED 20mV/div ILED (mA) 15 VOUT AC-COUPLED 200mV/div 10 5 VLX 10V/div fPWM = 200Hz TO 200kHz 0 0 10 20 30 40 50 60 70 80 90 100 400ns/div PWM DUTY CYCLE (%) BOOST STARTUP AND SHUTDOWN WAVEFORMS VCTRL VIN AC-COUPLED 2V/div VCTRL VIN AC-COUPLED 20mV/div 5V/div VOUT AC-COUPLED 50mV/div IIN 20mA/div 0V 100mA/div 4ms/div 40µs/div LCD BIAS SUPPLY CURRENT vs. INPUT VOLTAGE MAX1578toc09 1200 1000 MAX1578toc10 LCD BIAS STARTUP SEQUENCE 1400 IIN (µA) 2V/div 20mV/div VOUT IIN MAX1578toc08 MAX1578toc07 PWM-DIMMING WAVEFORMS VONBIAS 5V/div 5V/div VMAIN 800 600 5V/div VNEG 400 200 10V/div VPOS VCTRL = 0V 0 0 1 2 3 4 5 10ms/div INPUT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX1578/MAX1579 Typical Operating Characteristics (continued) (Circuit of Figure 3, VIN = 3.6V, ILED = 20mA, 4 LEDs, CTRL = IN, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 3, VIN = 3.6V, ILED = 20mA, 4 LEDs, CTRL = IN, TA = +25°C, unless otherwise noted.) MAIN OUTPUT LOAD REGULATION POS AND NEG LOAD REGULATION -0.5 -1.0 VOLTAGE DROOP (%) -0.1 -0.2 -0.3 -0.4 MAX1578toc12 0 MAX1578toc11 0 VOLTAGE DROOP (%) MAX1578/MAX1579 Complete Bias and White LED Power Supplies for Small TFT Displays -1.5 POS -2.0 -2.5 NEG -3.0 -3.5 -4.0 -0.5 -4.5 -0.6 -5.0 5 0 10 15 20 25 0 LOAD CURRENT (mA) 20 40 60 80 100 LOAD CURRENT (µA) Pin Description 6 PIN NAME FUNCTION 1 MAIN LDO Output and the POS and NEG Charge-Pump Inputs. VMAIN is regulated to 5V. Bypass to GND with a 1µF capacitor. Output is internally discharged with a 1kΩ resistor when VONBIAS = 0V. 2 CU1 POS Charge-Pump Capacitor Connection 1. Connect a 1µF capacitor between CU1 and PMP. 3 CU2 POS Charge-Pump Capacitor Connection 2. Connect a 1µF capacitor between CU2 and PMPB. 4 CU3 POS Charge-Pump Capacitor Connection 3. Connect a 1µF capacitor between CU3 and GND. 5 POS Output of Positive (3x) Charge Pump. Bypass POS to GND with a 1µF capacitor. POS is internally discharged with a 3kΩ resistor when VONBIAS = 0V. 6 PMP Charge-Pump Capacitor Connection. Connect a 1µF capacitor between PMP and CU1 and another 1µF capacitor between PMP and CD1. 7 PMPB 8 CD1 NEG Charge-Pump Capacitor Connection 1. Connect a 1µF capacitor and a 200Ω ±5% resistor in series between CD1 and PMP. 9 CD2 NEG Charge-Pump Capacitor Connection 2. Connect a 1µF capacitor and a 200Ω ±5% resistor in series between CD2 and PMPB. 10 NEG Output of Inverting (-2x) Charge Pump. Bypass NEG to GND with a 1µF capacitor. Output is internally discharged with a 1.5kΩ resistor when VONBIAS = 0V. 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. _______________________________________________________________________________________ Complete Bias and White LED Power Supplies for Small TFT Displays PIN NAME FUNCTION 11 ONBIAS 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. 12 GND 13 COMP 14 CS 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. 15 CTRL 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. 16 OUT 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 0.1µF capacitor. 17 LX 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. 18 PGND 19 C1N Main Charge-Pump Transfer Capacitor Negative Connection 1. Connect a 2.2µF capacitor between C1N and C1P. 20 C2N Main Charge-Pump Transfer Capacitor Negative Connection 2. Connect a 2.2µF capacitor between C2N and C2P. 21 IN Power-Supply Input. Connect to a 2.7V to 5.5V input supply. Bypass IN to GND with a 4.7µF capacitor. 22 C2P Main Charge-Pump Transfer Capacitor Positive Connection 2. Connect a 2.2µF capacitor between C2P and C2N. 23 C1P Main Charge-Pump Transfer Capacitor Positive Connection 1. Connect a 2.2µF capacitor between C1P and C1N. 24 VDD Regulated Main Charge-Pump Output. VDD is regulated to 5.5V. Bypass VDD to GND with a 4.7µF capacitor. VDD is connected to IN when ONBIAS is pulled low. — EP Ground. Connect to PGND and the exposed pad directly under the IC. LED Driver Compensation. Connect a 0.1µF from COMP to GND. CCOMP stabilizes the driver and sets the soft-start time. Power Ground. Connect to GND and the exposed pad directly under the IC. Exposed Paddle. Connect directly to a ground plane, GND, and PGND directly under the IC. _______________________________________________________________________________________ 7 MAX1578/MAX1579 Pin Description (continued) Complete Bias and White LED Power Supplies for Small TFT Displays MAX1578/MAX1579 Block Diagram 1MHz OSCILLATOR IN LX BIAS PWM CONTROL GND N PGND REF OSC OVERVOLTAGE PROTECTION COMP OUT CS CTRL TIMER 170mV CHARGE-PUMP CONTROL ONBIAS SHUTDOWN SEQUENCING C1P OSC MULTIMODE CHARGE PUMP 1.5X/2X DIVIDE BY FOUR C1N C2P C2N OSC VDD PMP DIVIDE BY 64 PMPB 5V LDO MAIN SHUTDOWN CU1 CD2 3X POSITIVE CHARGE PUMP -2X INVERTING CHARGE PUMP CD1 CU2 CU3 NEG N 8 N SHUTDOWN MAX1578 MAX1579 POS SHUTDOWN N _______________________________________________________________________________________ Complete Bias and White LED Power Supplies for Small TFT Displays 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. 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. 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 1kΩ resistor. 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 1µF ceramic 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. POS Charge Pump (+15V Supply) The POS charge pump uses capacitors at CU1, CU2, and CU3 to generate +15V (3 x VMAIN). Connect 1µF 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. ONBIAS VMAIN (5V) VPOS (+15V) VNEG (-10V) Figure 1. Charge-Pump Turn-On Sequence ONBIAS VMAIN (5V) VPOS (+15V) VNEG (-10V) Figure 2. Charge-Pump Turn-Off 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. _______________________________________________________________________________________ 9 MAX1578/MAX1579 Detailed Description MAX1578/MAX1579 Complete Bias and White LED Power Supplies for Small TFT Displays 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: 12µA × t SS CCOMP = 1.25V Applications Information Adjusting LED Current where tSS is the desired soft-start time in seconds. 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 +42°C. Once the temperature reaches +42°C, the maximum VCS declines by 6mV/°C until the minimum 40mV threshold is reached at +100°C. 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. 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 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 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 870µA when the boost converter is shut down and the charge pumps are enabled. 10 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 430µA when the charge pumps are shut down and the boost converter is enabled. 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. 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. 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 10µF cuts input ripple in half. Likewise, an output filter or higher output capacitance value reduces output ripple. ______________________________________________________________________________________ Complete Bias and White LED Power Supplies for Small TFT Displays 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 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) + 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. Chip Information TRANSISTOR COUNT: 3801 PROCESS: BiCMOS VIN(MIN) × 0.8µs 2 ×L 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 × 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 ______________________________________________________________________________________ 11 MAX1578/MAX1579 Component Selection Use only ceramic capacitors with an X5R, X7R, or better dielectric. See Table 1 for a list of recommended components. MAX1578/MAX1579 Complete Bias and White LED Power Supplies for Small TFT Displays Table 1. Recommended Components for the Typical Application Circuit DESIGNATION DESCRIPTION C1, C8 4.7µF, 6.3V X5R ceramic capacitors (0603) Murata GRM188R60J475KE19 C2 0.1µF, 6.3V X5R ceramic capacitor (0402) TDK C1005X5R1A104K C3 0.1µF, 50V X7R ceramic capacitor (0603) TDK C1608X7R1H104K C4, C5, C12 1µF, 16V X7R ceramic capacitors (0805) TDK C2012X7R1C105K L1 22µH INPUT 2.7V TO 5.5V C1 4.7µF PWM OR ANALOG DIMMING C6, C7 C9, C10, C11, C13, C14 D1 D2–D7 1µF, 6.3V X5R ceramic capacitors (0402) Murata GRM155R60J105KE19 40V, 0.5A Schottky diode International Rectifier MBRX0540 White LEDs Nichia NSCW215T L1 22µH, 250mA inductor (1210) Murata LQH32CN220K53 R1 22.1Ω ±1% resistor (0402) R2, R3 200Ω ±5% resistors (0402) LX PGND COMP C2 0.1µF D2–D7 MAX1578 MAX1579 ON OFF CS ONBIAS CU1 C6 2.2µF C1P C2N VDD C8 4.7µF MAIN OUTPUT +5V, 25mA CD1 C13 1µF CU2 MAIN C11 1µF C14 1µF R3 200Ω POS POSITIVE OUTPUT +15V, 100µA C4 1µF CU3 GND C12 1µF NEG C5 1µF Figure 3. Typical Application Circuit 12 R1 22.1Ω R2 200Ω PMPB CD2 C9 1µF C10 1µF PMP C2P C7 2.2µF C3 0.1µF OUT CTRL C1N 2.2µF, 6.3V X5R ceramic capacitors (0603) Taiyo Yuden JMK107BJ225KA OUTPUT UP TO 8 LEDS IN SERIES D1 IN ______________________________________________________________________________________ NEGATIVE OUTPUT -10V, 100µA Complete Bias and White LED Power Supplies for Small TFT Displays 24L QFN THIN.EPS 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 © 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1578/MAX1579 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.)