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19-5233; Rev 0; 4/10
TION KIT EVALUA BLE VAILA A
High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN
General Description
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. The MAX8848Y/MAX8848Z 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 (MAX8848Y) S 24mA to 0mA PWM Dimming (MAX8848Z) 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
MAX8848Y/MAX8848Z
Typical Operating Circuit
1F 1F
Applications
White LED Backlighting Cellular Phones PDAs, Digital Cameras, and Camcorders
INPUT 2.7V TO 5.5V 1F
C1P IN GND
C1N
C2P
C2N 1F NEG EP LED1 D1
Ordering Information
PART MAX8848YETE+T MAX8848ZETE+T DIMMING Serial pulse/ PWM PWM PIN-PACKAGE 16 Thin QFN-EP* 16 Thin QFN-EP* TOP MARK AHQ AHP
LED ENABLE AND DIMMING CONTROL
ENA (ENB)
MAX8848Y MAX8848Z
LED2
D2
LED3 0.22F LED4
D3
D4
Note: All devices are specified over the -40C to +85C extended temperature range. +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad.
LED5
D5
LED6
D6
LED7
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.
High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
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 (MAX8848Z) 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 (MAX8848Y)
2
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High-Performance Negative Charge Pump for 7 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
MAX8848Y/MAX8848Z
LED1-LED7 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 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
Typical Operating Characteristics
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. SUPPLY VOLTAGE (7 MATCHED LEDs)
MAX8848Y toc01
EFFICIENCY vs. Li+ BATTERY VOLTAGE (7 MATCHED LEDs)
MAX8848Y toc02
EFFICIENCY vs. SUPPLY VOLTAGE (7 MISMATCHED LEDs)
VIN FALLING 20.8mA/LED
MAX8848Y toc03
100
100 20.8mA/LED EFFICIENCY PLED/PBATT (%) 90 80 70 60 50 6.4mA/LED 1.6mA/LED 16mA/LED
100 90 80 70 60 50
20.8mA/LED
EFFICIENCY PLED/PBATT (%) 90 80 70 60 50 2.7 3.0 3.3 3.6
16.0mA/LED 6.4mA/LED 1.6mA/LED VIN FALLING
3.9 4.2
EFFICIENCY PLED/PBATT (%)
16.0mA/LED 6.4mA/LED 1.6mA/LED
4.2 3.9
3.8
3.7
3.6
3.5 3.4 3.0
2.7
3.0
3.3
3.6
3.9
4.2
SUPPLY VOLTAGE (V)
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
SUPPLY VOLTAGE (V)
EFFICIENCY vs. Li+ BATTERY VOLTAGE (7 MISMATCHED LEDs)
MAX8848Y toc04
SUPPLY CURRENT vs. SUPPLY VOLTAGE (7 MATCHED LEDs)
MAX8848Y toc05
SUPPLY CURRENT vs. Li+ BATTERY VOLTAGE (7 MATCHED LEDs)
MAX8848Y toc06
100 20.8mA/LED EFFICIENCY PLED/PBATT (%) 90 80 70 60 50 4.2 3.9 3.8 3.7 3.6 6.4mA/LED 1.6mA/LED 16mA/LED
250 200 SUPPLY CURRENT (mA) 150 100 50 0
VIN FALLING
250 200 SUPPLY CURRENT (mA) 150 100 50 0
20.8mA/LED 16.0mA/LED
20.8mA/LED 16.0mA/LED
6.4mA/LED 1.6mA/LED
6.4mA/LED 1.6mA/LED
3.5 3.4 3.0
2.7
3.0
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)
MAX8848Y toc07
INPUT CURRENT vs. Li+ BATTERY VOLTAGE (7 MISMATCHED LEDs)
MAX8848Y toc08
INPUT VOLTAGE RIPPLE (PEAK-TO-PEAK) vs. INPUT VOLTAGE
LEDS HAVE MISMATCHED VF 20.8mA/LED
INPUT VOLTAGE RIPPLE (mV) 30
MAX8848Y toc09
250 200 SUPPLY CURRENT (mA) 150 100 50 0 2.7 3.0 3.3 3.6
VIN FALLING
250 200 SUPPLY CURRENT (mA) 150 100 50 0
40
MAX8848Y 16.0mA/LED VIN FALLING
20.8mA/LED 16.0mA/LED
20.8mA/LED 16.0mA/LED
20
6.4mA/LED
10
6.4mA/LED 1.6mA/LED
3.9 4.2
6.4mA/LED 1.6mA/LED
1.6mA/LED
0 3.6 3.5 3.4 3.0 2.7 3.0 3.3 3.6 3.9 4.2
4.2 3.9
3.8
3.7
SUPPLY VOLTAGE (V)
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
INPUT VOLTAGE (V)
4
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High-Performance Negative Charge Pump for 7 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.)
LED CURRENT MATCHING vs. SUPPLY VOLTAGE
MAX8848Y toc10
MAX8848Y/MAX8848Z
LED CURRENT vs. TEMPERATURE
24mA/LED 25 LED CURRENT (mA) 20 15 10 5 0 -40 -15 10 35 60 85 TEMPERATURE (C)
MAX8848Y toc11 MAX8848Y toc13
17.0 16.8 16.6 LED CURRENT (mA) 16.4 16.2 16.0 15.8 15.6 15.4 15.2 15.0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 MAX8848Y 16mA/LED, VIN FALLING
30
5.5
SUPPLY VOLTAGE (V)
1x MODE OPERATING WAVEFORM (VIN = 4V)
MAX8848Y toc12
1.5x MODE OPERATING WAVEFORM (VIN = 3V)
100mV/div AC-COUPLED
VIN
VIN
100mV/div AC-COUPLED 100mA/div
IIN
IIN 100mA/div 0A
0A IILED1 20mA/div 0A 1s/div
IILED1
ALL LEDS ON 24mA/LED 1s/div
20mA/div 0A
ALL LEDS ON 24mA/LED
STARTUP AND SHUTDOWN RESPONSE (MAX8848Y)
MAX8848Y toc14
SINGLE-WIRE SERIAL-PULSE DIMMING RESPONSE (MAX8848Y)
MAX8848Y toc15
VENA/ENB
2V/div 200mV/div AC-COUPLED
VENA = VENB 1x MODE ALL LEDS OPERATING TOTAL ILED
5V/div 0V
VIN
50mA/div 0A
IN
200mA/div 0A ILED1 ENA = ENB 24mA/LED 2ms/div
IILED1
20mA/div 0A 10ms/div
20mA/div 0A
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5
High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
Typical Operating Characteristics (continued)
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25C, unless otherwise noted.)
LINE TRANSIENT RESPONSE (VIN = 4.3V TO 3.8V TO 4.3V)
4.3V VIN 3.8V
LINE TRANSIENT RESPONSE (VIN = 3.8V TO 3.4V TO 3.8V)
3.8V VIN
MAX8848Y toc16
MAX8848Y toc17
4.3V 1V/div
3.4V
3.8V 1V/div
IIN
200mA/div 0A IIN 200mA/div 0A 20mA/div 0A 1ms/div ILED1 20mA/div 0A 1ms/div
ILED1
ALL LEDS ON 24mA/LED
ALL LEDS ON 24mA/LED
LINE TRANSIENT RESPONSE (VIN = 3.5V TO 3.0V TO 3.5V)
3.5V VIN 3.0V
PWM DIMMING RESPONSE
MAX8848Y toc19
MAX8848Y toc18
3.5V 1V/div VENA 5V/div
IIN
100mA/div 0A
IIN 12mA
100mA/div 0A
16mA ILED1 ALL LEDS ON 16mA/LED 1ms/div 20mA/div 0A ILED1
20mA/div MAX8848Z 40s/div
PWM DIMMING LINEARITY
MAX8848Y toc20
25 20
LED CURRENT (mA)
15 10 5
MAX8848Z
0 0 20 40 60 80 100 PWM DUTY CYCLE (%)
6
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High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN
Pin Configuration
TOP VIEW
MAX8848Y/MAX8848Z
LED5
LED4
LED3 10
12
11
LED2 9
LED6 13 LED7 14 CPWM (ENB) 15 ENA 16
8 7
LED1 NEG C1N C2N
MAX8848Y MAX8848Z
+ EP
6 5
1 IN
2 GND
3 C1P
4 C2P ( ) FOR MAX8848Y ONLY
THIN QFN
Pin Description
PIN MAX8848Y 1 MAX8848Z 1 NAME 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.
IN
2 3 4 5 6
2 3 4 5 6
GND C1P C2P C2N C1N
7
7
NEG
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7
High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
Pin Description (continued)
PIN MAX8848Y MAX8848Z NAME FUNCTION 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 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. 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. Exposed Paddle. Connect EP to NEG directly under the IC.
8-14
8-14
LED1-LED7
15
--
ENB
--
15
CPWM
16
16
ENA
--
--
EP
8
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High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
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 LED1
LED2
C25 0.22F
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
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9
High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
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. 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.
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. 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. 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
Enable and Dimming Control Input (ENA, ENB)
Current Regulator Switchover
PWM Dimming Control (MAX8848Z)
Serial-Pulse Dimming Control (MAX8848Y)
Table 1. ENA and ENB Enable and Dimming Control
PART MAX8848Y MAX8848Z ENA LED1-LED5 enable and serial-pulse dimming control LED1-LED7 enable and PWM dimming control ENB LED6 and LED7 enable and serial-pulse dimming control --
10
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High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN
for the LED current values and the corresponding ENA/ ENB pulse count. Figure 2 shows a timing diagram for ENA/ENB. If dimming control is not required, ENA/ENB works as a simple on/off logic control. Drive ENA/ENB high for 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.
MAX8848Y/MAX8848Z
0 EN_ INITIAL tHI tINIT > 120s 24mA ILED_
1
2
3
4
5
26
27
28
29
30
31
22.4mA 20.8mA 19.2mA 17.6mA
tLO 1s TO 500s 16.0mA
> 1s
tHI 24mA 22.4mA
tSHDN 8ms (TYP)
SHDN
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. ENA/ENB Serial-Pulse Dimming Count and Programmed LED_ Currents
EN_ PULSE COUNT Startup or EN_high 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 EN_ 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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11
High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
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. 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 10k internal resistor. 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 +60C. The typical derating function characteristic is shown in the Typical Operating Characteristics.
Low LED Current Levels
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. The MAX8848Y/MAX8848Z 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
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 MAX8848Y/MAX8848Z 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 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.
Input Ripple
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 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 8 7 6 5 8 7 6 5 8 ILED (mA) 1.2 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 MAXIMUM ILED (mA)* 1.6 1.6 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
Capacitor Selection
PCB Layout and Routing
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.
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High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN
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
MAX8848Y/MAX8848Z
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High-Performance Negative Charge Pump for 7 White LEDs in 3mm x 3mm Thin QFN MAX8848Y/MAX8848Z
Revision History
REVISION NUMBER 0 REVISION DATE 4/10 Initial release DESCRIPTION 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.
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Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.

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