Boost Buck LED driver (Demo board description) DownloadLink 5306

Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
1.
Scope
This document describes the design and use of the MLX10803 Boost/Buck Evaluation Board. For a general
description about the functionality of the MLX10803 please refer to the MLX10803 data sheet.
Note: This document is valid for the MLX10803 Boost/Buck Evalboard Rev.2.2 or higher!
2.
Applications
The MLX10803 Boost/Buck Evaluation Board is intended to be used as an application example of the
MLX10803 Power LED driver. It was developed to show the possibility of driving a serial chain of LEDs with a
supply voltage that is lower than the total forward voltage of all LEDs. It works as a two stage step-up/step-down
converter by using 2 MLX10803 ICs. The board is suitable to be used in prototypes and mock ups to allow quick
implementation of the MLX10803 in a multiple LED lighting application.
3.
Other Components Needed
The board is optimized for a chain of serially connected LEDs with a total forward voltage of 20-24V and an
average LED current of 350mA. This corresponds to 6 white high brightness LEDs like Lumileds Luxeon I® ,
Cree XLamp™ 7090 or OSRAM Golden Dragon®. However, the board can be modified in order to suit other
applications with less or more LEDs.
4.
Application Schematic
Figure 1: MLX10803 Boost/Buck Evaluation board schematic
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
Page 1/8
Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
5.
Connector Pin Definitions
Connector J1 (VBAT)
Signal
Connection
Pin1
Pin2
Connector J2 (LED)
VBAT
GND
Signal
LED and IC supply, 9..18V DC
Supply ground
Connection
Pin1
Pin2
LED+
LED-
High Brightness LEDs anode
High Brightness LEDs cathode
6.
PCB Layout
Figure 2: MLX10803 Boost/Buck Evaluation board, dimensions and layout (top view)
Note:
Pin 1 of both connectors is marked with a square pad on the PCB. For connector J1 pin 1 is the upper pin, for connector J2
pin 1 is left (top view).
7.
Minimal Board Connections
The supply line VBAT (J1) must be connected to any standard DC supply. The VBAT voltage should be in the
range of 9..18V as the board is optimized for a 12V automotive net. The supply must be able to drive the peak
current of the board. For the original board a supply with a minimum current of ≥1A is recommended.
The LEDs must be connected to the LED connector (J2) according to the symbols ‘+’ (anode) and ‘-‘ (cathode).
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
Page 2/8
Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
8.
Description
For details on the function of the LED driver circuit, please refer to the MLX10803 IC specification.
The board consists of 2 relatively independent stages of MLX10803 driver ICs that work in different modes.
The 1st stage (U1) works as a pure voltage booster in order to generate the voltage that is necessary to drive
the chain of LEDs in the 2nd stage. The 2nd stage (U2) gets that boosted voltage as input voltage and drives the
LEDs with an average LED current of 350mA. This current is kept constant within the VBAT range of 9-18V.
8.1 Voltage Booster Stage
The 1st stage of the circuitry works as a voltage step-up converter. By switching ON transistor T1, inductor L1 is
charged and the voltage is boosted. The zener diode D4 limits the boosted voltage to a maximum of about 30V.
The input supply voltage is boosted to provide the necessary input voltage for the 2nd stage of the circuitry.
The sense resistor R3 (0.27Ω) and the setting resistor R2 (33kΩ) of the MLX10803 (U1) were selected in order
to limit the maximum peak current of coil L1 which is necessary to remain within its safe operation area
(saturation current Isat). We get:
U IREF 1 = IREF1 ⋅ R 2 = 50 µA ⋅ 33kΩ = 1.65V
U THRESH _ RSENSE = U IREF1 ÷ 5 = 330mV
I MAX _ RSENSE = U THRESH _ RESNSE ÷ R3= 1.22 A
When the threshold level UTHRESH_RSENSE is reached, T1 is switched OFF. With resistor R1 the drivers OFF time
(monoflop time Tmon) is set. During this time, energy is provided by the coil for supplying the 2nd stage of the
circuitry. As both stages do not work synchronously (i.e. with the same switching frequency), capacitor C3
serves as a temporary energy buffer. This way, C3 is charged in case the 2nd stage is in OFF time (driver FET
T2 is off) and discharged during T2 is ON.
Defining the value of R1 depends very much on the energy that is needed for the 2nd stage and thus on the
number and current of the connected LEDs and the input voltage VBAT. For this board, a value of 150kΩ was
chosen which generates a monoflop time of:
ROSC [kΩ] = 222.2 ⋅
Tmon[ µs ] = 12.5 ⋅
Tmon[ µs ]
− 0.02
12.5
(equation according to MLX10803 datasheet, page 10)
ROSC [kΩ] + 4.44
150 + 4.44
= 12.5 ⋅
222.2
222.2
Tmon = 8.7 µs
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
Page 3/8
Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
8.2 LED Driver Stage
The LED driver stage works in the same way as the standard MLX10803 EVA board; as a switching regulator.
The inductor L2 is charged until the desired threshold voltage on RSENSE (R9) is reached. This voltage is
proportional to the peak current that flows through the LEDs. The peak current in this application is:
U IREF 1 = IREF1 ⋅ R 2 = 50 µA ⋅ 30kΩ = 1.5V
U THRESH _ RSENSE = U IREF 1 ÷ 5 = 300mV
I MAX _ RSENSE = U THRESH _ RESNSE ÷ R9= 638mA
As soon as UTHRESH_RSENSE is reached, the MLX10803 (U2) shuts off the transistor T2 for a time defined by the
value of ROSC (monoflop time Tmon). During the OFF time of the circuit, L2 provides the previously stored energy
to the chain of LEDs via the flyback diode D3.
The OFF or monoflop time can be adjusted by ROSC (R7) resistor. With the selected 220kΩ resistor the
monoflop time is:
Tmon[ µs ] = 12.5 ⋅
ROSC [kΩ] + 4.44
220 + 4.44
= 12.5 ⋅
222.2
222.2
Tmon = 12.6 µs
With these both parameters -set by R7, R8 and R9- the MLX10803 Boost EVA board is currently optimized for
6 white high brightness LEDs (Luxeon, Osram Golden Dragon etc.) in series with a total forward voltage of
20-24V and a LED current of 350mA.
8.3 Efficiency
With this MLX10803 Boost/Buck Evaluation Board the following efficiency can be achieved (supply input power
versus output power of LEDs) for the configurations below at different supply input voltages:
[email protected]
[%]
[email protected]
[%]
[email protected]
[%]
20-24V
Number
of
LEDs
6
76
80
81
2
13-16V
4
78
79
80
3
34-40V
10
70
75
76
Configuration
Total UF_LEDs
[V]
1*
* default setting of the board
The efficiency for a certain application depends on the exact adaptation of the 1st stage to the energy needs of
the 2nd stage. Therefore, the booster voltage should always be in the range of 5-10V above the total forward
voltage of the LED chain.
Furthermore, the efficiency can be increased by using peripheral components with low intrinsic power losses.
Following components are subject for improvements in terms of efficiency:
•
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
Inductors L1, L2 (DC resistance)
Page 4/8
Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
•
•
•
Diodes D1, D2, D3 (forward voltage)
RSENSE resistors R3, R9 (DC resistance)
Transistors T1, T2 (RDSon))
8.4 User Modification of the board
As mentioned before, the board is optimized for a total LED forward voltage of 20-24V which corresponds to 6
white high brightness LEDs (Luxeon I®, Osram Golden Dragon® etc.).
However, the board can easily be modified to other customer requirements. The adaption is mainly
accomplished by a modification of the setting resistors of the 2nd stage, R7 and R8 and –if necessary- R1, R2
and D4 of the 1st stage of the circuit.
Generally, the modification for a certain application should be accomplished in 2 steps. First, the 2nd stage
should be adapted to the number and current of the LEDs to be driven. This can be done by the setting
resistors ROSC (oscillator frequency, R7) and IREF1 (peak current, R8). For details about how to set the
parameters please refer to the MLX10803 data sheet.
If the new load condition differs much from the default setting of the board it might also be necessary to modify
the 1st stage in order to adapt the booster stage to these new conditions.
This will be, for example, the case if the total forward voltage of the LED chain will be close or higher than the
boosted voltage of the 1st stage. As a general rule the boosted voltage should be around 5-10V higher than the
total forward voltage of the LED chain (measured at the respective LED peak current!). In case a higher booster
voltage is needed, the zener diode D4 must be adapted according to the desired booster voltage (e.g. Uz=43V
for 43V booster voltage).
If the delivered energy of the 1st stage is not sufficient to maintain the boosted voltage for the 2nd stage, more
energy has to be pumped into the inductor. This is possible by increasing the peak current of the inductor
(IREF1 Resistor R2) and/or by increasing the inductance of the coil.
The table below shows the configuration for applications with 4 and 6 (default) LEDs, driven at 350mA, and 10
LEDs, driven at 250mA, respectively. Other configurations are possible and have to be worked out by the user.
In such cases, attention has to be drawn to the maximum allowed parameters of peripheral components which
must not exceed their respective limitations (e.g. inductor current, voltage class of the caps etc.). Furthermore,
the thermal behaviour of the board must always be kept in mind.
Configuratio
n
Total UF_LEDs
[V]
R7
[KΩ
Ω]
R8
[KΩ
Ω]
R1
[KΩ
Ω]
R2
[KΩ
Ω]
D4
[Uz/V]
20-24V
Number
of
LEDs
6
1*
220
30
150
51
30
2
13-16V
4
300
30
220
39
22
3
34-40V
10
200
27
100
68
47
* default configuration of the board
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
Page 5/8
Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
9.
Used Components
Board
Part
Number
Type
Category
Part Name
Alternative
Data Sheet Download
(Manufacturer) Part
(Manufacturer)
D1
Rectifier
Schottky
Diode
SS24
(FCH)
many
www.fairchildsemiconductor.com
D2, D3
Rectifier
Ultrafast
Recovery
Diode
BYD77B
(PHI)
many, e.g.
ES2D (FCH)
www.fairchildsemiconductor.com
www.semiconductors.philips.com
D4
Zener
Diode
30V Zener
Diode
BZV55-C30
(PHI)
many
www.semiconductors.philips.com
D5, D6
Diode
Switching
Diode
MCL4148
(VS)
many, e.g.
1N4148
(PHI)
www.vishay.com
www.semiconductors.philips.com
T1, T2
Switching
Transistor
n-channel
MOSFET
BSP318S
(INF)
many, e.g.
NTF3055
(ON)
www.infineon.com
www.onsemi.com
L1
Inductor
Power choke
WE-PD XL
100µH
(WE)
many, e.g.
www.wuerth-elektronik.de
MSS1278-104ML www.coilcraft.com
(CC)
L2
Inductor
Power choke
WE-PD L
470µH
(WE)
many, e.g.
(CC)
www.wuerth-elektronik.de
www.coilcraft.com
C1
Capacitor
470nF/50V
Ceramics
1210 type
X7R
F3102X7R
(KMT)
many, e.g.
B37950
(EP)
www.kemet.com
www.epcos.com
C2
Capacitor
47µF/35V
Electrolytic,
low ESR
EEEFK1V470P
(PS)
many
www.panasonic.com/industrial
C3
Capacitor
47µF/50V
Electrolytic,
low ESR
EEEFK1H470P
(PS)
many
www.panasonic.com/industrial
C4, C5,C6
Capacitor
100nF/50V
Ceramics
0805 type
X7R
B37941
(EP)
many
www.epcos.com
R1, R2, R4,
R7, R8
Resistor
5% tolerance
R1: 150k, R2: 51k
R4: 30k, R7: 220k
R8: 30k
Resistor 0.5W
1% tolerance
R3: 0.27R
R9: 0.47R
Resistor
5% tolerance
R5, R6: 39R
0805 type
D12CRCW
(VS)
many, e.g.
Yageo RC 0805
(YG)
www.vishay.com
www.yageo.com
1206 type
LR1206-R33Fl
(WW)
many
www.welwyn-tt.co.uk
0805 type
D12CRCW
(VS)
many, e.g.
Yageo RC 0805
(YG)
www.vishay.com
www.yageo.com
R3, R9
R5, R6
manufacturer codes:
FCH = Fairchild Semiconductor
WE = Würth Elektronik
INF = Infineon
YG = Yageo
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
PHI = Philips Semiconductor
VS = Vishay
CC = Coilcraft
WW = Welwyn
Page 6/8
PS = Panasonic
ON = On Semiconductor
EP = Epcos
KMT = Kemet
Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
10. EMI Measurement of the MLX10803 Boost/Buck EVA board
The MLX10803 Boost/Buck Evaluation Board is designed for good electromagnetic emission behaviour. The
test was performed on the original board and the settings shown below. No additional filter components have
been used. The conducted electromagnetic emission measurement was accomplished according to IEC 61967
part 4. The RF was decoupled from line VBAT.
Settings MLX10803_Boost/Buck Evalboard:
VBAT
LEDs:
Board:
13.5V
6x Luxeon III @ ILED = 350mA (average)
according to Figure 1
Figure 3: Conducted electromagnetic emission MLX10803 Boost/Buck Evaluation board
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
Page 7/8
Application Note
MLX10803 Boost/Buck EVA board
Power LED driver for automotive applications
Disclaimer
Devices sold by Melexis are covered by the warranty and patent indemnification provisions appearing in its
Term of Sale. Melexis makes no warranty, express, statutory, implied, or by description regarding the
information set forth herein or regarding the freedom of the described devices from patent infringement. Melexis
reserves the right to change specifications and prices at any time and without notice. Therefore, prior to
designing this product into a system, it is necessary to check with Melexis for current information. This product
is intended for use in normal commercial applications. Applications requiring extended temperature range,
unusual environmental requirements, or high reliability applications, such as military, medical life-support or lifesustaining equipment are specifically not recommended without additional processing by Melexis for each
application.
The information furnished by Melexis is believed to be correct and accurate. However, Melexis shall not be
liable to recipient or any third party for any damages, including but not limited to personal injury, property
damage, loss of profits, loss of use, interrupt of business or indirect, special incidental or consequential
damages, of any kind, in connection with or arising out of the furnishing, performance or use of the technical
data herein. No obligation or liability to recipient or any third party shall arise or flow out of Melexis’ rendering of
technical or other services.
© 2002 Melexis NV. All rights reserved.
For the latest version of this document, go to our website at:
www.melexis.com
----------------------------------------------------------------------------Or for additional information contact Melexis Direct:
Europe and Japan:
Phone: +32-13-61 16 31
E-mail: [email protected]
All other locations:
Phone: +1-603-223 2362
E-mail: [email protected]
[email protected]-----------QS9000, VDA6.1 and ISO14001 Certified
Rev.1.0
Date: 31/07/2006
Author: SSZ/ANP
Page 8/8
Similar pages