Diodes PAM2845 User Manual

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PAM2845

40V HIGH POWER BOOST CONVERTER with 6 CHANNEL CONSTANT CURRENT SOURCE

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

string current-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 input voltage 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.

Features

• Six Constant-Current Output Channels

• Parallel Channels Allow Higher Current per LED String

• Maximum 40V Continuous Voltage Output Limit for Each

Channel

• Adjustable Constant Output Voltage

• Adjustable Constant LED Current

• Drives 10 or more LEDs Each String as Long as the String

Voltage Less Than 40V

• Internal 2.5A Power MOSFET

• Allows Digital PWM and Analog Dimming

• Wide (100:1) PWM Dimming Range without Color Shift

• Independent Dimming and Shutdown Control of the LED Driver

• ±1% Typical Current Matching between Strings

• Short LED Protection

• 3 Frequencies Selection: 1.6MHz/1MHz/500kHz

• Wide Input Voltage Range: 4.8V to 28V

• Over Temperature Protection

• Available in 24-pin 4mmx4mm QFN Package

• Pb-free Package

Pin Assignments

Applications

• White or RGB Backlighting for LCD, TV, LCD Monitor, Notebook,

Handy Terminals, and Avionics Displays Panels

• LED Lighting Devices

PAM2845

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Typical Applications Circuit

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PAM2845

Pin Descriptions

Number

1 VIN Supply Input

3 GND Ground

4 ENA Enable Input

5 PWMD PWM Dimming Control

6 LED1 LED1 Cathode Terminal

7 LED2 LED2 Cathode Terminal

8 LED3 LED3 Cathode Terminal

9 GND Ground

10 GND Ground

11 LED4 LED4 Cathode Terminal

12 LED5 LED5 Cathode Terminal

13 LED6 LED6 Cathode Terminal

16 VC Boost Stage Compensation Pin

18 FB Feedback

19 PGND Power Ground

20 PGND Power Ground

21 PGND Power Ground

22 SW Power MOS Drain

23 SW Power MOS Drain

24 SW Power MOS Drain

Pin Name

QFN4x4-24

-Driver

SET

-5V

SEL

5V Linear Regulator Output for Power MOS Driver

LED Current Adjustment Pin

5V Linear Regulator

Oscillator Frequency Selection Pin

Function

PAM2845

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Functional Block Diagram

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PAM2845

Absolute Maximum Ratings (@T

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.

Parameter Rating Unit

VIN, ENA -0.3 to +30

SW, LED -0.3 to +40

VCC -5V, VCC–Driver, VC

PWMD. F

Storage Temperature -65 to +170

Maximum Junction Temperature 170

Soldering Temperature 300, 5sec

PAM2845

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SEL

, FB, I

SET

= +25°C, unless otherwise specified.)

-0.3 to +6.0

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°C

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PAM2845

Recommended Operating Conditions (@T

Parameter Rating Unit

Supply Voltage Range 4.8 to 28 V

Operation Temperature Range -40 to +85

Junction Temperature Range -40 to +150

= +25°C, unless otherwise specified.)

°C

Thermal Information

Parameter Package Symbol Max Unit

Thermal Resistance (Junction to Case)

Thermal Resistance (Junction to Ambient)

Electrical Characteristics (@T

Parameter Test Conditions Min Typ Max Units

Input Voltage Range 4.8 28 V

Quiescent Current

VDD_5V 4.5 5 5.5 V

VDD_5V Current Limit 14 74 90 mA

VDD_5V UVLO Threshold 3.9 4.2 4.5 V

VDD_5V UVLO Hysteresis 40 mV

VDD_Driver 4.5 5 5.5 V

VDD_Driver Current_Limit 14 74 90 mA

VDD_Driver UVLO Threshold 3.9 4.2 4.5 V

VDD_Driver UVLO Hysteresis 70 mV

Feedback Voltage 1.2 V

Switch RDS(ON)

Switch Current Limit 2.5 A

Switch Leakage Current 1 µA

Switching Frequency

Minimum Duty Cycle

Maximum Duty Cycle 90 %

VC Source Current 60 µA

VC Sink Current 60 µA

PAM2845

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QFN4x4-24

= +25°C, VIN = ENA = 12V, L = 22µH, R

E NA = high (no switching frequency) 1 2

= high (1.6M switching frequency)

SEL

= high (1M switching frequency)

SEL

= high (500k switching frequency)

SEL

E NA = low 5 20 µA

_5V = 5V

= VCC_5V

SEL

= Open

SEL

= GND

SEL

= VCC_5V

SEL

= Open

SEL

= GND

SEL

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LDO Stage

Boost Stage

4 of 14

= 10kΩ, unless otherwise specified.)

SET

°C/W

0.2 Ω

1.6 MHz

1.0 MHz

500 kHz

20 %

10 %

5 %

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PAM2845

Electrical Characteristics (cont.) (@T

Parameter Test Conditions Min Typ Max Units

I = 190*1.2V/R

Full-Scale LED_Output Current

LED Current Matching -3 1 +3 %

ISET Voltage 1.2 V

Minimums LED Voltage 400 mV

Analog Dimming Range

PWM Dimming Frequency 100 1k Hz

LED_Overvoltage Threshold 4.6 4.9 5.1 V

LED_Overvoltage Hysteresis 1 V

Thermal-Shutdown 150 °C

Thermal-Shutdown Hysteresis 30 °C

EN High 1.5 V

EN Low 0.4 V

PWMD High 1.5 V

PWMD Low 0.4 V

FSEL High

FSEL Midlevel 1 2 V

FSEL Low 0.5 V

EN Min Pulse Width 0.5 µs

EN Max Pulse Width 10 µs

EN Off Delay 10 µs

PAM2845

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I = 190*1.2V/R

= +25°C, VIN = ENA = 12V, L = 22µH, R

LED Controller Stage

, R

ISET

= 7.68k

ISET

, R

= 11.3k

ISET

, R

= 22.6k

ISET

Fault Protection

Control Interface

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= 10kΩ, unless otherwise specified.)

SET

30 mA

20 mA

10 mA

I/32 I mA

_5V -

0.5

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Typical Performance Characteristics

(@TA = +25°C, VIN = ENA = 12V, L = 22µH, R

= 10kΩ, unless otherwise specified.)

SET

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PAM2845

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Typical Performance Characteristics (cont.)

(@TA = +25°C, VIN = ENA = 12V, L = 22µH, R

= 10kΩ, unless otherwise specified.)

SET

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PAM2845

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Typical Performance Characteristics (cont.)

(@TA = +25°C, VIN = ENA = 12V, L = 22µH, R

= 10kΩ, unless otherwise specified.)

SET

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PAM2845

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PAM2845

Application Information

The inductance, peak current rating, series resistance, and physical size should all be considered when selecting an inductor. These factors

affect the converter'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 determine the inductor value. Very high inductance

minimizes the current ripple, and therefore reduces the peak current, which decreases core losses in the inductor and I

power path. However, large inductor values also require more energy storage and more turns of wire, which increases physical size and I

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 minimized 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 2.2µF input capacitor is recommended for most applications. A

minimum output capacitor value of 10µF is recommended under normal operating conditions, while a 22µF 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 10µF, voltage rating (50V) capacitor is chosen.

Diodes Selection

PAM2845 is high switching frequency convertor, which demands high speed rectifier. It'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

2) PWM Input at PWMD

3) PWM Input at ENA

R losses in the entire

PAM2845

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Figure 1

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Application Information (cont.)

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PAM2845

• Method 1: LED Current Setting with External Resistor R

The most basic means of setting the LED current is connecting a resistor between R

= 228/ R

LED

ISET

and GND. The LED current is decided by I

ISET

Resistor.

SET

• Method 2: LED Current Setting Using PWM Signal to PWMD Pin

This circuit uses resistor R

PWMD pin is logic low. Average LED current is approximately equal to:

I = (t

*I)/ (tON + t

Also, the recommended PWM frequency is between 100Hz and 1kHz. Frequency <100Hz can cause the LEDs to blink visibly.

to set the on state current and the average LED current, then proportional to the percentage of on-time when the

ISET

)

OFF

• 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

width). Each pulse reduces the LED current by 1/32, so after one pulse the LED current is 31/32*I

to I

LED

Figure 1 shows a timing diagram for EN.

. Dimming is done by pulsing ENA low (500ns to 10µs pulse

LED

. The 32th pulse sets the LED current back

LED

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.

The recommend procedure is to choose R2 = 300kΩ and R1 = 9.2kΩ to set V

Generally the V

= 33V.

LED

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.

Normally V

make sure all LED pins’ voltage is less then 5V. With this function V

Note:

: LED pin voltage

LED

: Output voltage

OUT

: LED forward voltage

: The minimum LED numbers among all strings.

MIN

= VFB (1 + R2/R1)

OUT

must be higher than total LED voltage. For 10-LED application, R1 = 9.1kΩ, R2 = 270kΩ, V

OUT

= V

LED

– VfN

OUT

is around 0.4V and V

= 40V.

OUT(MAX)

= 36.8V, higher than

OUT

is decided by LED numbers. When one or more LED(s) is/are shorted, the PAM2845 will clamp V

OUT

will be clamped at (5V+ VfN

OUT

MIN

OUT

PAM2845

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Application Information (cont.)

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 noise spikes. Create an analog ground island (GND) consisting of the output voltage detection-divider ground connection, the I 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 Evaluation Board

PAM2845

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Ordering Information

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PAM2845

Part Number Package Type Standard Package

PAM2845KHR QFN4x4-24 3000 Units/Tape&Reel

Marking Information

PAM2845

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Package Outline Dimensions (All dimensions in mm.)

QFN4x4-24

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IMPORTANT NOTICE

LIFE SUPPORT

PAM2845

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