CrM Buck LED Driver Evaluation Board

AND8480/D
CrM Buck LED Driver
Evaluation Board
Prepared by: Fabien Franc
ON Semiconductor
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APPLICATION NOTE
Introduction
This document describes the CrM Buck LED driver
evaluation board. This board provides a step−down
converter for driving up to 6 strings of LEDs at 240 mA from
an unregulated high−voltage supply up to 250 V. Each
channel is regulated using the ON Semiconductor CAT4201
step−down LED driver. The solution provides a very high
overall efficiency around 95%, minimizing the heat
dissipation and is ideal for powering a large number of
LEDs, such as those found in LCD TV backlight
applications. Mismatch between LED string forward
voltages does not impact the overall efficiency as each string
has an independent buck converter.
evaluation board with the power supply. For an introduction
to the CAT4201 LED driver operation, please refer to pages
4, 5, and to the CAT4201 datasheet. Figures 2 and 3 show
pictures of the actual board.
The evaluation boards are set−up for driving LED strings
at a constant average current of 240 mA or 120 mA
depending on the board configuration. This note describes
the 240 mA board. Ideally, the LED string forward voltage
(VLED) is close to half of the supply voltage (VIN). For
example, a 100 V LED string voltage and a 200 V supply.
Figure 2 shows the recommended operating condition for a
range of supply and output voltages.
In order to support high supply voltages, each CAT4201
has an external cascode transistors connected between the
SW pin and the LED cathode. The LED current is set−up
independently on each channel by external resistors. On the
CrM buck 240 mA board, the LED current is set to 240 mA
on all channels. The board schematic is shown in Figure 3.
A PWM logic input (active high) allows to turn on all
6 channels together. The PWM can be used to control the
brightness of the LEDs by using a PWM signal where the
duty cycle sets the brightness. A frequency of 100 Hz is
recommended to get the best dimming resolution.
Board Description
The step−down conversion is achieved by a reverse
Critical conduction Mode (CrM) switching regulator
(CAT4201) using a high−voltage cascode MOSFET to
support supply voltages up to 250 V. The switching
frequency automatically adjusts itself depending on the
supply voltage and the LED load configuration.
The board requires a regulated low voltage supply VCC
(12 V) to bias the CAT4201 LED drivers (VBAT pin).
Figure 1 shows a simplified block diagram of the CrM buck
200 V Not Regulated VIN
VA C
Anode Voltage
400 V Half Bridge
LLC SMPS
PFC
90 V to
264 V
Cathode
Voltage
Power Supply
VIN
LED1
LED6
VCC
12 V Regulated
PWM
Dimming
Control
UVLO
and
Control
Logic
CAT4201
Step−down
LED Driver
........X 6 Channels........
CrM Buck LED Driver Evaluation Board
GND
Figure 1. CrM Buck LED Driver Block Diagram
© Semiconductor Components Industries, LLC, 2010
December, 2010 − Rev. 0
1
Publication Order Number:
AND8480/D
AND8480/D
Figure 2. CrM Buck LED Driver Board (Top Side)
Figure 3. CrM Buck LED Driver Board (Bottom Side)
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Detailed Operation
LED Brightness Setting
The LED current is set to 240 mA in each channel by the
12 kΩ resistors (Rx02) connected between each CAT4201
RSET pin and ground. For setting the LED current to
another value, please refer to the CAT4201 datasheet to find
out the desired resistor values.
The LEDs can be dimmed dynamically by applying a
100 Hz PWM signal to the PWM input. Figure 5 shows the
PWM waveform and the LED current.
Operating Voltage Range
The graph below shows the recommended operating
region for a range of VLED string voltage and supply voltage
VIN defined by the equation:
1.3
V LED t V IN t 2.5
V LED
For example, with VLED = 100 V, the recommended VIN
range is between 130 V minimum and 250 V maximum.
300
targeted
operation
VIN, SUPPLY VOLTAGE (V)
250
200
150
100
50
0
50
100
150
200
Figure 5. PWM Waveform with 5% Duty Cycle
VLED, LED STRING VOLTAGE (V)
Figure 4. Region of Operation
NOTE:
UVLO Set−up
NOTE:
Figure 6 shows the variation of the LED current versus the
PWM duty cycle.
The board is configured for operating VIN voltage of 150 V
minimum. It will not turn on for a supply below 150 V.
100
+ V CC
ǒ
+ 12 V
ǒ
+ 12 V
12.6 + 150 V
1)
(R 12 ) R 11)
(R 9 ) R 10)
LED CURRENT (%)
The high voltage power supply (VIN) should be always
greater than the LED forward voltage (VLED) to guarantee
proper regulation of the LED current. An undervoltage
lockout (UVLO) protection logic is included on the board to
shutdown all LED drivers when VIN drops below a defined
threshold (VUVLO). Figure 14 shows the control logic and
UVLO circuitry. The following formula shows how to
calculate the UVLO threshold voltage.
V UVLO + V IN(min)
Ǔ
(620 kW ) 470 k)
1)
(47 kW ) 47 kW)
The PWM input voltage should not exceed 5 V maximum.
The PWM logic high level should be 2.5 V or higher.
10
1
0.1
Ǔ
0.1
1
10
100
PWM DUTY CYCLE (%)
Figure 6. LED Current vs. PWM Duty Cycle
It is recommended to have VUVLO greater than the
VLEDmax + 10 V. The UVLO threshold can be changed, if
needed, by replacing resistor R11 or R12.
The board is configured for 12 V supply VCC, if a different
supply (must be below 15 V), some of the above resistors
must be changed accordingly.
There is also an undervoltage protection on the supply
VCC. If VCC drops below a threshold of about 7.6 V, the
CAT4201 CTRL signal is pulled low and all CAT4201
drivers are disabled.
LED Current Ripple
The LED current ripple depends on several parameters
including the LED current, the CAT4201 switching
frequency and the size of the output capacitors Cx00A on
each channel. A 0.47 mF capacitor is recommended for
240 mA LED current. For smaller LED current values, the
value of the output capacitors can be reduced. For 120 mA
LED current, a 0.22 mF capacitors can be used.
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100
EFFICIENCY [%]
98
4
150 V
200 V
96
94
VIN = 250 V
92
2
90
75
100
125
LED VOLTAGE [V]
150
Figure 8. Efficiency vs. VIN & VLED
Figure 9 shows the LED Current regulation versus VLED.
The LED current regulation is best when the supply VIN is
equal to twice VLED.
5
−4
Ch1
Ch2
Ch3
Ch4
CHANNEL
Ch5
Ch6
LED CURRENT VARIATION [%]
LED CURRENT VARIATION PERCENT [%]
The voltage rating of the output capacitors Cx00A should
be equal to or greater than the maximum supply voltage (VIN)
in order to handle the open LED conditions. For example, for
VIN supply voltages up to 250 V, use 250 V rated capacitors.
Figure 7 shows the LED current variation in the six
different channels; the 0% is referenced to the average of all
channles.
Figure 7. Channel Matching
Performance
Figure 8 shows the efficiency versus the total LED voltage
and for various VIN supply voltages. For example, at VIN =
200 V, VLED = 100 V, the efficiency is about 95%. Since the
efficiency is high, there is very little heat dissipated in the
components on the board.
4
3
2
VIN = 250 V
160 V
−2
200 V
−3
−4
−5
75
100
125
LED VOLTAGE [V]
150
Figure 9. LED Current Line Regulation
CAT4201 LED Driver Operation
Each LED string is controlled by a CAT4201 step−down LED driver which operates in Critical conduction mode (CRM)
or Zero current switching (ZCS) mode where the inductor current swings between zero and twice the average LED current.
Figure 10. Simplified CAT4201 Block Diagram
The external RSET resistor sets the average LED current.
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AND8480/D
Figure 11 shows the switching waveform of the CAT4201
SW pin and the current flowing in the inductor.
For duty cycle calculations use the following equation.
Duty Cycle + 1 *
ǒ Ǔ
V LED
V IN
In a typical 240 mA current setting and a 100 V string of
LEDs using the 1mH inductor and a 200 V supply, the
frequency should be about 90 kHz at a duty cycle of 62.5%.
We recommended the 1.0 mH inductor from Wurth
Elektronik part number 744732102. Switching frequencies
higher than 20 kHz are recommended to guarantee the best
regulation. A more detailed illustration of the switching
frequency is modeled by Figure 13.
160
FREQUENCY [kHz]
240 mA / 1 mH
Figure 11. SW Pin and Inductor Current Waveform
In the case of the CrM buck application, long strings of
LEDs require a high−voltage supply. Due to the limitation
on the CAT4201 SW pin voltage rating (maximum 40 V),
there is a need to add high−voltage cascode transistors
(Qx00) between the inductor and the CAT4201 SW pin. The
board is populated with 500 V N−channel transistors
NDD03N50ZT4G from ON Semiconductor.
Figure 12 shows the power up of the circuit with the
steady−state current ripple.
VIN = 250 V
120
80
200 V
150 V
40
90
75
100
125
150
LED VOLTAGE [V]
Figure 13. Frequency vs. Input Voltage
Open and Short LED Faults
The CrM Buck board can handle both open−LED and
shorted−LED fault conditions. Since each of the 6 strings are
independent from other strings, these fault conditions do not
cause any significant loss of efficiency or extra heat
dissipation and do not require external circuitry for
protection.
In case an LED string becomes disconnected, the
associated CAT4201 LED driver stops switching and the
LED cathode is pulled to Ground. The voltage across the
corresponding capacitor Cx00A is equal to the full VIN.
Therefore each capacitor should be rated to VIN. There is no
fault detection and no need for protection. The CAT4201
resumes normal operation as soon as the LED string is
reconnected.
In case some of the LEDs within a string become shorted,
the LED forward voltage is reduced and the switching
frequency will adjust itself and increase, but the average
LED current remains unchanged. The board can handle a
large number of shorted LEDs. It is not recommended to
operate with less than 4 LEDs per string as the LED current
may increase.
Figure 12. Power−Up Waveforms
Switching Frequency
For critical conduction mode, the switching frequency of
the LED driver is dictated by the input voltage VIN, forward
LED voltage drop VLED, output current ILED, and the
inductor (Lx00). This relationship can be closely modeled
by the following equation, with about 14% to account for
non−idealities and loss in the various components.
ȡV
ȧ2
Ȣ
LED *
Frequency + 0.86
ȣ
Lȧ
Ȥ
VLED 2
VIN
I LED
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AND8480/D
Figure 14. Schematic for Control Logic and One Channel
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AND8480/D
• Connect the PWM input to the connector CN02 P1. The
Test Procedure
Warning: Due to the high−voltage present on the board,
power supply and LED load; the set−up should be handled
with care.
The following steps are needed for the installation of the
board together with the two power supplies and the load. In
this procedure, we will limit the usage to a single channel
load. Any populated channel can be tested individually. The
load should be an LED string, or an equivalent resistive load,
with a voltage drop of about 50 V to 60 V when biased with
a 240 mA current. This test procedure is designed for
channel 1 and can be replicated for the other channels.
Figure 15 shows the correct test setup.
•
•
•
•
•
160 V Not Regulated VIN
Anode
Voltage
VIN
•
•
Cathode
Voltage
A Ammeter
VIN
12 V VCC
VIN
0−5 V
PWM
Dimming
Control
Function
Generator
UVLO
and
Control
Logic
•
•
LED1
CAT4201
Step−down
LED Driver
CAT4201
CTRL Pin
•
•
V Voltmeter
•
CrM Buck LED Driver
Evaluation Board
GND
•
Figure 15. Test Set−up
• Connect the 12 V DC external supply to the board
•
connector CN01 P2 (VCC). Connect the external supply
Ground to connector CN01 P3 (GND).
Connect a high−voltage supply ground to the connector
CN01 P4 (GND). Connect the high−voltage supply to
the connector CN01 P7 and P8 (VIN or V−Power). Set
the high−voltage power supply current limit to 500 mA,
(see Note 1).
•
•
•
PWM input should never exceed 5 V.
Before powering−up the board, an LED load (or
equivalent resistive load) should be connected to one of
the six LED channels on connector CN04. The
connector CN04 includes 6 LED cathode pins and 6
anode voltage pins connected together.
Connect the connector CN04 P2 (LED1 channel 1) to
either an LED string cathode, or to a resistive load.
Connect the connector CN04 P1 (VIN) to either an
LED string anode, or to the other side of a resistive
load.
Connect the PWM input to GND (disabled).
Turn on the 12 V supply (VCC).
Turn on the high−voltage supply (VIN) to 130 V to test
the under−voltage lockout (UVLO) function.
Set the PWM input to 5 V (enabled).
Make sure the LEDs do not turn on, and the CAT4201
CTRL pin (pin 1) voltage is under 0.5 V or logic low
(disabled).
Connect the PWM input to GND (disabled).
Set the high−voltage supply (VIN) to 160 V (above the
UVLO threshold).
Set the PWM input to 5 V (logic high).
Measure the CAT4201 CTRL pin voltage (IC100
pin 1), the pin voltage should be around 5.2 V, well
above the 3 V threshold to be in logic high (enabled).
Measure the current in the LED string (or resistive
load) with an ammeter, the average current should be
around 240 mA (120 mA for 120 mA board).
Using a function generator, set the PWM signal to have
a 300 Hz frequency, a 0 V to 5 V amplitude swing, and
a 50% duty cycle pulse train. Measure the average
current through the load which should be around half or
120 mA.
Connect the PWM input to GND (disabled).
Turn off all power sources starting with the
high−voltage supply (VIN).
When testing with a board setup for 120 mA maximum
LED current, the currents for 100% and 50% duty cycle
PWM should be around 120 mA and 60 mA
respectively.
1. If several LED channels are turned on, the high−voltage power supply current limit must be increased to the number of channels x 2 x 240 mA
(for 240 mA board).
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AND8480/D
Figure 16. CrM Buck Board 240 mA Schematic
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AND8480/D
BOARD LIST OF COMPONENTS
Table 1. CrM BUCK BOARD LIST OF COMPONENTS CONFIGURED FOR 240 mA LED CURRENT
Name
IC100, IC200, IC300,
IC400, IC500, IC600
Manufacturer
ON Semiconductor
Description
Part Number
Units
CAT4201 Buck LED Driver
CAT4201TD
6
450BXA47MEFG18X31.5
1
C01
RUBYCON
Electrolytic Cap, 47 mF, 450 V
C02
RUBYCON
Electrolytic Cap, 47 mF, 25 V
25ZL47M5X11
1
C100A, C200A, C300A,
C400A, C500A, C600A
Panasonic
Metal Polyester capacitor, 0.47 mF, 250 V
ECQ−E2474KB
6
C322C471JDG5TA
6
C0805F104K3RACTU
6
C101, C201, C301, C401,
C501, C601
Kemet
Ceramic capacitor, 470 pF, 1 kV
C102, C202, C302, C402,
C502, C602
Kemet
Ceramic capacitor, 0.1 mF, 25 V, 0805
CN01
8−pin connector
4324−08R
1
CN02
3−pin connector
4324−03R
1
12−pin connector
4324−12R
1
15 V zener diode, 500 mW, SOD−123
MMSZ15
1
High Speed Switching Diode
1N4148
1
MM3Z6V2
1
MUR140
6
NSR0340H
6
RC0805JR−070RL
6
744732102
6
BC848AL
3
CN04
D01
ON Semiconductor
Dx1
NXP
Dx2
ON Semiconductor
6.2 V zener diode, 200 mW, SOD−323
D100, D200, D300, D400,
D500, D600
ON Semiconductor
1 A, 400 V Ultrafast Rectifier
D101B, D201B, D301B,
D401B, D501B, D601B
ON Semiconductor
40 V, 0.3 A Schottky Diode, SOD−323
D103, D203, D303, D403.
D503, D603
L100, L200, L300, L400,
L500, L600
Yageo
Wϋrth Elektronik
0 Ω resistor, 1/8 W, 0805
1 mH inductor, 0.6 A, ±10%
Q01, Q02, Qx1
ON Semiconductor
NPN transistor, SOT23
Q03
ON Semiconductor
PNP transistor, SOT23
Q100, Q200, Q300, Q400,
Q500, Q600
ON Semiconductor
N−Channel 500 V, 2.3 A, DPAK
BC858AL
1
NDD03N50ZT4G
6
R01
Yageo
10 Ω resistor, 1%, 0805
RC0805FR−0710RL
1
R03, R05, R07, R08
Yageo
10 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−0710KL
4
R102, R202, R302, R402,
R502, R602
Yageo
12 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−0712KL
6
R04
Yageo
33 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−0733KL
1
R06
Yageo
1 MΩ resistor, 1%, 1/8 W, 0805
RC0805FR−071ML
1
R09, R10
Yageo
47 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−0747KL
2
R11
Yageo
470 kΩ resistor, 1% 1/8 W, 0805
RC0805FR−07470KL
1
R12
Yageo
620 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−07620KL
1
R100, R200, R300, R400,
R500, R600
Yageo
22 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−0722KL
6
Rx1, Rx3
Yageo
100 kΩ resistor, 1% 1/8 W, 0805
RC0805FR−07100KL
2
Rx2
Yageo
1 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−071KL
1
Rx4
Yageo
4.99 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−074K99L
1
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Listed below are only the components that differ from the 240 mA board configuration.
Table 2. CrM BUCK BOARD LIST OF COMPONENTS CONFIGURED FOR 120 mA LED CURRENT
Name
Manufacturer
Description
Part Number
Units
C100A, C200A, C300A,
C400A, C500A, C600A
Panasonic
Metal Polyester capacitor, 0.22 mF, 250 V
ECQ−E2224KF
6
D101B, D201B, D301B,
D401B, D501B, D601B
ON Semiconductor
MMSD4148
6
L100, L200, L300, L400,
L500, L600
Wϋrth Elektronik
1.0 mH inductor, 0.6 A, ±10% or
1.5 mH inductor, 0.3 A, ±10%
744732102 or
744732152
6
Yageo
24 kΩ resistor, 1%, 1/8 W, 0805
RC0805FR−0724KL
6
R102, R202, R302, R402,
R502, R602
100 V Switching Diode, SOD−123
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
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