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40V High Power Boost Convertor with 6 Channel Constant Current Source 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 Adjustable Constant Output Voltage 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 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 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
PAM2845
Description
The PAM2845 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 PAM2845 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. Separate feedback loops limit the output voltage if one or more LEDs open or short. The PAM2845 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 PAM2845 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 10 F
22 H
3A/40V
L SW SW SW FB R2
10 F
VIN ENA PWMD Fsel PWMD Fsel
R1 270k Total 10 LEDs per string
PAM2845
VC Vcc-5V Vcc-driver 100nF 1F Iset 1F 10k
GND
LED1 LED2 LED3 LED4 LED5 LED6
9.1k
PGND PGND PGND
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
1
40V High Power Boost Convertor with 6 Channel Constant Current Source Block Diagram
VIN
PAM2845
Vcc-5V Vcc-driver
LDO1 LDO2 UVLO
SLOPE COMPENSATION PWM COMPATATOR
FB
SOFT START 1.6MHz 800KHz 500KHz
Fsel
ENA
ENABLE
OTP Bandgap
1.2V
MAIN CONTROL
Level Shift Driver
+
SW
EA
CURRENT LIMIT
POR VC
CS AMP PWMD PWM DIMMING
PGND LED1 LED2 LED3 LED4 LED5 LED6
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 08/2008 Rev 1.0
2
40V High Power Boost Convertor with 6 Channel Constant Current Source Pin Configuration
QFN 4X4
24 1 2 3 4 5 6 7 8 9 10 11 12 23 22 21 20 19 18 17
PAM2845
P2845 XXXYW
16 15 14 13
X: Internal Code Y: Year W: Week
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 FB 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 Feedback Power ground Power ground Power ground Power MOS drain Power MOS drain Power MOS drain
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
3
40V High Power Boost Convertor with 6 Channel Constant Current Source Absolute Maximum Ratings
PAM2845
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 . VIN,ENA.....................................-0.3V To +30V Operating Temperature...............-40 OC to 125 OC SW ,LED.....................................-0.3V To +40V Storage Temperature..................-40 OC to 150 OC Vcc-5V,Vcc-driver,VC....................-0.3V To +6V Maximum Junction Temperature...............150 OC PWMD,Fsel,FB,Iset.......................-0.3V To +6V Soldering Temperature....................300 OC,5sec
Recommended Operating Conditions
Supply Voltage Range.......................4.8V to 28V Operation Temperature Range.........-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 20 37 Unit C/W C/W
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
4
40V High Power Boost Convertor with 6 Channel Constant Current Source 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 Feedback Voltage 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 1.2 0.2 2.5 1 1.6 1.0 500 20 10 5 90 V A A MHz MHz kHz % % % % A A 4.5 14 3.9 4.5 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 Conditions Min 4.8 1 10 6 3 5 20 A Typ Max 28 2 mA Units V
PAM2845
60 60
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
5
40V High Power Boost Convertor with 6 Channel Constant Current Source 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 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 100 0.5 10
Vcc_5V-0.5
PAM2845
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 1 150 30 1.5
I 1k 5.1
mA Hz V V C C V
0.4 1.5 0.4 1 2 0.5
V V V V V V s s s
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
6
40V High Power Boost Convertor with 6 Channel Constant Current Source Typical Performance Characteristic
T A=25C, V IN=ENA=12V, L=22 H, Rset=10k , unless otherwise noted.
100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 5
PAM2845
Input Voltage VS Efficiency
25 20
LED current(mA)
LED Current VS PWMD Duty cycle
Efficiency(%)
15 10 5 0
9
13 Input voltage(V)
17
21
20%
40%
60% Duty cycle
80%
100%
Shutdown current VS Input Voltage
6 Shutdown current(uA)
Supply Current (mA)
Quiescent Current VS Input Voltage
9 8 7 6 5
Fsw=1MHz Fsw=1.6MHz
5 4 3 2 1 0 0 10 20 30 Input voltage(V)
4 3 2 1 0 6 10
PWMD=0 Fsw=500KHz
14 18 22 Input Voltage (V)
26
Vout Line Regulation
41 40.5
Output Voltage (V)
Total LED Current Line regulation
133 132.5
40 39.5 39 38.5 38 4.5 8 12 16 20 24 28 Input Voltage (V)
LED Current (mA)
132 131.5 131 130.5 130 129.5 129 4.5 8 12 16 20 24 28 Input Voltage (V)
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
7
40V High Power Boost Convertor with 6 Channel Constant Current Source Typical Performance Characteristic
T A=25C, V IN=ENA=12V, L=22 H, Rset=10k , unless otherwise noted. Efficiency VS Input Voltage
100% 6x6 LED 8x6 LED 10x6 LED
Efficiency
PAM2845
Start up and Shutdown waveforms
95%
90%
12x6 LED
Vout DC coupling IL DC coupling ENA DC coupling
85%
80%
75% 4.5 8 12 16 20 Input Voltage(V) 24 28
Switching Waveforms
Inductor Current Waveforms( PWMD =20%)
Vsw DC coupling IL DC coupling
PWMD DC coupling V LED1 DC coupling
IL DC coupling
LED Current Waveforms( PWMD =20%)
PWMD DC coupling V LED1 DC coupling
Inductor Current Waveforms( PWMD =50%)
PWMD DC coupling V LED1 DC coupling
I LED DC coupling
IL DC coupling
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
8
40V High Power Boost Convertor with 6 Channel Constant Current Source Typical Performance Characteristic
T A=25C, V IN=ENA=12V, L=22 H, Rset=10k , unless otherwise noted. LED Current Waveforms(PWMD=50%)
PWMD DC coupling V LED1 DC coupling PWMD DC coupling V LED1 DC coupling
PAM2845
Inductor Current Waveforms( PWMD =80%)
I LED DC coupling
IL 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
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
9
40V High Power Boost Convertor with 6 Channel Constant Current Source 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 PAM2845. For good input decoupling, Low ESR (equivalent series resistance) capacitors should be used at the input. At least 2.2uF input capacitor is recommended for most applications. A minimum output capacitor value of 10uF is recommended under normal operating conditions, while a 22uF 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 10uF, voltage rating(50V) capacitor is chosen. Diode Selection PAM2845 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 PAM2845. Using a Schottky diode with a lower forward voltage drop is better to improve the power LED efficiency, and its voltage rating 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) PWM Input at ENA
PAM2845
>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 200us(typ)
shutdown
I LED_
shutdown
2/32
1/32
shutdown
Figure 1
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
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40V High Power Boost Convertor with 6 Channel Constant Current Source
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 Output Voltage The FB pin is connected to the center tap of a resistive voltage divider (R1 and R2 in Figure 1) from the high-voltage output. V OUT=V FB(1+R2/R1) The recommend procedure is to choose R2=300k and R1=9.2k to set V OUT MAX=40V. Generally the Vout must be higher than total LED voltage. For 10-LED application, R1=9.1k , R2=270k , Vout=36.8V, higher than V LED=33V. One or more of the LED1-6 pins could be floating if not used because the PAM2845 uses external resistor to set the output voltage. One or more LEDx pins floating is just like change of output loadings. LED Short Protection The PAM2845 uses LED FB 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 PAM2845 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 fN MIN). Note: V LED: LED pin voltage V OUT: Output voltage V f : LED forward voltage N MIN: The minimum LED numbers among all strings.
PAM2845
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
11
40V High Power Boost Convertor with 6 Channel Constant Current Source
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 PAM2845 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 PAM2845 Evaluation board for an example of proper board layout.
PAM2845
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
12
40V High Power Boost Convertor with 6 Channel Constant Current Source Ordering Information
PAM2845
PAM2845 X X X
Packing Number of Pins Package Type
Part Number PAM2845KHR
Package Type QFN 4mmx4mm
MOQ/Packing 3,000 Unites /Tape &Reel
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
13
40V High Power Boost Convertor with 6 Channel Constant Current Source Outline Dimensions
QFN 4mmx4mm
PAM2845
QFN
Note:all dimensions are in millimeters.
Power Analog Microelectronics , Inc
www.poweranalog.com 08/2008 Rev 1.0
14

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