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PAM2846
Integrated 6 String LED Boost Type Driver Features
n Six Constant-Current Output Channels n Parallel Channels Allow Higher Current per LED String n Maximum 40V Continuous Voltage Output Limit for Each Channel n Self-adaptive Vout to Fit Different LED Number n Adjustable Constant LED Current n Drives 10 or more LEDs Each String as Long as the String Voltage Less Than 40V n Internal 2.5A Power MOSFET n Low 400mV Feedback Voltage at Full Current Improves Efficiency n Allows Digital PWM and Analog Dimming n Wide (100:1) PWM Dimming Range without Color Shift n Independent Dimming and Shutdown Control of the LED Driver n 1% Typical Current Matching between Strings n Open LED Protection: Adjustable Clamp Voltage n Short LED Protection n 3 Frequencies Selection: 1.6MHz/1MHz/500kHz n Wide Input Voltage Range: 4.8V to 28V n Over Temperature Protection n Available in 24-pin 4mmx4mm QFN Package n Pb-free Package
Description
The PAM2846 is a high-efficiency boost type LED driver. It is designed for large LCD panel that employs an array of LEDs as back light source. The PAM2846 employs a current-mode step-up converter that drives six parallel strings of LEDs connected in multiple series. This built-in stringcurrent-control circuit achieves 1% typical current matching between strings, which ensures even brightness for all LEDs. The device is able to adapt to different numbers of LEDs in each string or different forward voltage for different types of LEDs and keep the string current constant. Separate feedback loops limit the output voltage if one or more LEDs open or short. The limit voltage of over voltage protection is adjustable by changing the feedback resistor ratio. The PAM2846 has features cycle-by-cycle current limit to provide consistent operation and soft-start capability. A thermal-shutdown circuit provides another level of protection. The PAM2846 has a wide +4.8V to +28V inputvoltage range and provides adjustable full-scale LED current. The switching frequency of this device can be selected among 500kHz, 1MHz and 1.6MHz according to the application requirements.
Applications
n White or RGB Backlighting for LCD TV, LCD Monitor, Notebook, Handy Terminals, and Avionics Displays Panels n LED Lighting Devices
Typical Application
VIN: 4.8V to 28V
22 H
3A/40V
L SW SW SW OVP R2
VIN ENA PWMD Fsel PWMD Fsel
R1 300k Total 10 LEDs per string
PAM2846
VC Vcc-5V Vcc-driver 0-10k 1 1 10K 100nF
GND
LED1 LED2 LED3 LED4 LED5 LED6
9.2k
Iset
PGND PGND PGND
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Block Diagram
VIN
Vcc-5V Vcc-driver
LDO1 LDO2 UVLO
1.2V SOFT START 1.6MHz 1MHz 500KHz
OVP Fsel
ENA
ENABLE
OVP
SLOPE COMPENSATION PWM COMPATATOR
OTP Bandgap POR VC
LED Feedback 1.2V
MAIN CONTROL
Level Shift Driver
+
SW
EA
CURRENT LIMIT
LED1-6
CS AMP
PGND LED1 LED2 LED3 LED4 LED5 LED6
PWMD
PWM DIMMING
led short protect
led short protect
led short protect
led short protect
led short protect
led short protect
I_set AMP
Iset
GND
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Pin Configuration
QFN 4mmX4mm
24 1 2 3 4 5 6 7 8 9 10 11 12 23 22 21 20 19 18 17
P2846 XXXYW
16 15 14 13
XXX: Internal Code Y: Year W: Weekly
Pin Descriptions
Pin Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Name VIN Vcc-driver GND ENA PWMD LED1 LED2 LED3 GND GND LED4 LED5 LED6 Iset Vcc-5V VC Fsel OVP PGND PGND PGND SW SW SW Description Supply input 5V linear regulator output for power MOS driver Ground Enable input PWM dimming control LED1 cathode terminal LED2 cathode terminal LED3 cathode terminal Ground Ground LED4 cathode terminal LED5 cathode terminal LED6 cathode terminal LED current adjustment pin 5V linear regulator output Boost stage compensation pin Oscillator frequency selection pin Over voltage protection Power ground Power ground Power ground Power MOS drain Power MOS drain Power MOS drain
Power Analog Microelectronics , Inc
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PAM2846
Integrated 6 String LED Boost Type Driver Absolute Maximum Ratings
These are stress ratings only and functional operation is not implied . Exposure to absolute maximum ratings for prolonged time periods may affect device reliability . All voltages are with respect to ground . O O VIN,ENA.....................................-0.3V To +30V Junction Temperature Range.......-40 C to 125 C O O SW ,LED.....................................-0.3V To +40V Storage Temperature..................-40 C to 150 C O Vcc-5V,Vcc-driver,VC....................-0.3V To +6V Maximum Junction Temperature...............150 C O PWMD,Fsel,OVP,Iset.....................-0.3V To +6V Soldering Temperature....................300 C,5sec
Recommended Operating Conditions
Supply Voltage Range.......................4.8V to 28V Operating Temperature.................-20 OC to 85 OC
Thermal Information
Parameter Thermal Resistance (Junction to Case) Thermal Resistance (Junction to Environment) Package 4x4mm QFN 4x4mm QFN Symbol jC jA Maximum 2 37 Unit C/W C/W
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Electrical Characteristic
T A=25C, V IN=ENA=12V, L=22 H, Rset=10k , unless otherwise noted
PARAMETER Input Voltage Range ENA=high (no switching) Fsel=high (1.6M switching frequency) Quiescent Current Fsel =high (1M switching frequency) Fsel =high (500k switching frequency) ENA=low LDO Stage Vcc_5V Vcc_5V current_limit Vcc_5V UVLO Threshold Vcc_5V UVLO Hysteresis Vcc_driver Vcc_driver current_limit Vcc_driver UVLO Threshold Vcc_driver UVLO Hysteresis Boost Stage Switch Rdson Switch Current Limit Switch Leakage Current Fsel =Vcc_5V Switching Frequency Fsel =Open Fsel =Gnd Fsel =Vcc_5V Minimums Duty Cycle Fsel =Open Fsel =Gnd Maximums Duty Cycle VC Source Current VC Sink Current Vcc_5V=5V 0.2 2.5 1 1.6 1.0 500 20 10 5 90 A A MHz MHz kHz % % % % A A Conditions Min 4.8 1 10 6 3 5 20 A Typ Max 28 2 mA Units V
No switching No switching No switching No switching No switching No switching No switching No switching
4.7 14 3.9 4.7 14 3.9
5 74 4.2 70 5 74 4.2 70
5.5 90 4.5 5.5 90 4.5
V mA V mV V mA V mV
60 60
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Electrical Characteristic
T A=25C, V IN=ENA=12V, L=22 H, Rset=10k , unless otherwise noted .
PARAMETER LED Controller Stage I=190*1.2V/Riset, Riset=7.68k Full-Scale LED_Output Current I=190*1.2V/Riset, Riset=11.3k I=190*1.2V/Riset, Riset=22.6k LED current matching Iset Voltage Minimums LED voltage Analog Dimming Range PWM Dimming Frequency Fault Protection LED_ Overvoltage Threshold LED_ Overvoltage Hysteresis Overvoltage Threshold Overvoltage Hysteresis Thermal-Shutdown Thermal-Shutdown Hysteresis Controll Interface EN High EN Low PWMD High PWMD Low Fsel High Fsel Midlevel Fsel Low EN Min pulse width EN Max pulse width EN off delay
Vcc_5V- 0.5
Conditions
Min
Typ 30 20 10
Max
Units mA mA mA
-3
1 1.2 400
+3
% V mV
I=190*1.2V /Riset
I/32 100 4.6 4.9 1 1.2 80 150 30
I 1k 5.1
mA Hz V V V mV C C
1.5 0.4 1.5 0.4
V V V V V 2 0.5 V V s 10 200 s s
1
single wire dimming low level single wire dimming low level single wire dimming low level
0.5
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Typical Performance Characteristic
T A=25C, V IN=ENA=12V, Rset=10k , L=22 H, Fsw=500kHz, 10x6 LEDs, unless otherwise noted
100% 90% 80% 70%
Efficiency
Input Voltage VS Efficiency
LED Current VS PWMD Duty cycle@100Hz
25 20
LED current (mA)
60% 50% 40% 30% 20% 10% 0% 5 9 13 17 21 25 Input Voltage (V) Fsw =500KHz Fsw =1MHz Fsw =1.6MHz
15 10 5 0 20% 40% 60% Duty cycle 80% 100%
Shutdown current VS Input Voltage
6
10
Quiescent current VS Input Voltage
Shutdown current (uA)
5
Supply Current (mA)
8
PWMD=0 Fsw=500KHz Fsw=1MHz Fsw=1.6MHz
4 3 2 1 0 0 10 20 30 Input voltage (V)
6
4
2
0 6 10 14 18 22 26 Input Voltage
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Typical Performance Characteristic
T A=25C, V IN=ENA=12V, Rset=10k , L=22 H, Fsw=500kHz, 10x6 LED, unless otherwise noted Total LED Current Line regulation
137.5 137
Efficiency VS Input Voltage
100% 95%
136.5
LED Current (mA)
Efficiency
136 135.5 135 134.5
90% 85% 80%
10x6 LED 8x6 LED 6x6 LED
134 133.5 4 8 12 16 20 24 28
75% 4 8 12 16 20 24 28
Input Voltage (V)
Input Voltage (V)
Analog Dimming Signal Step
25 20 15 10 5 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 ENA Dimming Step
Max Output Current Vs Output Voltage (Per Channel)
100
Max Output Current (mA)
LED Current (mA)
80 60 40 20
Vin=12V Vin=6V
0 16 20 24 28 32 36 40 Output Voltage (V)
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Typical Performance Characteristic
T A=25C, V IN=ENA=12V, Rset=10k , L=22 H, Fsw=500kHz, 10x6 LEDs, unless otherwise noted.
PWM Dimming (Duty Cycle=20%)
PWMD DC coupling V LED1 DC coupling
LED Current Waveforms (PWMD=50%)
PWMD DC coupling V LED1 DC coupling
I LED DC coupling
I LED DC coupling
LED Current Waveforms ( PWMD =80%)
Startup Waveforms
PWMD DC coupling V LED1 DC coupling
Vout DC coupling IL DC coupling
I LED DC coupling
ENA DC coupling
Start up and Shutdown Waveforms
Stable Operation
Vout DC coupling IL DC coupling ENA DC coupling
Vsw DC coupling IL DC coupling
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Application Information
Inductor Selection The inductance, peak current rating, series resistance, and physical size should all be considered when selecting an inductor. These factors affect the conver ter's operating mode, efficiency, maximum output load capability, transient response time, output voltage ripple, and cost. The maximum output current, input voltage, output voltage, and switching frequency d e t e r m i n e t h e i n d u c t o r v a l u e . Ve r y h i g h inductance minimizes the current rip ple, and therefore reduces the peak current, which 2 decreases core losses in the inductor and I R losses in the entire power path. However, large inductor values also require more energy storage and more turns of wire, which increases physical 2 size and I R copper losses in the inductor. Low inductor values decrease the physical size, but increase the current ripple and peak current. Finding the best inductor involves the compromises among circuit efficiency, inductor size, and cost. When choosing an inductor, the first step is to determine the operating mode: continuous conduction mode (CCM) or discontinuous conduction mode (DCM). When CCM mode is chosen, the ripple current and the peak current of the inductor can be minimized. If a small-size inductor is required, DCM mode can be chosen. In DCM mode, the inductor value and size can be mini mized but the inductor ripple current and peak current are higher than those in CCM. Capacitor Selection An input capacitor is required to reduce the input ripple and noise for proper operation of the PAM2846. For good input decoupling, low ESR (equivalent series resistance) capacitors should be used at the input. At least 2.2 F input capacitor is recommended for most applications. A minimum output capacitor value of 10 F is recommended under normal operating conditions, while a 22 F or higher capacitor may be required for higher power LED current. A reasonable value of the output capacitor depends on the LED current. The total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging on the output capacitor, and the ohmic ripple due to the capacitor's equivalent series resistance. The ESR of the output capacitor is the important parameter to determine the output voltage ripple of the converter, so low ESR capacitors should be used at the output to reduce the output voltage ripple. The voltage rating and temperature characteristics of the output capacitor must also be considered. So a value of 10 F, voltage rating (50V) capacitor is chosen. Diode Selection PAM2846 is high switching frequency convertor, w h i c h d e m a n d s h i g h s p e e d r e c t i f i e r. I t ' s indispensable to use a Schottky diode rated at 2A, 60V with the PAM2846. Using a Schottky diode with a lower forward voltage drop can improve efficiency of the convertor. The voltage rating of the diode should be greater than the output voltage. Methods for Setting LED Current There are three methods for setting and adjusting the LED current outlined here. The methods are: 1) RSET only 2) PWM Input at PWMD 3) Single wire logic signal at ENA
>10ms
0 1 2 3 4 5 27 28 29 30 31 32
ENA
t SOFT-START 32/32 31/32 500ns500ns 5/32 4/32 3/32 32/32
t 200s(typ)
shutdown
I LED_
shutdown
2/32
1/32
shutdown
Figure 1. Timing diagram for EN
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver
Method 1: LED Current Setting with External Resistor R ISET The most basic means of setting the LED current is connecting a resistor between R ISET and GND. The LED current is decided by I SET Resistor. I LED=228/ R ISET Method 2: LED Current Setting Using PWM Signal to PWMD Pin This circuit uses resistor R ISET to set the on state current and the average LED current, then proportional to the percentage of on-time when the PWMD pin is logic low. Average LED current is approximately equal to: I= ( t on*I)/ (t on+ t off) Also, the recommended PWM frequency is between 100Hz and 1kHz. Frequency <100Hz can cause the LEDs to blink visibly. Method 3: LED Current Setting with single wire logic to ENA Pin When the LEDs are enabled by high level, the LED current initially goes to I LED. Dimming is done by pulsing ENA low (500ns to 10 s pulse width). Each pulse reduces the LED current by 1/32, so after one pulse the LED current is 31/32*I LED. The 32th pulse sets the LED current back to I LED. Figure 1 shows a timing diagram for EN. Setting the Over Voltage Protection The OVP pin is connected to the center tap of a resistive voltage divider (R 1 and R 2 in Typical Application diagram) from the high-voltage output. V out_limit=V ov(1+R 2/R 1) The recommend procedure is to choose R 2=300k and R 1=9.2k to set V out_limit=40V. This OVP pin is used for over voltage protect, not for output voltage regulate. LED Short Protection The PAM2846 uses LED_OVP function to protect devices when one or more LED(s) is/are shorted. V LED = V OUT - V f * N Normally V LED is around 0.4V and V OUT is decided by LED numbers. When one or more LED(s) is/are shorted, the PAM2846 will clamp V OUT to make sure all LED pins' voltage is less then 5V. With this function V OUT will be clamped at (5V+ V f*N MIN). Note: V LED: LED pin voltage V OUT: Output voltage Vf : LED forward voltage N MIN: The minimum LED numbers among all strings. LED Open Protection The control loop is related to all six LED sinks. When one or more LED(s) is/are opened, the sink will have no current and the device will work in unstable open loop state. The V OUT voltage will be limited by external resistor divider or 5V+V f*N MIN, whichever is lower.
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
11
PAM2846
Integrated 6 String LED Boost Type Driver
PCB Layout Guidelines Careful PCB layout is important for proper operation. Use the following guidelines for good PCB layout: 1) Minimize the area of the high current switching loop of the rectifier diode and output capacitor to avoid excessive switching noise. 2) Connect high-current input and output components with short and wide connections. The high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the SW pin. The high-current output loop is from the positive terminal of the input capacitor through the inductor, rectifier diode, and positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. Avoid using vias in the high-current paths. If vias are unavoidable, use multiple vias in parallel to reduce resistance and inductance. 3) Create a ground island (PGND) consisting of the input and output capacitor ground and PGND pin. Connect all these together with short, wide traces or a small ground plane. Maximizing the width of the power ground traces improves efficiency and reduces output-voltage ripple and PAM2846 Evaluation Board noise spikes. Create an analog ground island (GND) consisting of the output voltage detection-divider ground connection, the I SET resistor connections, VCC-5V and VCC-driver capacitor connections, and the device's exposed backside pad. Connect the GND and PGND islands by connecting the GND pins directly to the exposed backside pad. Make sure no other connections between these separate ground planes. 4) Place the output voltage setting-divider resistors as close to the OVP pin as possible. The divider's center trace should be kept short. Avoid running the sensing traces near SW Pin. 5) Place the VIN pin bypass capacitor as close to the device as possible. The ground connection of the VIN bypass capacitor should be connected directly to GND pins with a wide trace. 6) Minimize the size of the SW node while keeping it wide and short. Keep the SW node away from the feedback node and ground. If possible, avoid running the SW node from one side of the PCB to the other. 7) Refer to the PAM2846 Evaluation board for an example of proper board layout.
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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PAM2846
Integrated 6 String LED Boost Type Driver Ordering Information
PAM2846 X X
Shipping Package Type
Part Number PAM2846KR
Marking P2846 XXXYW
Package Type QFN 4mmx4mm
Standard Package 3,000 Unites /Tape &Reel
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
13
PAM2846
Integrated 6 String LED Boost Type Driver Outline Dimensions
QFN 4mmx4mm
QFN
Note:all dimensions are in millimeters.
Power Analog Microelectronics , Inc
www.poweranalog.com 07/2008 Rev 1.2
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