19-5233; Rev 0; 4/10 TION KIT EVALUA BLE A IL A V A High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN The MAX8848Y/MAX8848Z negative charge pumps drive up to 7 white LEDs with regulated constant current for display backlight applications. By utilizing an inverting charge pump and extremely low-dropout adaptive current regulators, these ICs achieve very high efficiency over the full 1-cell Li+ battery voltage range even with large LED forward voltage mismatch. The 1MHz fixedfrequency switching allows for tiny external components. The regulation scheme is optimized to ensure low EMI and low input ripple. The MAX8848Y/MAX8848Z include thermal shutdown, open- and short-circuit protection. The MAX8848Y/MAX8848Z support independent LED on/off and dimming control. The MAX8848Y dimming ranges are pseudo-logarithmic from 24mA to 0.1mA and off in 32 steps. All devices include a temperature derating function to safely allow bright 24mA full-scale output current while automatically reducing current gradually to protect LEDs at high ambient temperatures above +60NC. Features S Negative 1x/1.5x Charge Pump S Adaptive Current Regulators S Independent Voltage Supply for Each LED S 24mA to 0.1mA Serial-Pulse Dimming (MAX8848Y) S 24mA to 0mA PWM Dimming (MAX8848Z) S 2% (max) LED Current Accuracy, 1% (typ) Matching S Low 120µA Quiescent Current S Low 0.4µA Shutdown Current S Inrush Current Limit S Temperature Derating Function S 16-Pin, 3mm x 3mm Thin QFN Packages Typical Operating Circuit 1µF The MAX8848Y/MAX8848Z are available in 16-pin, 3mm x 3mm thin QFN packages. Applications White LED Backlighting INPUT 2.7V TO 5.5V C1P 1µF C1N C2P 1µF IN NEG 1µF Cellular Phones C2N GND EP PDAs, Digital Cameras, and Camcorders Ordering Information PART MAX8848YETE+T MAX8848ZETE+T DIMMING PIN-PACKAGE Serial pulse/ PWM 16 Thin QFN-EP* PWM 16 Thin QFN-EP* TOP MARK LED ENABLE AND DIMMING CONTROL Note: All devices are specified over the -40°C to +85°C extended temperature range. +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. (ENB) LED1 LED2 LED3 AHQ AHP ENA MAX8848Y MAX8848Z <CPWM> 0.22µF LED4 LED5 LED6 LED7 D1 D2 D3 D4 D5 D6 D7 NOTE: ( ) FOR MAX8848Y, < > FOR MAX8848Z ________________________________________________________________ Maxim Integrated Products 1 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. MAX8848Y/MAX8848Z General Description MAX8848Y/MAX8848Z High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN ABSOLUTE MAXIMUM RATINGS IN to GND..............................................................-0.3V to +6.0V IN to NEG..............................................................-0.3V to +6.0V NEG, C2N to GND...................................................-6V to +0.3V C1P, C2P, CPWM, ENA, ENB to GND........-0.3V to (VIN + 0.3V) C2P to C1N.................................................-0.3V to (VIN + 0.3V) LED_, C1N, C2N, ENA, ENB to NEG..........-0.3V to (VIN + 0.3V) Continuous Power Dissipation (TA = +70°C) 16-Pin Thin QFN Multilayer PCB (derate 20.8mW/°C above +70°C)..........................1666.7mW Junction-to-Case Thermal Resistance (θJC) (Note 1)......10°C/W Junction-to-Ambient Thermal Resistance (θJA) (Note 1) Multilayer PCB...............................................................48°C/W 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 Soldering Temperature (reflow).......................................+260°C Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial. 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 = 3.6V, VGND = 0V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2) PARAMETER CONDITIONS IN Operating Voltage Undervoltage Lockout (UVLO) Threshold MIN VIN rising 2.35 Undervoltage Lockout Hysteresis IN Shutdown Supply Current TYP 2.7 2.45 MAX UNITS 5.5 V 2.55 V 100 VEN_ = 0V, all outputs off TA = +25NC 0.4 TA = +85NC 0.4 mV 2.5 Charge pump inactive, 2 LEDs enabled at 0.1mA setting 120 Charge pump active, 1MHz switching, all LEDs enabled at 0.1mA setting 1.6 mA Thermal Shutdown Threshold +160 NC Thermal Shutdown Hysteresis 20 NC IN Operating Supply Current 150 FA FA PWM DIMMING CONTROL (MAX8848Z) PWM Low-Level Input 0.4 PWM High-Level Input 1.4 EN_ PWM Input Signal Frequency Range CCPWM = 0.22FF PWM Dimming Filter Corner Frequency CCPWM = 0.22FF Current Dimming Range Duty cycle = 0 to 100% PWM Dimming Resolution 1% P duty cycle P 100% V V 0.2 200 2 0 kHz Hz 24 0.24 mA mA/% SERIAL-PULSE LOGIC (MAX8848Y) EN_ Logic Input High Voltage 1.4 V EN_ Logic Input Low Voltage EN_ Logic-Input Current 0.4 VIL = 0V or VIH = 5.5V TA = +25NC TA = +85NC -1 0.01 0.1 2 _______________________________________________________________________________________ +1 V FA High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN (VIN = 3.6V, VGND = 0V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2) PARAMETER CONDITIONS EN_ Low Shutdown Delay tSHDN EN_ tLO See Figure 2 EN_ tHI See Figure 2 Initial EN_ tINIT See Figure 2, first EN_ high pulse MIN TYP 5 8 1 MAX UNITS ms 500 Fs 1 Fs 120 Fs CHARGE PUMP Switching Frequency Soft-Start Time 4.3 1 MHz 0.5 ms 5 V Output Regulation Voltage VIN - VNEG Open-Loop NEG Output Resistance (VNEG - 0.5 x VIN)/INEG 2 NEG Shutdown Discharge Resistance VEN_ = 0V, all outputs off 10 4 I kI LED1–LED7 CURRENT REGULATOR Current Setting Range LED_ Current Accuracy Serial-pulse interface or PWM VLED_= 0.5V for charge pump inactive, VLED_ = -0.9V, VNEG_= -1.4V 0.1 24mA setting, TA = +25NC -2 24mA setting, TA = -40NC to derating function start temperature (Note 3) -5 1.6mA setting, TA = +25NC LED_ Dropout Voltage (Note 4) LED_ Current Regulator Switchover Threshold (Inactive to Active) From derating function start temperature Charge pump inactive, 24mA setting 85 Charge pump active, 24mA setting TA = +25NC 110 TA = +85NC 124 %/NC 125 95 135 150 mV 165 100 All LEDs off % NC -2.5 TA = +25NC TA = +85NC LED_ Current Regulator Switchover Hysteresis LED_ Leakage in Shutdown +5 +60 VLED_ falling mA +2 Q5 Derating Function Start Temperature Derating Function Slope 24.0 Q1 TA = +25NC 0.01 TA = +85NC 0.1 mV mV 5 FA Note 2: Limits are 100% production tested at TA = +25°C. Limits over the operating temperature range are guaranteed by design. Note 3: Guaranteed by design. Not production tested. Note 4: LED dropout voltage is defined as the LED_ to GND voltage at which current into LED_ drops 10% from the value at VLED_ = 0.5V. _______________________________________________________________________________________ 3 MAX8848Y/MAX8848Z ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.) EFFICIENCY vs. Li+ BATTERY VOLTAGE (7 MATCHED LEDs) 70 16.0mA/LED 6.4mA/LED 60 1.6mA/LED 3.0 2.7 3.3 3.6 3.9 80 6.4mA/LED 70 16mA/LED 1.6mA/LED 60 VIN FALLING 20.8mA/LED 90 80 70 16.0mA/LED 60 6.4mA/LED 50 50 4.2 4.2 3.9 3.8 3.7 3.6 3.0 2.7 3.5 3.4 3.0 3.3 3.6 3.9 SUPPLY VOLTAGE (V) Li+ BATTERY VOLTAGE (V, TIME WEIGHTED) SUPPLY VOLTAGE (V) EFFICIENCY vs. Li+ BATTERY VOLTAGE (7 MISMATCHED LEDs) SUPPLY CURRENT vs. SUPPLY VOLTAGE (7 MATCHED LEDs) SUPPLY CURRENT vs. Li+ BATTERY VOLTAGE (7 MATCHED LEDs) 6.4mA/LED 16mA/LED 1.6mA/LED 20.8mA/LED 150 16.0mA/LED 100 60 50 50 0 6.4mA/LED 250 200 20.8mA/LED 150 16.0mA/LED 100 6.4mA/LED 50 1.6mA/LED 4.2 3.9 3.8 3.7 3.6 1.6mA/LED 0 3.0 2.7 3.5 3.4 3.0 4.2 MAX8848Y toc06 200 SUPPLY CURRENT (mA) 80 VIN FALLING SUPPLY CURRENT (mA) 20.8mA/LED 90 70 250 MAX8848Y toc04 100 MAX8848Y toc03 90 100 1.6mA/LED VIN FALLING 50 3.3 3.6 3.9 4.2 4.2 3.9 3.8 3.7 3.6 3.5 3.4 3.0 Li+ BATTERY VOLTAGE (V, TIME WEIGHTED) SUPPLY VOLTAGE (V) Li+ BATTERY VOLTAGE (V, TIME WEIGHTED) SUPPLY CURRENT vs. SUPPLY VOLTAGE (7 MISMATCHED LEDs) INPUT CURRENT vs. Li+ BATTERY VOLTAGE (7 MISMATCHED LEDs) INPUT VOLTAGE RIPPLE (PEAK-TO-PEAK) vs. INPUT VOLTAGE 150 16.0mA/LED 100 6.4mA/LED 50 200 20.8mA/LED 150 16.0mA/LED 100 6.4mA/LED 50 1.6mA/LED 3.3 3.6 SUPPLY VOLTAGE (V) 3.9 20.8mA/LED 30 MAX8848Y 4.2 VIN FALLING 16.0mA/LED 20 6.4mA/LED 10 1.6mA/LED 0 0 3.0 LEDS HAVE MISMATCHED VF 1.6mA/LED 0 2.7 40 INPUT VOLTAGE RIPPLE (mV) 20.8mA/LED MAX8848Y toc08 200 250 SUPPLY CURRENT (mA) VIN FALLING MAX8848Y toc07 250 MAX8848Y toc09 EFFICIENCY PLED/PBATT (%) 20.8mA/LED EFFICIENCY PLED/PBATT (%) 80 MAX8848Y toc02 90 100 EFFICIENCY vs. SUPPLY VOLTAGE (7 MISMATCHED LEDs) MAX8848Y toc05 EFFICIENCY PLED/PBATT (%) 20.8mA/LED EFFICIENCY PLED/PBATT (%) 100 MAX8848Y toc01 EFFICIENCY vs. SUPPLY VOLTAGE (7 MATCHED LEDs) SUPPLY CURRENT (mA) MAX8848Y/MAX8848Z High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN 4.2 3.9 3.8 3.7 3.6 3.5 3.4 3.0 Li+ BATTERY VOLTAGE (V, TIME WEIGHTED) 2.7 3.0 3.3 3.6 INPUT VOLTAGE (V) 4 _______________________________________________________________________________________ 3.9 4.2 High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN LED CURRENT MATCHING vs. SUPPLY VOLTAGE LED CURRENT vs. TEMPERATURE 24mA/LED 25 16.4 LED CURRENT (mA) LED CURRENT (mA) 16.6 MAX8848Y toc11 MAX8848Y 16mA/LED, VIN FALLING 16.8 30 MAX8848Y toc10 17.0 16.2 16.0 15.8 15.6 15.4 20 15 10 5 15.2 15.0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 0 5.5 -40 -15 10 35 60 SUPPLY VOLTAGE (V) TEMPERATURE (°C) 1x MODE OPERATING WAVEFORM (VIN = 4V) 1.5x MODE OPERATING WAVEFORM (VIN = 3V) MAX8848Y toc12 85 MAX8848Y toc13 VIN 100mV/div AC-COUPLED IIN 100mA/div VIN 100mV/div AC-COUPLED IIN 100mA/div 0A 0A IILED1 20mA/div 0A ALL LEDS ON 24mA/LED IILED1 20mA/div 0A ALL LEDS ON 24mA/LED 1µs/div 1µs/div STARTUP AND SHUTDOWN RESPONSE (MAX8848Y) SINGLE-WIRE SERIAL-PULSE DIMMING RESPONSE (MAX8848Y) MAX8848Y toc14 VENA/ENB MAX8848Y toc15 2V/div 200mV/div AC-COUPLED VIN IN 5V/div VENA = VENB 0V 1x MODE ALL LEDS OPERATING TOTAL ILED 50mA/div 200mA/div 0A 0A 20mA/div 0A ILED1 IILED1 ENA = ENB 24mA/LED 2ms/div 20mA/div 0A 10ms/div _______________________________________________________________________________________ 5 MAX8848Y/MAX8848Z Typical Operating Characteristics (continued) (VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.) LINE TRANSIENT RESPONSE (VIN = 3.8V TO 3.4V TO 3.8V) LINE TRANSIENT RESPONSE (VIN = 4.3V TO 3.8V TO 4.3V) MAX8848Y toc17 MAX8848Y toc16 4.3V VIN 3.8V 4.3V 3.8V 1V/div 3.8V 3.4V VIN 1V/div 200mA/div IIN 0A 200mA/div IIN 0A ILED1 ILED1 20mA/div 0A ALL LEDS ON 24mA/LED 20mA/div 0A ALL LEDS ON 24mA/LED 1ms/div 1ms/div LINE TRANSIENT RESPONSE (VIN = 3.5V TO 3.0V TO 3.5V) PWM DIMMING RESPONSE MAX8848Y toc19 MAX8848Y toc18 3.5V VIN 3.0V 3.5V 1V/div 5V/div VENA 100mA/div 0A IIN 100mA/div IIN 0A 12mA 16mA ILED1 ALL LEDS ON 16mA/LED 20mA/div ILED1 20mA/div 0A MAX8848Z 40µs/div 1ms/div PWM DIMMING LINEARITY MAX8848Y toc20 25 20 LED CURRENT (mA) MAX8848Y/MAX8848Z High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN 15 10 5 MAX8848Z 0 0 20 40 60 80 100 PWM DUTY CYCLE (%) 6 _______________________________________________________________________________________ High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN LED5 LED4 LED3 LED2 TOP VIEW 12 11 10 9 LED6 13 8 LED1 LED7 14 7 NEG 6 C1N 5 C2N MAX8848Y MAX8848Z CPWM (ENB) 15 EP 2 3 4 C2P IN 1 C1P + GND ENA 16 THIN QFN ( ) FOR MAX8848Y ONLY Pin Description PIN NAME FUNCTION 1 IN Supply Voltage Input. The input voltage range is 2.7V to 5.5V. Bypass IN to GND with a 1FF ceramic capacitor as close as possible to the IC. IN is high impedance during shutdown. Connect IN to the anodes of all the LEDs. 2 2 GND Ground. Connect GND to system ground and the input bypass capacitor as close as possible to the IC. 3 3 C1P Transfer Capacitor 1 Positive Connection. Connect a 1FF ceramic capacitor from C1P to C1N. 4 4 C2P Transfer Capacitor 2 Positive Connection. Connect a 1FF ceramic capacitor from C2P to C2N. 5 5 C2N Transfer Capacitor 2 Negative Connection. Connect a 1FF ceramic capacitor from C2P to C2N. An internal 10kI resistor pulls C2N to GND during shutdown. 6 6 C1N Transfer Capacitor 1 Negative Connection. Connect a 1FF ceramic capacitor from C1P to C1N. 7 7 NEG Charge-Pump Negative Output. Connect a 1FF ceramic capacitor from NEG to GND. In shutdown, an internal 10kI resistor pulls NEG to GND. Connect the exposed pad to NEG directly under the IC. MAX8848Y MAX8848Z 1 _______________________________________________________________________________________ 7 MAX8848Y/MAX8848Z Pin Configuration MAX8848Y/MAX8848Z High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN Pin Description (continued) PIN MAX8848Y 8–14 MAX8848Z 8–14 NAME FUNCTION LED1–LED7 LED Current Regulators. Current flowing into LED_ is based on the ENA/ENB input. Connect LED_ to the cathodes of the external LEDs. LED_ is high impedance during shutdown. Short any unused LED_ to IN prior to power-up to disable the corresponding current regulator. 15 — ENB Enable and Serial-Pulse Dimming Control Input B for the MAX8848Y. ENB controls LED6 and LED7. Drive ENB high to turn on the LED6 and LED7 current regulators at 24mA. Drive ENB low for greater than 8ms to turn off the current regulators or drive both ENA and ENB low to place the IC in shutdown. In addition to the enable function, ENB can also be used to control the LED6 and LED7 serial-pulse dimming. — 15 CPWM Filter Capacitor Connection for PWM Dimming for the MAX8848Z. Connect a capacitor from CPWM to GND to form a filter with the internal 360kI resistor. The recommended capacitor for a 2Hz corner frequency is 0.22FF. Enable and PWM/Serial-Pulse Dimming Control Input A. ENA controls LED1– LED5 for the MAX8848Y, and LED1–LED7 for MAX8848Z. Drive ENA high to turn on all the controlled LED current regulators at 24mA. Drive ENA low for greater than 8ms to turn off the current regulators or drive both ENA and ENB low to place the IC in shutdown. Drive ENA with a PWM signal from 200Hz to 200kHz to dim LED1–LED7 for the MAX8848Z. See the PWM Dimming Control (MAX8848Z) section. For the MAX8848Y, ENA controls the LED1–LED5 serial-pulse dimming. 16 16 ENA — — EP Exposed Paddle. Connect EP to NEG directly under the IC. 8 _______________________________________________________________________________________ High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN C1P VIN 2.7V TO 5.5V C3 1µ F C1N IN C2P C2N NEG INVERTING CHARGE PUMP C1 1 µF FB GND 1MHz OSCILLATOR C4 1µF EP SEL MIN VIN CURRENT REGULATOR ENABLE AND DIMMING CONTROL INPUTS ENA LED ENABLE LED1 (ENB) CURRENT REGULATOR PWM DIMMING LED2 <CPWM> C25 0.22µF 360kI CURRENT REGULATOR LED3 CURRENT SOURCE CONTROL CURRENT REGULATOR LED4 THERMAL DERATING AND PROTECTION CURRENT REGULATOR LED5 CURRENT REGULATOR LED6 MAX8848Y MAX8848Z CURRENT REGULATOR LED7 NOTE: () FOR MAX8848Y ONLY, < > FOR MAX8848Z ONLY Figure 1. Functional Diagram and Application Circuit _______________________________________________________________________________________ 9 MAX8848Y/MAX8848Z C2 1µ F MAX8848Y/MAX8848Z High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN Detailed Description The MAX8848Y/MAX8848Z have an inverting charge pump and seven current regulators capable of 24mA each to drive up to 7 white LEDs. The current regulators are matched to within 1% (typ) providing uniform white LED brightness for LCD backlight applications. To maximize efficiency, the current regulators operate with as little as 0.15V voltage drop. Individual white LED current regulators conduct current to GND or NEG to extend usable battery life. In the case of mismatched forward voltage of white LEDs, only the white LEDs requiring higher voltage are switched to pull current to NEG instead of GND, further raising efficiency and reducing battery current drain. Current Regulator Switchover When VIN is higher than the LED forward voltage plus the 150mV dropout voltage of the current regulator, the LED current returns through GND. If this condition is satisfied for all active white LEDs, the charge pump remains inactive. When the input voltage drops so that the current regulator voltage (VLED_) cannot be maintained for any of the individual white LEDs, the inverting charge pump activates and generates a voltage on NEG that is no greater than 5V below VIN. For any current regulator that is detected at the switchover threshold voltage of 150mV (typ, VIN falling), internal circuitry switches that current regulator’s return path from GND to NEG to provide enough voltage across that regulator to overcome dropout. When VLED_ rises to 250mV (typ), the return of that current regulator is switched back from NEG to GND. Each current regulator is independently monitored to detect when switchover is required. Since the LED current is switched for only the individual LED current regulators requiring higher voltage, power consumption is minimized. Enable and Dimming Control Input (ENA, ENB) ENA and ENB inputs have dual functions: LED on/off control and PWM or serial-pulse dimming control. See Table 1 for details. For the MAX8848Y, ENA functions as on/off control and serial-pulse dimming control for LED1– LED5. ENB functions as on/off control and serial-pulse dimming control for LED6 and LED7. For the MAX8848Z, only ENA functions as on/off control as well as PWM dimming control for LED1–LED7. PWM Dimming Control (MAX8848Z) When VIN is above its undervoltage lockout threshold, UVLO, apply a PWM signal to ENA to set the corresponding WLED current (see Table 1) that is proportional to the signal duty cycle (0% duty cycle corresponds to zero LED current and 100% duty cycle corresponds to full LED current). The allowed PWM frequency range is from 200Hz to 200kHz. If PWM dimming control is not required, ENA works as a simple on/off control. Serial-Pulse Dimming Control (MAX8848Y) The MAX8848Y uses ENA/ENB as a serial-pulse control interface to program the intensity of LED1–LED7. When the LEDs are enabled by driving ENA/ENB high, the MAX8848Y ramps LED current to 24mA. Subsequent pulses on ENA/ENB reduce the LED current from 24mA to 0.1mA in 31 steps. After the current reaches 0.1mA, the next pulse restores the current to 24mA. See Table 2 Table 1. ENA and ENB Enable and Dimming Control PART ENA MAX8848Y LED1–LED5 enable and serial-pulse dimming control MAX8848Z LED1–LED7 enable and PWM dimming control ENB LED6 and LED7 enable and serial-pulse dimming control — 10 ������������������������������������������������������������������������������������� High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN If dimming control is not required, ENA/ENB works as a simple on/off logic control. Drive ENA/ENB high for 0 1 2 3 4 at least 120Fs to enable the LED current regulators, or drive ENA/ENB low for greater than 8ms (typ) to place the LED current regulators in shutdown. The LED current regulators operate at 100% brightness and off under these conditions. 5 26 27 28 29 30 31 INITIAL tHI tINIT > 120µs EN_ 24mA ILED_ tLO 1µs TO 500µs 22.4mA 20.8mA 19.2mA 17.6mA tSHDN tHI > 1µs 24mA 22.4mA 8ms (TYP) 16.0mA 0.6mA 0.5mA 0.4mA 0.3mA 0.2mA 0.1mA SHDN SHDN Figure 2. Timing Charateristics for LED Serial-Pulse Dimming Control Table 2. ENA/ENB Serial-Pulse Dimming Count and Programmed LED_ Currents EN_ PULSE COUNT PROGRAMMED LED_ CURRENT (mA) EN_ PULSE COUNT PROGRAMMED LED_ CURRENT (mA) Startup or EN_high 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 24.0 22.4 20.8 19.2 17.6 16.0 14.4 12.8 11.2 9.6 8.0 6.4 5.6 4.8 4.0 3.2 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 2.8 2.4 2.0 1.6 1.4 1.2 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 24.0 ______________________________________________________________________________________ 11 MAX8848Y/MAX8848Z for the LED current values and the corresponding ENA/ ENB pulse count. Figure 2 shows a timing diagram for ENA/ENB. MAX8848Y/MAX8848Z High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN Low LED Current Levels The MAX8848Y internally generates a PWM signal to obtain higher resolution at lower currents. See the SingleWire Serial-Pulse Dimming Response (MAX8848Y) graph in the Typical Operating Characteristics section. When the LED current is set below 6.4mA, the IC adjusts not only LED DC current, but the duty cycle that is controlled by the PWM signal. The frequency of the PWM dimming signal is set at 16kHz with a minimum duty cycle of 1/8 to avoid the LED flickering effect to human eyes and also to avoid interference in the audio frequency range. Table 3 shows the current level and the corresponding duty cycle. Shutdown Mode The MAX8848Y/MAX8848Z are in shutdown mode when both ENA and ENB are held low for 8ms or longer. In shutdown, NEG is pulled to GND with a 10kΩ internal resistor. Temperature Derating Function The MAX8848Y/MAX8848Z contain a derating function that automatically limits the LED current at high temperatures in accordance with the recommended derating curve of popular white LEDs. The derating function enables the safe usage of higher LED current at room temperature, thus reducing the number of LEDs required to backlight the display. The derating circuit lowers the LED current at approximately 2.5%/°C once the die temperature is above +60°C. The typical derating function characteristic is shown in the Typical Operating Characteristics. Table 3. Internal PWM Duty Cycle vs. LED Set Current ILED (mA) DUTY MAXIMUM CYCLE ILED (mA)* (n/8) ILED (mA) MAXIMUM ILED (mA)* DUTY CYCLE (n/8) 6.4 6.4 8 1.2 1.6 6 5.6 6.4 7 1.0 1.6 5 4.8 6.4 6 0.8 0.8 8 4.0 6.4 5 0.7 0.8 7 3.2 3.2 8 0.6 0.8 6 2.8 3.2 7 0.5 0.8 5 2.4 3.2 6 0.4 0.8 4 2.0 3.2 5 0.3 0.8 3 1.6 1.6 8 0.2 0.8 2 1.6 7 0.1 0.8 1 1.4 *Maximum ILED is the full reference current when the internal PWM signal has 100% duty cycle at the lower level currents. Power-Up LED Short Detection and Open-Fault Protection The MAX8848Y/MAX8848Z contain special circuitry to detect short-circuit conditions at power-up and disable the corresponding current regulator to avoid wasting battery current. Connect any unused LED_ to IN to disable the corresponding current regulator. If an LED fails short-circuit detection after startup, the current regulator continues the current regulated operation until IC power is cycled and the short circuit is detected during the subsequent startup. An open-circuit LED failure drives the voltage on the corresponding LED current regulator output below the switchover threshold, enabling the negative charge pump. Thermal Shutdown The MAX8848Y/MAX8848Z include a thermal-limit circuit that shuts down the IC above approximately +160°C. The IC turns on after it cools by approximately 20°C. Applications Information Input Ripple For LED drivers, input ripple is more important than output ripple. The amount of input ripple depends on the source supply’s output impedance. Add a lowpass filter to the input of the MAX8848Y/MAX8848Z to further reduce input ripple. Alternatively, increasing CIN from 1.0µF to 2.2µF (or 4.7µF) cuts input ripple in half (or in fourth) with only a small increase in footprint. Capacitor Selection Ceramic capacitors are recommended due to their small size, low cost, and low ESR. Select ceramic capacitors that maintain their capacitance over temperature and DC bias. Capacitors with X5R or X7R temperature characteristics generally perform well. Recommended values are shown in the Typical Operating Circuit. Using a larger value input capacitor helps to reduce input ripple (see the Input Ripple section). PCB Layout and Routing The MAX8848Y/MAX8848Z are high-frequency switchedcapacitor voltage inverters. For best circuit performance, use a solid ground plane and place all capacitors as close as possible to the IC. Use large traces for the power-supply inputs to minimize losses due to parasitic trace resistance and to route heat away from the device. Refer to the MAX8848Z evaluation kit data sheet for an example PCB layout. 12 ������������������������������������������������������������������������������������� High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN PROCESS: BiCMOS Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 16 Thin QFN-EP T1633+5 21-0136 ______________________________________________________________________________________ 13 MAX8848Y/MAX8848Z Chip Information MAX8848Y/MAX8848Z High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN Revision History REVISION NUMBER REVISION DATE 0 4/10 DESCRIPTION Initial release PAGES CHANGED — 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. 14 © 2010 Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.