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19-5196; Rev 0; 4/10
TION KIT EVALUA BLE VAILA A
High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN
General Description
The MAX8847Y/MAX8847Z negative charge pumps drive up to 6 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 MAX8847Y/MAX8847Z include thermal shutdown, open- and short-circuit protection. The MAX8847Y/MAX8847Z support independent LED on/off and dimming control. The MAX8847Z has PWM dimming control for LED1-LED6. The MAX8847Y has PWM dimming control for LED1-LED4 and serial-pulse dimming control for LED5 and LED6. The serial-pulse 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 fullscale output current while automatically reducing current gradually to protect LEDs at high ambient temperatures above +60C. The MAX8847Y/MAX8847Z are available in 16-pin, 3mm x 3mm thin QFN packages.
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 (MAX8847Y) S 24mA to 0mA PWM Dimming (MAX8847Z) S 2% (max) LED Current Accuracy, 1% (typ) Matching S Low 120A Quiescent Current S Low 0.4A Shutdown Current S Inrush Current Limit S Temperature Derating Function S 16-Pin, 3mm x 3mm Thin QFN Packages
Features
MAX8847Y/MAX8847Z
Typical Operating Circuit
1F 1F
INPUT 2.7V TO 5.5V 1F
C1P IN GND
C1N
C2P
C2N 1F NEG EP LED1 D1
Applications
White LED Backlighting Cellular Phones PDAs, Digital Cameras, and Camcorders
LED ENABLE AND DIMMING CONTROL ENA ENB
MAX8847Y MAX8847Z
LED2
D2
Ordering Information
PART MAX8847YETE+T MAX8847ZETE+T DIMMING Serial pulse/ PWM PWM PIN-PACKAGE 16 Thin QFN-EP* 16 Thin QFN-EP* TOP MARK AHQ AHP
CPWM 0.22F
LED3
D3
LED4
D4
LED5
D5
Note: All devices are specified over the -40C to +85C extended temperature range. +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad.
LED6
D6
_______________________________________________________________ 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.
High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN MAX8847Y/MAX8847Z
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 = +70C) 16-Pin Thin QFN Multilayer PCB (derate 20.8mW/C above +70C)..........................1666.7mW Junction-to-Case Thermal Resistance (JC) (Note 1) .....10C/W Junction-to-Ambient Thermal Resistance (JA) (Note 1) Multilayer PCB..............................................................48C/W Operating Temperature Range .......................... -40C to +85C Junction Temperature .....................................................+150C Storage Temperature Range............................ -65C to +150C Lead Temperature (soldering, 10s) ................................+300C Soldering Temperature (reflow) ......................................+260C
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 = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 2) PARAMETER IN Operating Voltage Undervoltage Lockout (UVLO) Threshold Undervoltage Lockout Hysteresis IN Shutdown Supply Current VEN_ = 0V, all outputs off TA = +25NC TA = +85NC VIN rising CONDITIONS MIN 2.7 2.35 2.45 100 0.4 0.4 120 1.6 +160 20 0.4 1.4 CCPWM = 0.22FF CCPWM = 0.22FF Duty cycle = 0 to 100% 1% P duty cycle P 100% 1.4 0.4 VIL = 0V or VIH = 5.5V TA = +25NC TA = +85NC -1 0.01 0.1 +1 0 0.24 0.2 2 24 200 150 2.5 TYP MAX 5.5 2.55 UNITS V V mV FA FA mA NC NC V V kHz Hz mA mA/% V V FA
Charge pump inactive, 2 LEDs enabled at 0.1mA setting IN Operating Supply Current Thermal Shutdown Threshold Thermal Shutdown Hysteresis PWM DIMMING CONTROL PWM Low-Level Input PWM High-Level Input EN_ PWM Input Signal Frequency Range PWM Dimming Filter Corner Frequency Current Dimming Range PWM Dimming Resolution EN_ Logic Input High Voltage EN_ Logic Input Low Voltage EN_ Logic-Input Current Charge pump active, 1MHz switching, all LEDs enabled at 0.1mA setting
SERIAL-PULSE LOGIC (MAX8847Y LED5 and LED6 only)
2
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, VGND = 0V, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 2) PARAMETER EN_ Low Shutdown Delay tSHDN EN_ tLO EN_ tHI Initial EN_ tINIT CHARGE PUMP Switching Frequency Soft-Start Time Output Regulation Voltage Open-Loop NEG Output Resistance NEG Shutdown Discharge Resistance Current Setting Range VIN - VNEG (VNEG - 0.5 x VIN)/INEG VEN_ = 0V, all outputs off 4.3 1 0.5 5 2 10 4 MHz ms V I kI See Figure 2 See Figure 2 See Figure 2, first EN_ high pulse CONDITIONS MIN 5 1 1 120 TYP 8 500 MAX UNITS ms Fs Fs Fs
MAX8847Y/MAX8847Z
LED1-LED6 CURRENT REGULATOR Serial-pulse interface or PWM VLED_= 0.5V for charge pump inactive, VLED_ = -0.9V, VNEG_= -1.4V 24mA setting, TA = +25NC 24mA setting, TA = -40NC to derating function start temperature (Note 3) 1.6mA setting, TA = +25NC 0.1 -2 -5 Q5 +60 From derating function start temperature Charge pump inactive, 24mA setting Charge pump active, 24mA setting VLED_ falling TA = +25NC TA = +85NC TA = +25NC TA = +85NC 135 -2.5 85 95 110 124 150 165 mV 125 mV NC %/NC Q1 24.0 +2 +5 % mA
LED_ Current Accuracy
Derating Function Start Temperature Derating Function Slope LED_ Dropout Voltage (Note 4) LED_ Current Regulator Switchover Threshold (Inactive to Active) LED_ Current Regulator Switchover Hysteresis LED_ Leakage in Shutdown All LEDs off TA = +25NC TA = +85NC
100 0.01 0.1 5
mV FA
Note 2: Limits are 100% production tested at TA = +25C. 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.
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3
High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN MAX8847Y/MAX8847Z
Typical Operating Characteristics
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. SUPPLY VOLTAGE (6 MATCHED LEDs)
MAX8847Y toc01
EFFICIENCY vs. Li+ BATTERY VOLTAGE (6 MATCHED LEDs)
MAX8847Y toc02
EFFICIENCY vs. SUPPLY VOLTAGE (6 MISMATCHED LEDs)
MAX8847Y toc03
100 20.8mA/LED EFFICIENCY PLED /PBATT (%) 90 80 70 6.4mA/LED 60 1.6mA/LED 50 2.7 3.0 3.3 3.6 3.9 VIN FALLING 16.0mA/LED
100
20.8mA/LED EFFICIENCY PLED /PBATT (%)
100 90 80 70 16.0mA/LED 60 50 1.6mA/LED 2.7 3.0 6.4mA/LED 3.3 3.6 VIN FALLING 3.9 20.8mA /LED
80
16mA/LED
70
1.6mA/LED
6.4mA/LED
60 50
4.2
4.2 3.9
3.8
3.7
3.6
3.5 3.4 3.0
EFFICIENCY PLED /PBATT (%)
90
4.2
SUPPLY VOLTAGE (V)
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
SUPPLY VOLTAGE (V)
EFFICIENCY vs. Li+ BATTERY VOLTAGE (6 MISMATCHED LEDs)
MAX8847Y toc04
SUPPLY CURRENT vs. SUPPLY VOLTAGE (6 MATCHED LEDs)
VIN FALLING SUPPLY CURRENT (mA) 150 20.8mA/LED 100 16.0mA/LED
MAX8847Y toc05
SUPPLY CURRENT vs. Li+ BATTERY VOLTAGE (6 MATCHED LEDs)
MAX8847Y toc06
100 20.8mA/LED EFFICIENCY PLED /PBATT (%) 90 80 70 1.6mA/LED 60 50 4.2 3.9 3.8 3.7 3.6 6.4mA/LED 16mA/LED
200
200
SUPPLY CURRENT (mA)
150 20.8mA /LED 100 16mA /LED
50
6.4mA/LED 1.6mA/LED
50
6.4mA /LED 1.6mA /LED
0
0
2.7 3.0 3.3 3.6 3.9 4.2
3.5 3.4 3.0
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 (6 MISMATCHED LEDs)
VIN FALLING SUPPLY CURRENT (mA) 150 20.8mA/LED 100 16.0mA/LED
MAX8847Y toc07
SUPPLY CURRENT vs. Li+ BATTERY VOLTAGE (6 MISMATCHED LEDs)
MAX8847Y toc08
200
200
SUPPLY CURRENT (mA)
150 20.8mA /LED 100 16mA /LED
50
6.4mA/LED 1.6mA/LED
50
6.4mA /LED 1.6mA /LED
0 2.7 3.0 3.3 3.6 3.9 4.2 SUPPLY VOLTAGE (V)
0 4.2 3.9 3.8 3.7 3.6 3.5 3.4 3.0 Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
4
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN
Typical Operating Characteristics (continued)
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX8847Y/MAX8847Z
INPUT VOLTAGE RIPPLE (PEAK TO PEAK) vs. INPUT VOLTAGE
MAX8847Y toc09
LED CURRENT MATCHING vs. SUPPLY VOLTAGE
17.6 17.2 LED CURRENT (mA) 16.8 16.4 16.0 15.6 15.2 14.8 14.4 14.0 MAX8847Z 16mA/LED, VIN FALLING
MAX8847Y toc10
30 25
20.8mA/LED
18.0
INPUT VOLTAGE RIPPLE (mV)
MAX8847Z VIN FALLING
16.0mA/LED 20 15 10 5 0 2.7 3.0 3.3 3.6 3.9 4.2 INPUT VOLTAGE (V) 1.6mA/LED LEDS HAVE MISMATCHED VF 6.4mA/LED
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
SUPPLY VOLTAGE (V)
LED CURRENT vs. TEMPERATURE
MAX8847Y toc11
30 25 LED CURRENT (mA) 20 15 10 5 24mA/LED 0 -40 -15 10 35 60
1x MODE OPERATING WAVEFORM (VIN = 4V)
MAX8847Y toc12
VIN
50mV/div AC-COUPLED 100mA /div
IIN 0A ILED1
85
ALL LEDS ON 24mA /LED 1s/div
20mA /div 0A
TEMPERATURE (C)
1.5x MODE OPERATING WAVEFORM (VIN = 3V)
VIN
STARTUP AND SHUTDOWN RESPONSE (MAX8847Y)
MAX8847Y toc14
MAX8847Y toc13
100mV/div AC-COUPLED
VENA/ENB
ENA = ENB 24mA/LED
2V/div
100mA/div IIN 0A 20mA /div ILED1 1s/div ALL LEDS ON 24mA/LED 0A
VIN
200mV/div AC-COUPLED
IIN
100mA/div 0A 20mA /div 0A 20ms/div
ILED1
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5
High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN MAX8847Y/MAX8847Z
Typical Operating Characteristics (continued)
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25C, unless otherwise noted.)
SINGLE-WIRE SERIAL-PULSE DIMMING RESPONSE (MAX8847Y)
MAX8847Y toc15
LINE TRANSIENT RESPONSE (VIN = 4.3V TO 3.8V TO 4.3V)
5V/div 0V 4.3V VIN 3.8V
MAX8847Y toc16
VENB
4.3V 1V/div
96mA TOTAL ILED 96mA MAX8847Y, 1x MODE ALL LEDS OPERATING 24mA FOR LED1-LED4 50mA /div 0A 20mA /div 0A 10ms/div 1ms/div ILED1 ALL LEDS ON 24mA/LED IIN 100mA /div 0A 20mA /div 0A
ILED5
LINE TRANSIENT RESPONSE (VIN = 3.8V TO 3.4V TO 3.8V)
VIN 3.8V 3.4V
MAX8847Y toc17
LINE TRANSIENT RESPONSE (VIN = 3.5V TO 3.0V TO 3.5V)
1V/div VIN 200mA /div 3.5V
MAX8847Y toc18
3.8V
3.0V
3.5V
1V/div
IIN
0A 24mA
IIN
200mA /div 0A
ILED1 ALL LEDS ON 24mA/LED 1ms/div
20mA /div 0A
ILED1 ALL LEDS ON 16mA/LED 1ms/div
20mA /div 0A
PWM DIMMING LINEARITY
MAX8847Y toc19 MAX8847Y toc20
PWM DIMMING RESPONSE
25 20
VENA
LED CURRENT (mA)
5V/div
15 10 5
IIN 12mA
100mA /div 0A
ILED1
20mA /div 40s/div
0 0 20 40 60 80 100 PWM DUTY CYCLE (%)
6
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN
Pin Configuration
LED5 LED4 LED3 LED2
MAX8847Y/MAX8847Z
TOP VIEW
12
11
10
9
LED6 13 ENB 14 CPWM 15 ENA 16
8 7
LED1 NEG C1N C2N
MAX8847Y MAX8847Z
+ EP
6 5
1 IN
2 GND
3 C1P
4 C2P
THIN QFN
Pin Description
PIN 1 2 3 4 5 6 7 NAME IN GND C1P C2P C2N C1N NEG FUNCTION 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. Ground. Connect GND to system ground and the input bypass capacitor as close as possible to the IC. Transfer Capacitor 1 Positive Connection. Connect a 1FF ceramic capacitor from C1P to C1N. Transfer Capacitor 2 Positive Connection. Connect a 1FF ceramic capacitor from C2P to 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. Transfer Capacitor 1 Negative Connection. Connect a 1FF ceramic capacitor from C1P to C1N. 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. 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. Enable or Serial-Pulse Dimming Control Input B. ENB controls LED5 and LED6. For the MAX8847Z, ENB functions as on/off control for LED5 and LED6. For the MAX8847Y, except on/off control function, ENB can also be used to control the LED5 and LED6 serial-pulse dimming. Drive ENB high to turn on the LED5 and LED6 current regulators at 24mA. Drive ENB low for greater than 8ms to turn off the LED5 and LED6 current regulators or drive both ENA and ENB low to place the IC in shutdown. For the MAX8847Y LED5 and LED6 serial-pulse diming control, see the Serial-Pulse Dimming Control (MAX8847Y) section for details.
8-13
LED1-LED6
14
ENB
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN MAX8847Y/MAX8847Z
Pin Description (continued)
PIN 15 NAME CPWM FUNCTION Filter Capacitor Connection for PWM Dimming. 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 or PWM Dimming Control Input A. For the MAX8847Y, ENA functions as on/off control for LED1-LED4 and PWM dimming control for LED1-LED4. For the MAX8847Z, ENA functions as on/ off control for LED1-LED4 and PWM dimming control for LED1-LED6. Drive ENA high to turn on the LED1-LED4 current regulators at 24mA each. Drive ENA low for greater than 8ms to turn off the LED1- LED4 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 the LEDs. See the PWM Dimming Control section. Exposed Paddle. Connect EP to NEG directly under the IC.
16
ENA
--
EP
Detailed Description
The MAX8847Y/MAX8847Z have an inverting charge pump and six current regulators capable of 24mA each to drive up to 6 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. 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.
Current Regulator Switchover
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 MAX8847Y, ENA functions as an on/off control and PWM dimming control for LED1- LED4. ENB functions as on/off control and serial-pulse dimming control for LED5 and LED6. For the MAX8847Z, ENA functions as an on/off control for LED1-LED4 as well as PWM dimming control for LED1-LED6. ENB is used for on/off control for LED5 and LED6.
Enable and Dimming Control Input (ENA, ENB)
Table 1. ENA and ENB Enable and Dimming Control
PART MAX8847Y MAX8847Z ENA LED1-LED4 enable and PWM dimming control LED1-LED4 enable and LED1-LED6 PWM dimming control ENB LED5 and LED6 enable and serial-pulse dimming control LED5 and LED6 enable control
8
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN MAX8847Y/MAX8847Z
C2 1F C3 1F
C1P VIN 2.7V TO 5.5V C1 1F GND IN
C1N
C2P
C2N
INVERTING CHARGE PUMP FB 1MHz OSCILLATOR EP SEL MIN CURRENT REGULATOR
NEG C4 1F
VIN
ENABLE AND DIMMING CONTROL INPUTS
ENA LED ENABLE ENB CURRENT REGULATOR PWM DIMMING CPWM LED1
LED2
C25 0.22F
360kI
CURRENT REGULATOR
LED3 CURRENT SOURCE CONTROL
CURRENT REGULATOR
LED4
THERMAL DERATING AND PROTECTION
CURRENT REGULATOR
LED5
CURRENT REGULATOR
MAX8847Y MAX8847Z
LED6
Figure 1. Functional Diagram and Application Circuit
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9
High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN MAX8847Y/MAX8847Z
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.
PWM Dimming Control
MAX8847Y ramps LED5 and LED6 current to 24mA. Subsequent pulses on ENB reduces the LED5 and LED6 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 for the LED current values and the corresponding ENB pulse count. Figure 2 shows a timing diagram for ENB. If dimming control is not required, ENB works as a simple on/off logic control. Drive ENB high for at least 120s to enable the LED5 and LED6 current regulators, or drive ENB low for greater than 8ms (typ) to place the LED5 and LED6 current regulators in shutdown. The LED current regulators operate at 100% brightness and off under these conditions.
Serial-Pulse Dimming Control (MAX8847Y)
The MAX8847Y uses ENB as a serial-pulse control interface to program the intensity of LED5 and LED6. When LED5 and LED6 are enabled by driving ENB high, the
0 ENB INITIAL tHI tINIT > 120s 24mA ILED5/ILED6 SHDN
1
2
3
4
5
26
27
28
29
30
31
22.4mA 20.8mA 19.2mA 17.6mA 16.0mA
tLO 1s TO 500s
tHI > 1s
24mA
22.4mA
tSHDN 8ms (TYP)
0.6mA 0.5mA 0.4mA 0.3mA 0.2mA 0.1mA
SHDN
Figure 2. Timing Charateristics for LED Serial-Pulse Dimming Control
Table 2. ENB Serial-Pulse Dimming Count and Programmed LED_ Currents
ENB PULSE COUNT Startup or ENB high 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 10 PROGRAMMED LED_ CURRENT (mA) 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 ENB PULSE COUNT 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 PROGRAMMED LED_ CURRENT (mA) 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
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN
The MAX8847Y internally generates a PWM signal to obtain higher resolution at lower currents. See the SingleWire Serial-Pulse Dimming Response (MAX8847Y) 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. The MAX8847Y/MAX8847Z 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 10k internal resistor. The MAX8847Y/MAX8847Z 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 +60C. The typical derating function characteristic is shown in the Typical Operating Characteristics.
Low LED Current Levels
The MAX8847Y/MAX8847Z 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. The MAX8847Y/MAX8847Z include a thermal-limit circuit that shuts down the IC above approximately +160C. The IC turns on after it cools by approximately 20C.
Power-Up LED Short Detection and Open-Fault Protection
MAX8847Y/MAX8847Z
Shutdown Mode
Thermal Shutdown
Temperature Derating Function
Applications Information
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 MAX8847Y/MAX8847Z to further reduce input ripple. Alternatively, increasing CIN from 1.0F to 2.2F (or 4.7F) cuts input ripple in half (or in fourth) with only a small increase in footprint. 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). The MAX8847Y/MAX8847Z 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.
Input Ripple
Capacitor Selection
Table 3. Internal PWM Duty Cycle vs. LED Set Current
ILED (mA) 6.4 5.6 4.8 4.0 3.2 2.8 2.4 2.0 1.6 1.4 DUTY MAXIMUM CYCLE ILED (mA)* (n/8) 6.4 6.4 6.4 6.4 3.2 3.2 3.2 3.2 1.6 1.6 8 7 6 5 8 7 6 5 8 7 ILED (mA) 1.2 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 MAXIMUM ILED (mA)* 1.6 1.6 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 DUTY CYCLE (n/8) 6 5 8 7 6 5 4 3 2 1
PCB Layout and Routing
*Maximum ILED is the full reference current when the internal PWM signal has 100% duty cycle at the lower level currents.
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN MAX8847Y/MAX8847Z
Chip Information
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 16 Thin QFN-EP PACKAGE CODE T1633+5 DOCUMENT NO. 21-0136
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High-Performance Negative Charge Pump for 6 White LEDs in 3mm x 3mm Thin QFN
Revision History
REVISION NUMBER 0 REVISION DATE 4/10 Initial release DESCRIPTION PAGES CHANGED --
MAX8847Y/MAX8847Z
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.
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(c)
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2010 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.

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