ST AN2259 Application note

AN2259

APPLICATION NOTE

High intensity LED driver using the L5970D/L5973D

Introduction

High brightness LEDs are becoming a prominent source of light and often have better efficiency and reliability than conventional light sources. While LEDs can operate from an energy source as simple as a battery and resistor, most applications require an efficient energy source not only for the reduction of losses, but also to maintain the brightness of the LED itself. For applications that are powered from low voltage AC sources typically used in landscape lighting or low voltage DC sources that may be used in automotive applications or to meet safety requirements, high efficiency DC-DC converters configured for constant output current provide a high efficiency driver that can operate over a relatively wide range of input voltages to drive series strings of one to several LEDs

This application note describes a DC-DC converter circuit that can easily be configured to drive LEDs at several different output currents and can be configured for either AC or DC input. The circuit uses the L5973D monolithic step down converter configured to drive a series string of LEDs in a constant current mode.

L5970D is a step down monolithic power switching regulator capable of delivering 1A while the L5973D is able to deliver 2A at output voltages from 1.25V to 35V. Both devices use internal P-

Channel D-MOS transistors (with typical RDS(on) of 250mΩ) as the switching element to minimize the size of external components. An internal oscillator fixes the switching frequency

at 250kHz.

The brightness of the LED (Light Emitting Diode), or light intensity as measured in Lumens, is proportional to the forward current flowing through the LED. Since the forward voltage drop of the LED can vary from device to device it is important to drive the LEDs with a constant current driver to be able to get good matching of the light output, especially when they are located side by side where variations in light intensity are quickly noticed. A typical way to drive LEDs in the constant current mode is to use a DC-DC converter configured to give a constant current output. The circuit shown in Figure 7. uses the L5973D in a constant current configuration to drive LEDs

	Rev 1.0
AN2259/1105	1/38

www.st.com

AN2259

Figure 1. Board Layout

2/38

AN2259

Contents

1 DESCRIPTION OF BOARD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 INPUT CAPACITOR SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 CURRENT FEEDBACK LOOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 INDUCTOR SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 BOARD LAYOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6 REFERENCE DESIGN VERSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7 REVISION HISTORY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3/38

AN2259

Figures

Figure 1. Board Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Figure 2. Current feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 3. Ripple Current (One 1W LED). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 4. Ripple current (One 5W LED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 5. Top side of Board (not in scale) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 6. Bottom side of Board (not in scale) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 7. Board Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 8. 12Vac Input 1W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 9. 12Vac Input 3W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 10. 12Vac Input 5W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 11. 6 to 12Vdc Input 1W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 12. 6 to 12Vdc Input 3W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 13. 6 to 12Vdc Input 5W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 14. 6 to 24Vdc Input 1W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . 30 Figure 15. 6 to 24Vdc Input 3W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . 32 Figure 16. 6 to 24Vdc Input 5W LED Driver Schematic . . . . . . . . . . . . . . . . . . . . . . . 34

4/38

AN2259

Tables

Table 1. Bill of Matarials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 2. Components Changes For Different Configuration . . . . . . . . . . . . . . . . . . 16 Table 3. 12Vac Input 1W LED Driver Bill of Materials. . . . . . . . . . . . . . . . . . . . . . . 19 Table 4. 12Vac Input 3W LED Driver Bill of Materials. . . . . . . . . . . . . . . . . . . . . . . 21 Table 5. 12Vac Input 5W LED Driver Bill of Materials. . . . . . . . . . . . . . . . . . . . . . . 23 Table 6. 6 to 12Vdc Input 1W LED Driver Bill of Materials . . . . . . . . . . . . . . . . . . . 25 Table 7. 6 to 12Vdc Input 3W LED Driver Bill of Materials . . . . . . . . . . . . . . . . . . . 27 Table 8. 6 to 12Vdc Input 5W LED Driver Bill of Materials . . . . . . . . . . . . . . . . . . . 29 Table 9. 6 to 24Vdc Input 1W LED Driver Bill of Materials . . . . . . . . . . . . . . . . . . . 31 Table 10. 6 to 24Vdc Input 3W LED Driver Bill of Materials . . . . . . . . . . . . . . . . . . . 33 Table 11. 6 to 24Vdc Input 5W LED Driver Bill of Materials . . . . . . . . . . . . . . . . . . . 35

5/38

1 DESCRIPTION OF BOARD	AN2259

1 DESCRIPTION OF BOARD

The evaluation board shown in Figure 1. was designed so that it can be configured to accept several different input voltages that are common for automotive and lighting applications. The most common input voltages are 12Vac, 12Vdc (for automotive) and 24Vdc. The board also allows the user to select the output current using the jumpers J2 and J4 on the board without having to change any components on the evaluation board. The standard configuration of the board includes a full wave bridge rectifier that is required for an AC input

6/38

AN2259	2 INPUT CAPACITOR SELECTION

2 INPUT CAPACITOR SELECTION

For DC input, the input capacitor, C1, is selected based on its ripple current rating for the capacitor. The ripple current is calculated based on its duty cycle as outlined below.

D = Vo

Vin

Where D = duty cycle

Vo = output voltage

Vin = input voltage

The RMS current through the capacitor therefore is:

Iripple = Io D −	2	D2	+	D2
		η		η2

= efficiency

For an AC input voltage, the input capacitor is selected primarily to have enough capacity to supply the LED between the peaks of the AC input. The capacitor must be selected so that the minimum voltage at the input to the L5973D is maintained during each half cycle of the AC input.

Vpeak = 2 Vin

If the application is driving only one LED, the Vmin is determined by the minimum operating voltage specification for the L5973D (4.4V). When driving more than one LED in series, the minimum input voltage is determined by the output voltage and the minimum differential input to

output voltage for the regulator (the drop out voltage). In this case Vmin = (x * Vf) + (Iout * Rsense) + VDO

Where: x = number of LED in series

Vf = forward voltage of one LED

Io = LED drive current

VDO = Drop out voltage

The capacitor can then be selected using the equation:

	C =			5 10−3 Io Vo
		η (	1	Vpeak2 −	1	V min 2 )
			2
				2

The ripple current rating will have two parts where in the low frequency range, the capacitor will be charged by 120Hz while at the high frequency range the capacitor is discharged by 250kHz. For the low frequency part, it is approximately the same as the input RMS current and the power factor is approximately 0.7 for a full wave rectifier.

7/38

2 INPUT CAPACITOR SELECTION			AN2259
	Iin =	Io Vo
		Vin η 0.7

For the high frequency part (ignoring output current ripple), we have:

Iripple = Dav − 2 Dav2 + Dav2 Io

η η2

where Dav is the average duty cycle.

We will use the average duty cycle assuming that the voltage on the capacitor changes from the peak to the minimum voltage linearly.

Dav =	Vo	Vav =	Vpeak + V min
	Vav
				2
Iripple =		Dav − 2 Dav2	+ Dav2	Io
		η	η2

The equivalent series resistance of an aluminum capacitor has different frequency characteristics. There is a coefficient associated with different frequencies. Typically, for 120Hz, Kfl=1; for frequency greater than 10 kHz, Kfh=1.5.

Icap =	Iin	2		Iripple 2
			+
	Kfl			Kfh

Therefore, the ripple current rating of the capacitor has to be greater than Icap

8/38

AN2259	3 CURRENT FEEDBACK LOOP

3 CURRENT FEEDBACK LOOP

To drive LEDs in a constant current mode, the feedback for the regulator is taken by sensing the voltage drop across the current sense resistor, Rs, as shown in Figure 2. The voltage divider between the sense resistor and the feedback pin (R1 and R2) scales the voltage at the feedback pin so that it equals the internal reference voltage at the desired current level.

Figure 2. Current feedback

In order to get Io = 350 mA, the values of R1, R2 and Rs are selected based on the following values.

Vref = 3.3V ; Vfb = 1.235V ; Ifb_bias = 2.5 10-6 A ; Rs=0.68Ω

Vsense=Rs.Io

Using the superposition method:

Vfb = Vref

R2

+ Vsense

R1

+ Ifb _ bias

R1 R2

R1+ R2

R1+ R2

R1+ R2

Vsense = Vfb − (Vref − Vfb)

R2

− Ifb _ bias R2

Io =

Vsense

R1

Rs

Vfb − (Vref − Vfb)

R2

− Ifb _ bias R2

Io =

R1

Rs

Since Vref and Vfb come from same band gap, they are directly correlated. K=Vref/Vfb=2.672. Therefore, the equation can be simplified to:

		1− (K − 1)	R2	Vfb − Ifb _ bias R2
			R1
	Io =

Rs

For 350mA output the selected values are:

R1 = 2.74kΩ,

R2 =1.30kΩ and

Rs = 0.68Ω.

9/38

3 CURRENT FEEDBACK LOOP	AN2259

For an output current of 700mA the value of Rs would be 0.34Ω. If R1 and R2 are small enough, the effect of the bias current can be ignored.

On the evaluation board, the value of Rs is selected by jumpers J2 and J4. When both J2 and J4 are open, the output current is set to 350mA. Inserting each jumper connects a 0.68Ω resistor in parallel with the 0.68Ω Rs. With J2 shorted, the output current will be set to 700mA and the output current becomes 1A with both J2 and J4 shorted.

10/38

AN2259	4 INDUCTOR SELECTION

4 INDUCTOR SELECTION

The output inductor is selected to limit the ripple current in the LEDs.

For example, for a given DC input voltage and an output current of 350mA, the peak current can be fixed to 500mA. This implies a ∆Imax = 300mA.

For an output current of 700mA, the peak current can be fixed to1000mA. This implies a ∆Imax= 600mA

Lmin= Vin − Vo Ton

∆ Im ax

Figure 3. Ripple Current (One 1W LED)

Figure 3. shows the ripple current measured with one 1W LED (warm white) at the output with 12Vac input. The measured ripple current is 180mA.

11/38

4 INDUCTOR SELECTION	AN2259

Figure 4. Ripple current (One 5W LED)

Figure 4. shows the ripple current driving one 5W LED at 1.05 A from a 12Vac input. The input current is 269mA.

12/38