19-0865; Rev 0; 8/07 KIT ATION EVALU E L B AVAILA Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package The MAX8901A/MAX8901B step-up converters drive from two to six series-connected white LEDs (WLEDs) with constant current to provide uniform WLED intensity for LCD backlighting in cell phones, PDAs, and other handheld devices. The MAX8901_ operate at a fixed 750kHz (typ) switching frequency, allowing for tiny external components, and are optimized for the highest possible efficiency over the full 1-cell Li+/Li-Poly battery range. These converters use a single input (ON) to enable the IC and to control WLED intensity. The MAX8901A requires a direct PWM input to regulate WLED intensity with the WLED current proportional to the PWM duty cycle. The MAX8901B uses single-wire, serial-pulse dimming that reduces the WLED intensity in 32 linear steps. Full-scale WLED current for serial-pulse dimming is 24.75mA (MAX8901B, 0.75mA/step). The MAX8901_ features an internal soft-start to eliminate inrush currents during startup, input overvoltage protection, WLED overvoltage protection, and a shutdown mode with 0.01µA (typ) shutdown current. No WLED current is present in shutdown provided the WLED forward voltage is greater than the input supply voltage. Additional features include undervoltage lockout (UVLO) and thermal shutdown. The MAX8901_ are available in tiny 8-pin, 2mm x 2mm TDFN-EP packages (0.8mm max height). Applications Display Backlight (from 2 to 6 WLEDs) Features ♦ High Efficiency, Up to 91% ♦ 2.6V to 5.5V Input Voltage Range ♦ Fixed-Frequency Operation ♦ Supplies from 2 to 6 WLEDs with 1% LED Current Accuracy ♦ Flexible Dimming Control Direct PWM Dimming (MAX8901A) 32-Step, 1-Wire Serial Dimming (MAX8901B) ♦ Input Undervoltage Lockout ♦ Input Overvoltage Lockout ♦ WLED Overvoltage Protection (25V typ) ♦ 0.01µA (typ) Shutdown Current ♦ No WLED Current in Shutdown ♦ Internal Soft-Start and Thermal Shutdown Ordering Information PIN-PACKAGE PKG CODE TOP MARK MAX8901AETA+T 8 TDFN-EP* 2mm x 2mm T822-1 +ABA MAX8901BETA+T 8 TDFN-EP* 2mm x 2mm T822-1 +ABB PART Note: All devices are specified over the -40°C to +85°C operating temperature range. +Denotes a lead-free package. *EP = Exposed paddle. T = Tape and reel. Cellular Phones PDAs and Smartphones MP3 and Portable Media Players Portable Navigation Devices Pin Configuration appears at end of data sheet. Digital Cameras Typical Operating Circuits INPUT 2.6V TO 5.5V INPUT 2.6V TO 5.5V 3 LX IN 7 OVP GND PGND PWM DIMMING CONTROL 2 3 LX IN 4 OUTPUT UP TO 25V MAX8901A COMP 6 4 OUTPUT UP TO 25V ON EP CS MAX8901B 1 7 5 8 SERIAL-PULSE DIMMING CONTROL 6 2 OVP COMP GND PGND ON EP CS 1 5 8 ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com. 1 MAX8901A/MAX8901B General Description MAX8901A/MAX8901B Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package ABSOLUTE MAXIMUM RATINGS IN to GND .................................................................-0.3V to +7V CS, COMP, ON to GND ...............................-0.3V to (VIN + 0.3V) OVP, LX to GND .....................................................-0.3V to +28V PGND to GND .......................................................-0.3V to +0.3V LX Current ..................................................................770mARMS Continuous Power Dissipation (multilayer board at +70°C) 8-pin, 2mm x 2mm TDFN (derate above +70°C by 11.9mW/°C) ..........................953mW 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 (VIN = VON = VOVP = 3.6V, VPGND = VGND = 0V, COMP, CS, and LX are unconnected, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1) PARAMETER Input Voltage Input Undervoltage Lockout Threshold Input Overvoltage Lockout Threshold Shutdown Input Current Quiescent Current Output Voltage Range OVP Overvoltage Protection Threshold OVP Input Current CS Regulation Voltage SYMBOL CONDITIONS MAX UNITS 2.6 5.5 V VIN rising 2.25 2.50 VIN falling 2.20 2.53 VIN rising 6.2 6.3 VIN falling 6.0 6.3 VIN VIN_UVLO VIN_OVLO ISHDN IQ VON = 0V VCS = 0.55V, no load (not switching) MIN TA = +25°C 0.01 TA = +85°C 0.1 MAX8901A 70 135 MAX8901B 115 185 VOUT VDIODE = external boost diode voltage drop VOV 6 LEDs with 25V OVP option IOV VOVP = 20V VCS VIN VDIODE VOV V 26 V 0.50 0.525 V 1.33 1.50 ms 20 ON High Voltage VON_HI 2.6V < VIN < 5.5V 1.3 ON Low Voltage VON_LO 2.6V < VIN < 5.5V ON = IN µA V 0.4 TA = +25°C 0.01 TA = +85°C 0.1 1 V µA First ON high pulse to enable IC (MAX8901B) (Figure 1) 40 µs µs ON High Pulse Width tHI MAX8901B (Figure 1) 0.5 ON Low Pulse Width tLO MAX8901B (Figure 1) 0.5 2 µA 0.475 1.18 tHI_INIT µA No dimming Time VON is below low threshold until shutdown (Figure 1) Initial ON High Pulse Width V 25 tSHDN ION 1 V 24 ON Shutdown Delay ON Input Current TYP _______________________________________________________________________________________ 500.0 µs Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package (VIN = VON = VOVP = 3.6V, VPGND = VGND = 0V, COMP, CS, and LX are unconnected, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1) PARAMETER SYMBOL Serial Dimming Full-Scale LED Current Accuracy CONDITIONS ILED = 24.75mA (MAX8901B) PWM Frequency for PWM Dimming Control fPWM MAX UNITS TA = +25°C MIN -1 +1 % TA = -40°C to +85°C -3 +3 % MAX8901A 100 CS to COMP Transconductance 40 tSOFT- Soft-Start Interval START TYP CCOMP = 0.022µF (Figure 1) Thermal Shutdown kHz 60 80 µS 10 ms 160 °C OSCILLATOR Operating Frequency fSW Maximum Duty Cycle DMAX VCS = 0.4V ILXLKG VLX = 27V 700 750 90 92 800 kHz % n-CHANNEL SWITCH LX Leakage Current n-Channel Switch On-Resistance RLX n-Channel Current Limit ILIM TA = +25°C 0.1 TA = +85°C 0.1 0.63 5 µA 0.7 1.4 Ω 0.70 0.77 A Note 1: Specifications to -40°C are guaranteed by design and characterization and are not production tested. STEP tHI_INIT > 40μs 1 0 2 3 4 5 28 29 30 31 32 33 ON tHI tSOFT-START FULL tLO 500ns TO 500μs 32/33 31/33 30/33 29/33 ILED_ tSHDN 32/33 1.33ms (typ) 28/33 6/33 SHUTDOWN FULL > 500ns 5/33 4/33 3/33 2/33 1/33 SHUTDOWN Figure 1. MAX8901B Timing Diagram _______________________________________________________________________________________ 3 MAX8901A/MAX8901B ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1µF, C2 = 1µF, TA = +25°C, unless otherwise noted.) LED EFFICIENCY vs. INPUT VOLTAGE 3 WLEDs 4 WLEDs 80 6 WLEDs 85 75 3 WLEDs 80 6 WLEDs 75 INDUCTOR: TOKO 1069AS-220M DIODE: CENTRAL SEMI CMDSH05-4 INDUCTOR: TOKO 1098AS-100M DIODE: NXP PMEG3005EB 70 70 3 4 5 6 3 4 5 6 INPUT VOLTAGE (V) LED EFFICIENCY vs. LED CURRENT (6 WLEDs) LED EFFICIENCY vs. LED CURRENT (6 WLEDs) 85 80 VIN = 3.6V 75 VIN = 4.2V 90 VIN = 3V 70 65 VIN = 5V VIN = 5.5V 85 80 EFFICIENCY (%) VIN = 5.5V VIN = 5V MAX8901A/B toc03 90 VIN = 4.2V VIN = 3.6V 75 VIN = 3V 70 65 60 60 INDUCTOR: TOKO 1069AS-220M DIODE: CENTRAL SEMI CMDSH05-4 55 55 INDUCTOR: TOKO 1098AS-100M DIODE: NXP PMEG3005EB 50 50 5 10 15 20 0 25 5 10 15 20 LED CURRENT (mA) LED CURRENT (mA) LED EFFICIENCY vs. LED CURRENT (4 WLEDs) LED EFFICIENCY vs. LED CURRENT (4 WLEDs) VIN = 5V VIN = 5.5V 85 VIN = 4.2V VIN = 3.6V 80 75 VIN = 3V 70 90 VIN = 5V 65 VIN = 5.5V 85 80 EFFICIENCY (%) 90 MAX8901A/B toc05 0 VIN = 4.2V 75 25 MAX8901A/B toc06 EFFICIENCY (%) 2 INPUT VOLTAGE (V) MAX8901A/B toc04 2 VIN = 3.6V VIN = 3V 70 65 60 60 INDUCTOR: TOKO 1069AS-220M DIODE: CENTRAL SEMI CMDSH05-4 55 55 INDUCTOR: TOKO 1098AS-100M DIODE: NXP PMEG3005EB 50 50 0 5 10 15 LED CURRENT (mA) 4 MAX8901A/B toc02 4 WLEDs EFFICIENCY (%) 85 EFFICIENCY (%) LED EFFICIENCY vs. INPUT VOLTAGE 90 MAX8901A/B toc01 90 EFFICIENCY (%) MAX8901A/MAX8901B Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package 20 25 0 5 10 15 20 LED CURRENT (mA) _______________________________________________________________________________________ 25 Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package LED AND BOOST EFFICIENCY vs. INPUT VOLTAGE IN CURRENT (IIN) vs. SUPPLY VOLTAGE 90 MAX8901A/B toc08 BOOST MAX8901B 700 600 LED MAX8901A 500 85 IIN (μA) EFFICIENCY (%) 800 MAX8901A/B toc07 95 80 400 300 200 75 6WLEDs, INDUCTOR: TOKO A915AY-220M, DIODE: CENTRAL SEMI CMDSH05-4 70 2 3 4 5 100 0 2 6 3 4 5 6 7 SUPPLY VOLTAGE (V) INPUT VOLTAGE (V) INPUT OVERVOLTAGE PROTECTION RESPONSE SWITCHING WAVEFORMS MAX8901A/B toc9 MAX8901A/B toc10 50mV/div AC-COUPLED VI 6.4V VIN 5.4V VOUT VLX 1V/div (AC-COUPLED) 10V/div 10V/div 0V 100mV/div AC-COUPLED VOUT 0V ILEDs 20mA/div 0mA 100mA/div ILX 0mA 20mA/div VLX 1μs/div 0V 20ms/div STARTUP AND SHUTDOWN RESPONSE (MAX8901B) STARTUP AND SHUTDOWN RESPONSE (MAX8901A) MAX8901A/B toc12 MAX8901A/B toc11 C2 = 0.1μF VON 2V/div 0V 2V/div 0V VON 10V/div 10V/div VOUT 0V VOUT 0V 100mA/div 100mA/div ISUPPLY 0mA ILEDs 20mA/div 0mA 10ms/div ISUPPLY 0mA 20mA/div ILEDs 0mA 10ms/div _______________________________________________________________________________________ 5 MAX8901A/MAX8901B Typical Operating Characteristics (continued) (Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1µF, C2 = 1µF, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1µF, C2 = 1µF, TA = +25°C, unless otherwise noted.) PWM DIMMING RESPONSE (MAX8901A) SERIAL PULSE DIMMING RESPONSE (MAX8901B) MAX8901A/B toc13 MAX8901A/B toc14 2V/div VON D = 90% D = 10% VIN VCS 2V/div 0V VON 0V 50mV/div (AC-COUPLED) VIN 50mV/div (AC-COUPLED) 500mV/div VCS 1V/div 0V 0V VOUT VOUT 20mV/div fON_PWM = 100kHz, DUTY CYCLE CHANGE: 90% TO 10%, C2 = 0.1μF ILEDs 0V ILEDs 20mA/div 20mA/div 0mA 4ms/div 10ms/div LED CURRENT vs. PWM DIMMING (MAX8901A) VCS vs. LED CURRENT STEP (MAX8901B) ILED = 24.75mA 500.8 500.6 VCS (mV) 20 15 MAX8901A/B toc16 501.0 MAX8901A/B toc15 25 ILED (mA) 20mV/div 0V 0mA 30 500.4 500.2 500.0 10 499.8 5 499.6 RCS = 20Ω, fPWM = 100kHz 0 20 40 60 80 ILED = 0.75mA 499.4 100 0 5 10 15 20 25 30 PWM DUTY CYCLE (%) LED CURRENT STEP MAXIMUM LED CURRENT vs. TEMPERATURE (MAX8901B) MINIMUM LED CURRENT vs. TEMPERATURE (MAX8901B) 750 MAX8901A/B toc17 25.0 24.8 745 ILED (μA) 24.6 35 MAX8901A/B toc18 0 ILED (mA) MAX8901A/MAX8901B Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package 740 24.4 735 24.2 730 24.0 -40 -15 10 35 TEMPERATURE (°C) 6 60 85 -40 -15 10 35 60 85 TEMPERATURE (°C) _______________________________________________________________________________________ Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package PIN NAME 1 OVP FUNCTION WLED Overvoltage Protection Input. OVP monitors voltage at the WLEDs. Connect OVP to the positive terminal of the output capacitor. If an OVP condition is detected, the MAX8901_ latches off. Cycle VIN or toggle VON to restart the IC. Enable and Dimming Control Input. Drive ON high to enable the IC. Drive ON low for greater than 1.33ms (typ) to shutdown the WLED current regulator. 2 ON For the MAX8901A: After VIN is above VUVLO and ON is driven high, the MAX8901A enters soft-start and increases the WLED current to full brightness. Apply a minimum 30kHz (500kHz max) PWM signal to ON to adjust the WLED brightness from 100% to off, proportional to the duty cycle of the PWM signal. See the PWM Dimming Control (MAX8901A) section for more details. For the MAX8901B: After ON is driven high for 40µs (min), the MAX8901B enters soft-start and increases the WLED current to full brightness. Subsequent pulses on ON cause the MAX8901B WLED current to decrease in 32 equal steps. See the Serial-Pulse Dimming Control (MAX8901B) section for more details. IN Power Supply Input. Bypass IN to GND with a 1µF ceramic capacitor placed as close as possible to the IC. If VIN exceeds the input overvoltage lockout threshold (6.5V max, VIN rising), the IC stops switching and no WLED current flows (if the forward voltage of the WLED string is greater than VIN). When VIN falls below the overvoltage lockout hysteresis level (6.0V min, VIN falling), soft-start is initiated and normal operation resumes. 4 LX Boost Inductor Node. Connect an inductor between IN and LX. LX is high impedance in shutdown. 5 PGND 6 GND 3 7 COMP Power Ground. Connect to GND and the exposed paddle (EP) with a short, wide trace. Analog Ground. Connect GND to the exposed paddle with a short, wide trace. WLED Boost Compensation Node. Connect a 0.022µF ceramic capacitor from COMP to GND. CCOMP stabilizes the converter and sets the soft-start time. COMP discharges to GND when in shutdown. 8 CS WLED Current Sense Input. For the MAX8901A, connect a current-sense resistor from CS to GND. Voltage sensed at CS regulates the WLED current. For the MAX8901B, do not connect a sense resistor from CS to GND. The MAX8901B provides an internal current source from CS to GND to program the WLED current. The MAX8901B regulates VCS to 0.5V (typ) for all ILED. The MAX8901A regulates VCS to 0.5V (typ) for maximum duty cycle only. — EP Exposed Metal Paddle. Connect EP to GND. For good thermal dissipation, the exposed paddle must be connected to a large ground plane. _______________________________________________________________________________________ 7 MAX8901A/MAX8901B Pin Description MAX8901A/MAX8901B Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A GND REF THERMAL SHUTDOWN IC POWER BOOST CONTROL LOGIC INTERNAL 750kHz OSC IN LX SHUTDOWN TIMER PGND OVP CS Gm D*0.5V PWM COMP ON EP Figure 2. MAX8901A Block Diagram Detailed Description The high efficiency and tiny size of the MAX8901A/ MAX8901B WLED step-up converters make them ideally suited for driving LCD backlighting in cell phones, PDAs, and other portable and handheld devices. The MAX8901_ drive from 2 to 6 series-connected WLEDs with constant current for uniform WLED intensity and are optimized for the highest possible efficiency (up to 91%) over the full 1-cell Li+/Li-Poly battery range. These tiny 2mm x 2mm devices operate at a constant 750kHz 8 switching frequency, and use small external components to achieve minimal input and output ripple while occupying the smallest possible footprint. These converters use a single input (ON) to enable the IC and to control WLED intensity. The MAX8901A requires a direct PWM input to modulate WLED intensity, with WLED current proportional to the PWM duty cycle. The MAX8901B uses single-wire, serial-pulse dimming that reduces the WLED intensity in 32 equal steps. Full-scale WLED current for serial-pulse dimming is 24.75mA. _______________________________________________________________________________________ Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B MAX8901B GND REF THERMAL SHUTDOWN IC POWER BOOST CONTROL LOGIC INTERNAL 750kHz OSC SHUTDOWN TIMER IN LX PGND OVP CS Gm 0.5V PULSE DIMMING COMP ON EP Figure 3. MAX8901B Block Diagram The MAX8901_ features: • Internal soft-start that gradually illuminates the WLEDs to eliminate inrush current during startup. • Input overvoltage protection (6.5V max) that prevents IC switching when exceeded. • WLED overvoltage protection (25V typ) that latches off the IC. • Shutdown mode that reduces current to 0.01µA (typ). No WLED current is present in shutdown or during an overvoltage condition if the WLED forward voltage is greater than the input supply voltage. Figures 2 and 3 show the block diagrams for the MAX8901A and MAX8901B, respectively. Input Overvoltage Lockout When V IN exceeds 6.2V (min), input overvoltage lockout (OVLO) is engaged to protect the MAX8901_ from high-input voltage conditions. Once input OVLO occurs, the MAX8901_ stops switching and no WLED current flows, provided the forward voltage of the WLED string is greater than VIN. When VIN falls below 6V (min), the input OVLO condition is cleared, and the IC is enabled and enters soft-start. WLED Overvoltage Protection WLED overvoltage protection (OVP) occurs when the WLED output voltage rises above the WLED OVP threshold. The WLED OVP protection circuitry latches off the IC and the IC enters shutdown when the WLED OVP threshold is exceeded. After OVP is engaged, cycle VIN or toggle ON to reenable the IC and enter soft-start. _______________________________________________________________________________________ 9 MAX8901A/MAX8901B Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package Startup and Soft-Start PWM Dimming Control (MAX8901A) The MAX8901_ use a single input (ON) to enable the IC and to control WLED intensity. After ON is driven high, the MAX8901_ enter soft-start and gradually increase the WLED current to full brightness. When the MAX8901_ is first turned on, C COMP is charged to 1.25V with a 60µA current source. When VCOMP rises above 1.25V, CCOMP is charged with a 4µA current source, and the internal MOSFET begins switching at a reduced duty cycle. When VCOMP rises above 2.25V, the duty cycle is at its maximum. The VCOMP where the IC exits soft-start depends on the final duty cycle required by the load. Typical shutdown timing characteristics are shown in the Typical Operating Characteristics section. A PWM signal (MAX8901A) or serial pulses (MAX8901B) are applied to dim the WLEDs. See the PWM Dimming Control (MAX8901A) and Serial-Pulse Dimming Control (MAX8901B) sections for more details. After VIN is above UVLO, apply a PWM signal to ON for a WLED current that is proportional to the PWM signal’s duty cycle (0% duty cycle corresponds to zero LED current and 100% duty cycle corresponds to full LED current). Restrict PWM frequency to between 30kHz to 500kHz for a maximum ILED current accuracy. If dimming control is not required, ON works as a simple on/off control. Drive ON high to enable the IC, or drive ON low for shutdown. A resistor connected from CS to GND programs maximum ILED. See the Current-Sense Resistor (MAX8901A Only) section for more details. Shutdown The MAX8901_ enter shutdown when VON is held low for more than 1.33ms (typ), or when an output overvoltage condition is engaged. In shutdown, the supply current is reduced to 0.01µA (typ) by powering down the entire IC except for the ON voltage-detection circuitry. C COMP is discharged to GND during shutdown, allowing the device to reinitiate soft-start when it is re-enabled. Although the internal n-channel MOSFET does not switch in shutdown, there is still a DC current path between the input and the WLEDs through the inductor and Schottky diode. In shutdown, the minimum forward voltage of the WLED string must exceed the maximum input voltage to ensure that the WLEDs remain off. Typically, with two or more WLEDs, the forward voltage is large enough to keep leakage current low, less than 1µA (typ). Shutdown timing characteristics are shown in the Typical Operating Characteristics. Serial-Pulse Dimming Control (MAX8901B) After the MAX8901B is enabled by driving ON high for the minimum initial ON high pulse width (40µs, min), soft-start is engaged and brings WLED current to maximum brightness. After soft-start is completed, dim the MAX8901B by pulsing ON low (500ns to 500µs pulse width). Each pulse reduces the WLED current by 0.75mA. The maximum WLED current is 24.75mA for the MAX8901B (0.75mA/step). The overall dimming range for serial-pulse dimming control is from maximum WLED current down to 1/33 of maximum WLED current. The WLEDs are turned off by holding ON low for at least 1.33ms (typ). See Figure 1 for the serial-pulse dimming control timing diagram. Applications Information Inductor Selection Recommended inductor values range from 10µH to 47µH. A 22µH inductor optimizes the efficiency for most applications while maintaining low input voltage ripple. With input voltages near 5V, a larger value of inductance may 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: Thermal Shutdown The MAX8901 includes a thermal shutdown feature that protects the IC by turning it off when the die temperature reaches +160°C (typ). After thermal shutdown occurs, the MAX8901_ must be restarted by toggling the ON pin low, then high, or by cycling the input voltage. 10 IPEAK = VOUT(MAX) × ILED(MAX) 900 x VIN(MIN) + VIN(MIN) x 1.2 where ILED(MAX) is in mA and L is in µH. ______________________________________________________________________________________ 2xL Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package Current-Sense Resistor (MAX8901A Only) The MAX8901A uses a sense resistor (RCS) connected from CS to GND to program the maximum WLED current for 100% PWM duty cycle. The MAX8901A regulates VCS to 0.5V (typ) for 100% duty cycle. Calculate RCS (in ohms) using the following equation: IDIODE (RMS) = ILED × IPEAK RCS = 500 ILED(MAX) where I LED(MAX) is the maximum WLED current in milliamps. Maximum WLED current is programmed to 25mA using a 20Ω resistor. PCB Layout Capacitor Selection Ceramic capacitors with X5R, X7R, or better dielectric are recommended for stable operation over the entire operating temperature range. The exact values of input (C1) and output (C2) capacitors are not critical (see Figures 4 and 5). The typical value for the input capacitor is 1µF, and the typical value for the output capacitor is 0.1µF (MAX8901A) or 1µF (MAX8901B). Higher value capacitors can reduce input and output ripple, but at the expense of size and higher cost. The compensation capacitor (C3) stabilizes the converter and controls soft-start. The compensation capacitor is typically chosen to be 0.022µF for most applications. Due to fast switching waveforms and high-current paths, careful attention is required when the PCB layout is designed. Minimize trace lengths between the IC and the inductor, the diode, the input capacitor, and the output capacitor. For the MAX8901A, minimize the trace length between the IC and R CS. Keep traces short, direct, and wide. Keep noisy traces, such as the LX node trace, away from CS. The input bypass capacitor (CIN) should be placed as close as possible to the IC. The ground connections of CIN and COUT should be as close together as possible. PGND and GND should be connected together at the input capacitor ground terminal. Refer to the MAX8901 EV Kit for an example layout. ______________________________________________________________________________________ 11 MAX8901A/MAX8901B Schottky Diode Selection The high switching frequency of the MAX8901_ demands a high-speed rectification diode for optimum efficiency. 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 rating exceeds the average output current and peak inductor current. In addition, the diode’s reverse breakdown voltage must exceed VOUT. The RMS diode current can be approximated from the following equation: MAX8901A/MAX8901B Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package L1 22μH INPUT 2.6V TO 5.5V C1 1μF 3 LX IN 4 C2 0.1μF MAX8901A 7 C3 0.022μF 6 COMP OVP GND PGND PWM DIMMING CONTROL 2 ON EP OUTPUT UP TO 25V D1 CS 1 D2–D7 5 8 R1 (RCS) 20Ω Figure 4. MAX8901A Application Circuit L1 22μH INPUT 2.6V TO 5.5V C1 1μF 3 7 LX IN COMP SERIAL-PULSE DIMMING CONTROL OVP 6 2 PGND GND ON EP OUTPUT UP TO 25V D1 C2 1μF MAX8901B R1 0Ω C3 0.022μF 4 CS 1 5 8 Figure 5. MAX8901B Application Circuit 12 ______________________________________________________________________________________ Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package Chip Information PROCESS: BiCMOS TOP VIEW CS 8 COMP GND PGND 7 6 5 MAX8901 *EP + 1 2 3 4 OVP ON IN LX TDFN-EP (2mm × 2mm) *EXPOSED PAD ______________________________________________________________________________________ 13 MAX8901A/MAX8901B Pin Configuration 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.) 8L TDFN EXPOSED PADS.EPS MAX8901A/MAX8901B Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package PACKAGE OUTLINE 6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm 21-0168 14 ______________________________________________________________________________________ D 1 2 Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package COMMON DIMENSIONS SYMBOL A MIN. MAX. 0.70 0.80 D 1.90 2.10 E 1.90 2.10 A1 0.00 0.05 L 0.20 0.40 k 0.25 MIN. A2 0.20 REF. PACKAGE VARIATIONS PKG. CODE N D2 E2 e b r [(N/2)-1] x e T622-1 6 0.90±0.10 1.60±0.10 0.65 TYP. 0.30±0.05 0.150 1.30 REF T822-1 8 0.70±0.10 1.30±0.10 0.50 TYP. 0.25±0.05 0.125 1.50 REF T822-2 8 0.80±0.10 1.20±0.10 0.50 TYP. 0.25±0.05 0.125 1.50 REF PACKAGE OUTLINE 6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm 21-0168 D 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 ____________________ 15 © 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc. MAX8901A/MAX8901B Package Information (continued) (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.)