AF1502

Step-Down White LED Driver

General Description Features

Input voltage: 4.2V to 23V. Low Feedback Voltage 0.21V.

Driving up to 15 LEDs (1W 3S-5P) at 12V IN Oscillation frequency: 300KHz typ.

Soft-start, Current limit, Enable function Great Output Capability

PWM or Analog Dimming Control Thermal Shutdown function Built-in internal P-channel MOS SOP-8P Package. Application

High Power LED Driver Backlight Application

General Lighting Solution

Constant Current Source

Pin Define ( SOP-8P )

Marking Information

AF1502 is a step down LED driver that is designed to meet maximum 2A constant current for high power LED application, and utilizes PWM control scheme that switches with 300Khz fixed frequency.

The input voltage range of AF1502 is from 4.2V to 23V. It is suitable for series-parallel 1W, 3W, or 5W high power LED application due to the high operation voltage and output capability. At 12V input voltage, this device can drive up to 15pcs 1W LED (3S-5P) with constant 350mA LED current.

The PWM control circuit is able to very the duty ratio linearly from 0 up to 100%. This device also contains

an error amplifier circuit as well as a soft-start circuit

that prevents overshoot at startup. An enable function, an over current protect function and a short circuit protect function are built inside, and when OCP or SCP happens, the operation frequency will be reduced from

300KHz to 60KHz. Also, an internal compensation block is built in to minimum external component count. The package is available in a standard SOP-8P .

Step-Down White LED Driver

Typical Application Circuit

Pin Description

Pin

Symbol

Description

FB

1

Feedback pin Connect to a resistor to set LED current by following formula :

I LED = 0.21V/R FB EN 2 Enable / Disable pin

H: Normal operation mode. (Step-down operation) L: Shutdown mode.

(All circuits deactivated)

OCSET 3 Add an external resistor to set max output current. V IN 4 IC power supply pin

Output 5、6 Switch Pin. Connect external inductor/diode here. Minimize trace area at this pin to reduce EMI. GND

7、8

GND Pin

Ordering Information

Part Ordering No.

Part Marking

Package

Unit

Quantity

AF1502S8RG AF1502 SOP-8P Tape & Reel

3000 EA

ϡʳ A Lot code ϡʳ B Date code

ϡʳAF1502S8RG : 13” Tape & Reel ; Pb- Free ; Halogen- Free

Step-Down White LED Driver Block Diagram

Absolute Maximum Ratings (T A=25к Unless otherwise noted)

The following ratings designate persistent limits beyond which damage to the device may occur.

Symbol Parameter Value Unit

V IN(1)V IN Pin Voltage V SS - 0.3 to V SS + 25 V

V FB Feedback Pin Voltage V SS - 0.3 to V CC V

V ON/OFF(1)ON/OFF Pin Voltage V SS - 0.3 to V IN + 0.3 V

V OUTPUT Switch Pin Voltage V SS - 0.3 to V IN + 0.3 V

P D Power Dissipation Internally limited mW

T OPR Operating Temperature Range -20 to +125 o C

T STG Storage Temperature Range -40 to +150 o C

1. The absolute maximum ratings are rated values exceeding which the product could suffer physical damage. These values must therefore not be exceeded under any conditions.

Step-Down White LED Driver

Electrical Characteristics

(T A=25к, V CC=12V, unless otherwise specified.)

Symbol Parameter Conditions Min. Typ. Max. Unit V IN Input Voltage 4.2 23 V

V FB Feedback Voltage IOUT=0.1A 0.1995 0.21 0.220

5 V

I FB Feedback Bias Current IOUT=0.1A 0.1 0.5 µA

I SW Switch Current 2.5 A

I GND Current Consumption During

Power Off VON/OFF=0V 1 10 µA

I OCSET OCSET Pin Bias Current 75 90 105 µA

∆V OUT

/V OUT Line Regulation VIN=5V~23V 1 2 % ∆V OUT

/V OUT Load Regulation IOUT=0.1 to 2A 0.2 0.5 %

f OSC Oscillation Frequency Measure waveform at SW pin 240 300 360 KHz

f OSC1Frequency of Current Limit or

Short Circuit Protect Measure waveform at SW pin 30 60 90 KHz V ENH EN Pin Input Voltage Evaluate oscillation at SW pin 2.0 V V ENL Evaluate oscillation stop at SW pin 0.8

I ENH EN Pin Input Leakage Current 20 µA

I ENL-10 µA

T SS Soft-Start Time 0.3 2 5 ms R DS(ON)Internal MOSFET Rdson VIN=5V, VFB=0V 80 90 mΩ

VIN=12V, VFB=0V 50 60

ˤEfficiency VIN=12V, VOUT = 5V

IOUT=2A 96 %

Step-Down White LED Driver

Function Description

PWM Control

The AF1502 consists of DC/DC converters that employ a pulse-width modulation (PWM) system.

In converters of the AF1502 the pulse width varies in a range from 0 to 100%, according to the load current. The ripple voltage produced by the switching can easily be removed through a filter because the switching frequency remains constant. Therefore, these converters provide a low-ripple power over broad ranges of input voltage and load current.

R DS(ON) Current Limiting

The current limit threshold is setting by the external resistor connecting from V IN supply to O CSET . The internal 100uA sink current crossing the resistor sets the voltage at the pin of O CSET . When the PWM voltage is less than the voltage at O CSET , an over-current condition is triggered.

Short circuit protection

The device includes short protection. When the output is shorted to GND, the protection circuit will be triggered and force the oscillation frequency down to approximately 60Khz. The oscillation frequency will return to the normal value once the output voltage or the feedback voltage rises above 0V.

Delay Start-up

The following circuit uses the EN pin to provide a time delay between the input voltage is applied and the output voltage comes up. As the instant of the input voltage rises, the charging of the capacitor C DELAY pulls the EN pin low, keeping the device of. Once the capacitor voltage rises above the EN pin threshold voltage, the device will start to operate.

Step-Down White LED Driver

For example, setting V IN

=12V, R

DELAY =100K

Ω

, C DELAY

=0.1uF. The start-up delay time can be calculated as below:

Step-Down White LED Driver

Application Information

LED Current Sensing

This device is a constant current buck regulator that develops 0.21V reference voltage between the feedback pin

and GND pin. Therefore, the use 1% chip resistors to attain the better current accuracy.

The LED current is given by the following equation:

Step-Down White LED Driver

If the V

DIM is taken below the V FB

, the inverse will happen and brightness will increase.

The analog dimming circuit can be tailored for different resistor value using the following equation:

Step-Down White LED Driver

Choose the value of RC network by the following procedure: 1. Measure the voltage ringing by the frequency (f R ) of the SW pin.

2. Find a small capacitor and place it across the SW pin and the GND pin to damp the ringing frequency by half.

3.

The parasitical capacitor (C

PAR

) at the SW pin is 1/3 the value of the added capacitor above. The parasitical

inductance (L PAR

) at the SW pin is:

Step-Down White LED Driver

Where: D : Duty Cycle

f S : Switchin

g Frequency

V GS : Power MOSFET Gate Voltage I Q : Quiescent Current

The tr, tf and Q GATE are the rising, falling time, and gate charge of the power MOSFET . The typical value of (tr+tf) is approximately 28ns, and the Q GATE is approximately 10nC. The V GS

is approximately equal V IN .

The Maximum power dissipation of this device depends on the thermal resistance of the IC package and PCB

layout, the temperature difference between the junction and ambient air, and the rate of airflow. The maximum

power dissipation can be calculated by the following equation:

Step-Down White LED Driver

3. The feedback path of the voltage divider should be close to FB pin and keep noisy traces away; also keep them

separate using grounded copper.

4. The input capacitors should be close to the regulator and rectifier diode. The output capacitors should be close

to the load.

5.

Keep the (-) plates of input and output capacitor as close as possible.

Inductor Selection

The conduction mode of power stage depends on input voltage, output voltage, LED current, and the value of the inductor. Select an inductor to maintain this device operating in continuous conduction mode (CCM). The minimum

value of inductor can be determined by the following procedure.

1. Calculate the minimum duty ratio

Where R L

is the DC resistance of the external inductor, V

F is the forward voltage of external diode, and T

S is the

switching period. This equation can be simplified to

3. Calculate the inductor ripple current(∆I L ). In steady state conditions, the inductor ripple current increase, (∆I L +),

during the ON time and the current decrease, (∆I L -

), during the OFF time must be equal.

Step-Down White LED Driver

4. Calculate the minimum value of the inductor use maximum input voltage. That is the worst case condition

because it gives the maximum

∆

I L

.

Coil inductor and surface mount inductors are all available. The surface mount inductors can reduce the board size but they are more expensive and its larger DC resistance results in more conduction loss. The power dissipation is due to the DC resistance can be calculated as below :

P D_INDUCTOR = I LED 2 x R L

Output Rectifier Diode Selection

The rectifier diode provides a current path for the inductor current then the internal power switch of the converter turns off. The best solution is Schottky diode, and some parameters about the diode must be take care as below: 1. The forward current rating of diode must be higher than the continuous LED current. 2. The reverse voltage of diode must be higher then the maximum input voltage. 3. The lower forward voltage of the diode will reduce the conduction loss.

4. The fast reverse recovery time of diode will reduce the switching loss, but it is very small compared to

conduction loss.

5. The power dissipation can be calculated by the forward voltage and LED current for the time that the diode is

conducting.

P D_DIODE = I LED 2 x V F x (1-D)

Output Capacitor Selection

The functions of the output capacitor are to store energy and maintain the output voltage. The low ESR (Equivalent Series resistance) capacitors are preferred to reduce the output ripple voltage (

∆V OUT

) and conduction loss. The

output ripple voltage can be calculated as below:

Step-Down White LED Driver

The ESR of the aluminum electrolytic or tantalum output capacitor is an important parameter to determine the output ripple voltage. But the manufactures usually do not specify ESR in the specifications, Assuming the

capacitor is enough results in the output ripple voltage is due to the capacitor can be ignored, the ESR can be

calculated as below :

Choose the output capacitor by the average value of the RC product as below :

P D_COUT = (I RMS_COUT )2 x E SR_COUT

Besides, the capacitor’s ESL (Equivalent Series inductance) maybe causes ringing in the low Mhz region. Choose low ESL capacitors, and parallel connecting several smaller capacitors to replace with a larger one will reduce the ringing phenomenon.

Input Capacitor Selection

He input capacitor is required to supply current to regulator and maintain the DC input voltage. Low ESR capacitors

are preferred those provide the better performance and the less ripple voltage.

1. The input capacitors need an adequate RMS current rating. It can be calculated by following equation and

should not be exceed.

Step-Down White LED Driver

This equation has a maximum at V

IN =2V OUT . That is the worse case and the above equation can be simplified to :

Therefore, choose a suitable capacitor at input whose ripple current rating must greater than half of the maximum LED current.

2. The input ripple voltage(

∆

V IN

) mainly depends on the input capacitor’s ESR and its capacitance. Assuming the

input current of the regulator is constant, the required input capacitance for a given input ripple voltage can be

calculated as below :

If using aluminum electrolytic or tantalum input capacitors, parallel connecting a 0.1uF ceramic capacitor as close to the V IN pin of the regulator as possible. If using ceramic capacitor, make sure the capacitance is enough to prevent the excessive input ripple current.

3. The power dissipation of the input capacitor causes a small conduction loss can be calculated as below :

P D_CIN = (I RMS_CIN )2 x E SRF_CIN

Step-Down White LED Driver

Package Information ( SOP-8P )

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Tel : 886 2) 8791 0162 Fax : 886 2) 8791 7482 ©http://www.alfa-mos.com