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Common anode topology with Zetex hysteretic converters
George Goh & Silvestro Russo, Diodes Incorporated
Introduction
ZETEX hysteretic converters are widely used to drive LEDs due to their ease of use and inherent
stability. While this simple topology can be used in different configurations, a simple variant of
the buck configuration is the ‘common anode topology’ that provides several advantages
including; ease of wiring, linear or PWM dimming control, and regulation. Common Anode
topology can be easily implemented with the family of ZETEX hysteretic converters; the choice
depending on the level of current and voltage required. In particular, ZXLD1350 and ZXLD1360
are suited for 30V applications with currents from 350mA to 1A, while ZXLD1356, ZXLD1362 and
ZXLD1366 are suited for 60V application with currents from 550mA to 1A. Finally, for applications
that require a voltage lower than 20V with a current up to 1.5A, the ZXLD1320 is suggested.
Description
This application note specifically describes a driver solution developed using the Zetex ZXLD1360
LED driver IC to drive a series of LEDs in common anode topology, in conjunction with the high
thermally stable Super Barrier Rectifier SBR2A40P1 with 2A 40V capability and housed in the
proprietary PowerDI123 package.
The ZXLD1360 hysteretic converter features can be summarized as:
•
wide input voltage range 7V to 30V, with internal switch
•
Up to 1A output current
•
Capable of driving up to 8 series connected 3 Watt LEDs
•
High efficiency
•
Brightness control using DC voltage or PWM (low or high frequency)
•
Optional soft-start; up to 1MHz switching frequency
For more details about the hysteretic converter, please refer to the web site applications page.
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Common anode connection
For Buck LED controllers high sided current sense is preferred which usually positions the LED
after the current sense resistor and inductor. The simplicity of the hysteretic converters provides
flexibility to drive LEDs with a common anode scheme. The common anode circuit is shown in
Figure 1, and consists of connecting the anode of the LED directly to the supply voltage. The LED
string is still in series with the sense resistor and the inductor allowing for higher accuracy. The
Common Anode name usually refers to a configuration with a single LED (or a set of LEDs in
parallel), but the concept could be extended to a series of LEDs, or several chains of LED that
share the same V+ rail.
.
.
VIN
.
.
R sense
ZXLD1360
.
Figure 1 - Common anode connection
This configuration has several advantages mainly related to the circuit performance, but also to
installation convenience and component count in the system. From the performance point of
view, the circuit shows an improved load regulation compared to the standard buck topology.
Thermal management also becomes simpler for multiple LED chain systems as the anodes can
all sit on one heat sink at the same potential. Finally, since the voltage variation on the input is
reduced, the common anode configuration allows for a smaller input capacitor.
The common anode topology simplifies the installation in signage and light wall applications,
where drivers are usually separated from the LED chains. In this case, the first anode of each chain
is directly connected to the power supply, so a single wire is needed to connect all the chains
nonetheless a separate wire is still needed to connect the cathodes of each chain.
Finally, the common anode enables savings not only in the wiring side, but also on the
component side. There is usually a capacitor in parallel with the string of LEDs which is not
necessary, since the input capacitor already filters the ripple across the LEDs.
The main disadvantage of the common anode connection is that the amount of voltage available
for the load is lower compared to the standard buck configuration.This reduces the number of
LEDs that can be driven by the system, since VLOADmax= VSUPPLY - VINmin.
An application example of the common anode configuration has been chosen with ZXLD1360
driving a set of LEDs from 1 to 6.
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Circuit diagram
In this application note, a specific design of a 1A LED driver is chosen to demonstrate the common
anode features with a ZETEX based hysteretic converter (figure 2).
D1
R1
V IN
U1
VIN
ISEN SE
L1
Z XL D 1360
C1
ADJ
C3
AD J
LX
GN D
R2
C2
GN D
Figure 2 -common anode circuit schematic with ZXLD1360 and SBR2A40P1
Circuit description
In normal operation, when voltage is applied at +VIN, the ZXLD1360 internal NDMOS switch is
turned on. Current starts to flow through the LED, sense resistor R1, and the inductor L1. The
current ramps up linearly, and the ramp rate is determined by the input voltage +VIN and the
inductor L1. This rising current produces a voltage ramp across R1. The internal circuit of the
ZXLD1360 senses the voltage across R1 and applies a proportional voltage to the input of the
internal comparator. When this voltage reaches an internally set upper threshold, the NDMOS
switch is turned off. The inductor current continues to flow through the LED, R1, L1, and the
Schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined
by the forward voltage drop of the LEDs and the schottky diode. This decaying current produces
a falling voltage at R1, which is sensed by the ZXLD1360. A voltage proportional to the sense
voltage across R1 is applied at the input of the internal comparator. When this voltage falls to the
internally set lower threshold, the NDMOS switch is turned on again. This switch-on-and-off cycle
continues to provide the average LED current set by the sense resistor R1. Please refer to the
datasheets for the threshold limits, ZXLD1360 internal circuits, electrical characteristics and
parameters.
Circuit design
The ZXLD1360 and SBR2A40P1 are configured to the reference design in Figure 2.
The operating voltage is a nominal 30V, while the nominal current is set at 1A with a 0.1W sense
resistor R1. With these parameters, the system operates in continuous mode at 150kHz
approximately , with a 100uH inductor and a single LED.
The ADJ pin has a low pass filter within the ZXLD1360 chip to provide some decoupling and soft
start but an external capacitor C2 (100nF) is used to provide additional decoupling to reduce any
high frequency noise as well as providing an extra amount of soft start . The soft-start time will
be nominally 0.5ms without capacitor C2. Adding the capacitor C2 will increase the soft start time
by approximately 0.5ms/nF.
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Component choice and design can be done using the circuit calculator downloadable from the
Diodes website.
Circuit performance
The circuit performance is measured in terms of
•
Switching frequency
•
Duty Cycle
•
Load regulation
•
Line regulation
•
Efficiency
The following trends are worth noting for this system at a given supply voltage.
In this case, the switching frequency increases with the number of LEDs. This idea is clearly
confirmed by figure 3, for systems using up to 3 LEDs, but seems to fail for a greater number of
LEDs. In fact, systems with 4, 5 and 6 LEDs have a lower frequency than expected when the supply
voltage is 25V or 30V.
The reason for this trend is that the voltage VSUPPLY – VLOAD tends to be closer to the minimum
driver input voltage, reducing its switching frequency.
C o m m o n A n o d e s w itc h in g fre q u e n c y
350
1 LE D
300
2 LE D s
3 LE D s
F req u en cy [kHz ]
.
250
4 LE D s
5 LE D s
200
6 LE D s
150
100
50
0
0
5
10
15
20
25
30
35
S u p p ly V o lta g e [V ]
Figure 3
In figure 4, the duty cycle value in relation to the number of LEDs and the input voltage value is
shown.
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C o m m o n A n o d e D u ty C y c le
80
1 LE D
Du ty Cycle [% ]
70
2 LE D s
60
3 LE D s
50
5 LE D s
4 LE D s
6 LE D s
40
30
20
10
0
0
5
10
15
20
25
30
35
S u p p ly V o lta g e [V ]
Figure 4
Load and line regulation, have good performance in common anode configuration. Line
regulation is in the range of 1mA/V for LEDs between 1 and 5. This means that for this specific
case, the percentage variation of output current for each volt of variation of the input voltage is
about 0.3%. (Figure 5)
C o m m o n A n o d e L in e R e g u la tio n
1040
1 LED
1030
2 LED
In p u t C u rren t [m A ]
.
1020
3 LED
4 LED
1010
5 LED
1000
6 LED
990
980
970
960
950
940
0
5
10
15
20
25
30
35
V Su p p ly [V ]
Figure 5
Load regulation is about 5mA for each LED (Figure 6) but this strongly depends on the LED’s
characteristics, namely its forward voltage drop.
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C o mmo n An o d e - L o ad re g u latio n (V S u p p ly = 30V)
1000
1100
1080
S w itc hing frequenc y
700
1060
1040
LE D c urrent
600
1020
500
1000
400
980
300
960
200
940
100
920
0
.
800
L E D Cu rren t [m A]
S w itch in g freq u en cy [kHz ]
.
900
900
0
1
2
3
4
5
6
7
N u m b e r o f L ED s
Figure 6
The efficiency is between 70% and 80% in the case of a single LED, while is higher than 90% when
the common anode configuration drives three or more LEDs, as shown in figure 7.
C o m m o n A n o d e Efficie n cy
100
90
Efficien cy [% ] .
80
70
60
1 LED
50
2 LED
40
3 LED
30
4 LED
20
5 LED
10
6 LED
0
0
5
10
15
20
25
30
35
V Su p p ly [V ]
Figure 7
Both DC and PWM dimming can be achieved by driving the ADJ pin.
For DC dimming, the ADJ pin may be driven between 0.3V and 1.25V with an analog signal. In
particular, driving the ADJ pin below 0.2V will shutdown the output current. On the other hand,
for PWM dimming, an external open-collector NPN transistor or open-drain N-channel MOSFET
can be used to drive the ADJ pin.
Due to the internal filter features, PWM dimming can be performed in two ways. Firstly, if the
PWM frequency is low, around 100Hz to 1kHz, the PWM signal effectively turns ON and OFF the
LED current, with a maximum PWM resolution of 8bits. Secondly If the PWM frequency is high,
between 10kHz to 50kHz, the PWM signal is effectively filtered by the ADJ pin internal filter,
providing an analog signal to the input of the internal comparator.
For low frequency PWM, C2 should be removed on the evaluation board to give a more accurate
duty cycle. In figure 8, the PWM dimming linearity is shown, in the case of a dimming frequency
of 200Hz and a load of 6 LEDs. The system is able to provide good performance in terms of
linearity for a duty cycle higher than 3%.
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Com m on Anode - P W M
D im m in g L in e a rity @ 200H z
1000
900
6 LEDs @ 30V
.
800
L ED cu rren t [m A ]
700
600
500
400
300
200
100
0
0
10
20
30
40
50
60
70
80
90
100
Du ty C ycle [%]
Figure 8
This system is able to guarantee a high level of efficiency for a wide range of duty cycle values
(10% to 100%) when PWM dimming is performed. For Duty Cycles lower than 10%, the efficiency
tends to be low, but the level of power involved is small, allowing it to keep the system in a
thermally safe condition.
C o m m o n A n o d e - P W M D im m in g a n d P o w e r Efficie n cy
100
25
ef f ic ienc y
90
60
15
50
40
10
30
20
5
T o tal o u tp u t p o w er [W ]
D im m in g efficien cy [% ]
.
20
70
.
pow er LEDs
80
10
0
0
10
20
30
40
50
60
70
80
90
0
100
Du ty C ycle [%]
Figure 9
PCB and layout considerations
In Figure 10, the layout of the common anode board is provided. From a layout perspective, the
most important suggestions are:
•
Current sense resistor R1, has to be connected to the Vin and Vsense pins using short and
thin tracks, providing an high impedance path for the currents
•
Inductor L1 and freewheeling diode D1, should be connected to ZXLD1360 using short tracks
•
The ADJ pin needs special care, since it is a high impedance pin. Therefore, a ground ring
around the pin is preferable, and a small capacitor connected to ground (C2) could help to
avoid any noise on the pin.
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•
A small capacitor (C3), should be placed across Vin and GND, to filter any high frequency
noise on the Vin pin and improve accuracy.
Figure 10
Bill of materials
From table 1, the Bill of Materials for the common anode board, it is evident that the common
anode topology maintains the same characteristic of reduced component count as the standard
buck configuration.
Table 1
Ref
Value
Part Number
Manufacturer
R1
0.1R1 1%
NCST12FR100FTRF
R2
1K
generic
C1
2.2uF, 50V
50V, 1210 X7R
NMC1210X7R225K50F
NIC
components
C2
1uF, 50V
50V, 1206 X7R
NMC1206X7R105K50F
NIC
components
C3
100nF
50V, 0805 X7R
NMC0805X7R104K50F
NIC
components
Notes
NIC
components
C4
-
Not fitted
L1
100uH
MSS1038
Coilcraft
1.46A rms
Inductor
D1
40V, 1A
SBR2A40P1
Diodes
Super Barrier
Rectifier
U1
ZXLD1360
ZXLD1360E5TA
Zetex
DC-DC
converter
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Conclusion
Common anode is a useful configuration, which when combined with the inherent flexibility of
the hysteretic converter, allows us to achieve optimum thermal and electrical performance . This
configuration is particularly well suited to installations with long cable runs, reducing the wiring
and RF emissions.
Further information on the devices can be found at www.diodes.com
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Definitions
Product change
Diodes Incorporated reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or
service. Customers are solely responsible for obtaining the latest relevant information before placing orders.
Applications disclaimer
The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
assumed by Diodes Zetex with respect to the accuracy or use of such information, or infringement of patents or other intellectual property
rights arising from such use or otherwise. Diodes Zetex does not assume any legal responsibility or will not be held legally liable (whether
in contract, tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business,
contract, opportunity or consequential loss in the use of these circuit applications, under any circumstances.
Life support
Diodes Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express
written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labelling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to
cause the failure of the life support device or to affect its safety or effectiveness.
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representation relating to the products or services concerned.
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All products are sold subjects to Diodes Zetex’ terms and conditions of sale, and this disclaimer (save in the event of a conflict between the
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For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Diodes Inc. sales office .
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Diodes Zetex Semiconductors Limited is an ISO 9001 and TS16949 certified semiconductor manufacturer.
To ensure quality of service and products we strongly advise the purchase of parts directly from Diodes Inc. or one of our regionally
authorized distributors. For a complete listing of authorized distributors please visit: www.diodes.com
Diodes Zetex Semiconductors does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales
channels.
ESD (Electrostatic discharge)
Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices.
The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.
Devices suspected of being affected should be replaced.
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exceeding regulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use of hazardous substances and/or emissions.
All Diodes Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance
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Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
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“Obsolete”
Production has been discontinued
Datasheet status key:
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This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version”
This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
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