High Intensity LED Drivers Using NCP3065/NCV3065

AND8298
High Intensity LED Drivers
Using NCP3065/NCV3065
Prepared by: Petr Konvicny
ON Semiconductor
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Introduction
High brightness LEDs are a prominent source of light and
have better efficiency and reliability than conventional light
sources. Improvements in high brightness LEDs present the
potential for creative new lighting solutions that offer an
improved lighting experience while reducing energy
demand. LEDs require constant current driver solutions due
to their wide forward voltage variation and steep V/I transfer
function. 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, high efficiency driver that can operate over
wide range of input voltages to drive series strings of one to
several LEDs.
OSRAM OSTAR™, TopLED™ and Golden Dragon™.
Configurations like this are found in 12 VDC track lighting
applications, automotive applications, and low voltage AC
landscaping applications as well as track lighting such as
under−cabinet lights and desk lamps that might be powered
from standard off−the−shelf 5 VDC and 12 VDC wall
adapters. The NCP3065/NCV3065 can operate as a switcher
or as a controller. These options are shown bellow.
The brightness of the LEDs or light intensity is measured
in Lumens and is proportional to the forward current flowing
through the LED. The light efficiency can vary with the
current flowing through the LED string.
The NCP3065 is rated for commercial/industrial
temperature ranges and the NCV3065 is automotive
qualified.
Demo Board Design Versions
The demo boards are designed to display the full
functionality and flexibility of NCP3065 as a driver to drive
various LEDs at the low voltage AC and DC sources. The
components are selected for the 15 W LED driver
application. Based on this circuit, there are many possible
configurations with different input voltages and output
power levels that could be derived by making some minor
components changes. Table 1 shows these different circuit
solutions. Each application is described by the schematic
and the bill of material and it has the option of LED dimming
by using an external PWM signal.
Component Selection
Inductor
When selecting an inductor there is a trade off between
inductor size and peak current. In normal applications the
ripple current can range from 15% to 100%. The trade off
being that with small ripple current the inductance value
increases. The advantage is that you can maximize the
current out of the switching regulator.
Figure 1. Buck Demo Board
NCP/NCV3065 Demo Board
This application note describes a DC−DC converter
circuits that can easily be configured to drive LEDs at
several different output currents and can be configured for
either AC or DC input. The NCP3065/NCV3065 can be
configured in a several driver topologies to a drive string of
LEDs: be it traditional low power LEDs or high brightness
high power LEDs such as the Lumileds Luxeon™ K2 and
Rebel series, the CREE XLAMP™ 4550 or XR series, the
© Semiconductor Components Industries, LLC, 2009
April, 2009 − Rev. 1
With Output Capacitor Operation
A traditional buck topology includes an inductor followed
by an output capacitor which filters the ripple. The capacitor
is placed in parallel with the LED or array of LEDs to lower
LED ripple current. With this approach the output
inductance can be reduced which makes the inductance
1
Publication Order Number:
AND8298/D
AND8298
Output Capacitor
smaller and less expensive. Alternatively, the circuit could
be run at lower frequency with the same inductor value
which improves the efficiency and expands the output
voltage range. Equation 2 is used to calculate the capacitor
size based on the amount of LED ripple.
When you choose output capacitor we have to think about
its value, ESR and ripple current.
C OUT +
No Output Capacitor Operation
V IN * V OUT
DI MAX
T ON
(eq. 2)
Current Feedback Loop
A constant current buck regulator such as the NCP3065
focuses on the control of the current through the load, not the
voltage across it. The switching frequency of the NCP3065
is in the range of 100 kHz − 300 kHz which is much higher
than the human eye can detect. This allows us to relax the
ripple current specification to allow higher peak to peak
values. This is achieved by configuring the NCP3065 in a
continuous conduction buck configuration with low peak to
peak ripple thus eliminating the need for an output filter
capacitor. The important design parameter is to keep the
peak current below the maximum current rating of the LED.
Using 15% peak−to−peak ripple results in a good
compromise between achieving max average output current
without exceeding the maximum limit. This saves space and
reduces part count for applications that require a compact
footprint. For the common LED currents such as the
350 mA, 700 mA, 1000 mA we setup inductor ripple
current to the $52.5 mA, $105 mA, $150 mA. With
respect these requirements we are able to select inductor
value (Equation 1).
L+
V IN * (1 * D) * D
DI
+
DV * 8 * f
8 * L * f 2 * DV OUT
To drive LEDs in a constant current mode, the feedback
for the regulator is taken by sensing the voltage drop across
the sensing resistor R12, see Figures 2 or 8. The RC circuit
(R10 & C5) between the sense resistor and the feedback pin
improves converter transient response. The low feedback
reference voltage of 235 mV allows the use of low power
and lower cost sense resistor. Equation 3 calculates the sense
resistor value.
I OUT +
LED current
(mA)
R sense
+
0.235 V
R sense
[A]
(eq. 3)
Sensing resistor value
(mW)
350
680
1/4W
700
330
1/4W
1000
220
1/4W
(eq. 1)
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2
V REF
AND8298
Table 1. COMPONENTS CHANGES FOR DIFFERENT CONFIGURATIONS
LED Driver
VIN
ILED
VF
L
COUT
R8
Application
(V)
(mA)
(V)
(mH)
(mF)
(W)
12 VDC 1 W LED
10 − 14
350
3.6
47
100
12k
150
0
3k3
47
100
16k
150
0
12k
12 VDC 3 W LED
10 − 14
12 VDC 5 W LED
10 − 14
24 VDC 5 W LED
700 or 350
700 or 1000
21 − 27
24 VDC 10 W LED
350
21 − 27
700
3.6 or 7.2
7.2 or 3.6
14
14
BUCK
12 VAC 1 W LED
14 − 20
12 VAC 3 W LED
14 − 20
12 VAC 5 W LED
Vout
NCP3065
14 − 20
350
700 or 350
700 or 1000
3.6 or 7.2
7.2 or 3.6
12 VAC 5 W
14 − 20
350
14
12 VAC 15 W
21 − 27
1000
14
100
12k
0
12k
68
100
160k
220
0
39k
68
100
150k
220
0
100k
47
100
7k5
220
0
7k5
47
100
22k
220
0
22k
47
100
36k
220
0
100k/16k
47
100
NU
220
0
NU
47
100
82k
average current through the LEDs Figure 5. The component
value of the RC filter are dependent on the PWM frequency.
Due to this, the frequency has to be higher. Figure 17
illustrates the linearity of the digital dimming function with
a 200 Hz digital PWM. The dimming frequency range for
digital input mode is basically from 200 Hz to 1 kHz. For
frequencies below 200 Hz the human eye will see the flicker.
The low dimming frequencies are EMI convenient and an
impact to it is small.
The Figure 3 shows us an example of solution A, which
uses the COMP pin to perform the dimming function and
Figure 4 show us an example of solution B. The behavior of
the NCP3065 with dimming you can see in Figures 15
and 16 and dimming linearity in the Figure 17. As you can
see in these figures there aren’t any delays in the rise or fall
edges, which give us the required dimming linearity.
I = 350 mA
700 mA, 1000 mA
Comp
GND
3.6
47
150
Rsense
Figure 2. NCP3065 Current Feedback
Dimming Possibility
The emitted LED light is proportional to average output
(LED) current. The NCP3065 is capable of analog and
digital PWM dimming. For the dimming we have three
possibilities how to create it. We basically use a PWM signal
with variable duty cycle for the managing output current
value. The COMP or IPK pin of the NCP3065 is used to
provide dimming capability. In digital input mode the PWM
input signal inhibits switching of the regulator and reducing
the average current through the LEDs. In analog input mode
a PWM input signal is RC filtered and the resulting voltage
is summed with the feedback voltage thus reduces the
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AND8298
NCP3065
NCP3065
J2
+VIN
J3
NC
R1
+
0R10
C2
GND
J5
IPK
+VIN
VCC
J3
ON/OFF 1k2
Q2
BC817−LT1G
R9
10k
+VIN
J3
+
0R10
10k
GND
J5
IPK
VCC
C2
COMP
R11
ON/OFF 1k2
Q2
BC817−LT1G
R10
1k
0805
1k
C9
R10
J5
LED
R12
Rsense $1%
Figure 5. NCP3065 Dimming Solution C
NC
R9
R19
1k
0805
R12
Rsense $1%
NCP3065
R1
IPK
COMP
R11
ON/OFF
Figure 3. NCP3065 Dimming Solution A
J2
NC
VCC
C2
J5
R10
1k
0805
0R10
+
GND
COMP
R11
R1
J2
R12
Rsense $1%
Figure 4. NCP3065 Dimming Solution B
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AND8298
BOARD LAYOUT
The layout of the evaluation board and schematic is shown below in Figure 6 and Figure 7.
Figure 6. Demo board layout Top (Not in Scale)
Figure 7. Demo Board Silk Screen Top
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6
Figure 8. 12 VDC and 24 VDC Input LED Driver Schematic
R11
R11
ON/OFF 1k2
J6
ON/OFF 1k2
J6
+VAUX
J4
GND
J3
+VIN
J2
R3 R4
R5
R6
R7
220 F/50V
0.1 F
Q1
IPK
NC
SWC
SWE
1k
C5
15k
R13
NU
1k
0805
100pF
1.8nF
C3
10k
CT
R8
MMSD4148
D2
R10
R14
NU
NCP3065
SOIC8
VCC TCAP
COMP COMP
GND
VCC
IPK
NC
U1
R15
R9
Q2
BC817−LT1G
C2
+
C4
1206 12061206 1206 1206 1206
R2
1%R
BC807−LGT1G
0R10
R1
6x 1R0
Q5
MMBT3904LT1G
Q4
NTF2955
C1
R12
Rsense
1%
MBRS140LT3G 0.1 F
1206
D1
L1
GND
J7
−LED
J5
+LED
C6
NU
+
J1
AND8298
CON3
J2
D3
D5
7
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1k2
0805
R3
C5
R2
1k
0805
100pF
1.8nF
C2
10k
0805
NCP3065
SOIC8
CT
R5
100nF C3
220mF/35V
+
SWC
SWE
COMP VCC TCAP
COMPGND
IPK
NC
U1
Q1
BC817−LT1G
R4
VCC
MBRS2040LT3 MBRS2040LT3
D4
MBRS2040LT3 MBRS2040LT3 C4
D2
VCC
0.15R/0.5W
R1
R6
0.68W
1206
J4
LED
C1
1mF
1206
R7
0.68W
1206
R8
0.68W
1206
Jumper1 Jumper2
J3
MBRS2040LT3
D1
L1
OUTPUT
J1
AND8298
Figure 9. Schematic NCP3065 as Switcher in the AC Input LED Driver Application
AND8298
Table 2. 12 VDC INPUT 1 W LED DRIVER WITHOUT OUTPUT CAPACITOR BILL OF MATERIALS
Qty
Reference
Part Description
2
C1, C4
100 nF, Ceramic Capacitor
Mfg P/N
1
C2
220 mF/50 V, Electrolytic Capacitor
Package
Mtg
1206
SMD
G, 10x10.2
SMD
1
C3
1.8 nF, Ceramic Capacitor
0805
SMD
1
C5
100 pF, Ceramic Capacitor
0805
SMD
1
D1
1 A, 40 V Schottky Rectifier
1
D2
1
L1
1
Q4
1
EEEVFK1H221P
Mfg
Panasonic
MBRS140LT3G
ON Semiconductor
SMB
SMD
Switching Diode
MMSD4148T1G
ON Semiconductor
SOD123
SMD
Surface Mount Power Inductor
DO3340P−154MLD
Coilcraft
Power MOSFET, P−Channel
NTF2955T1G
ON Semiconductor
SOT223
SMD
Q5
General Purpose Transistor
MMBT3904LT1G
ON Semiconductor
SOT23
SMD
1
R1
100 mW, 0.5 W
2010
SMD
1
R8
3k3, Resistor
0805
SMD
1
R9
10 kW, Resistor
0805
SMD
2
R10, R15
1 kW, Resistor
0805
SMD
1
R11
1.2 kW, Resistor
0805
SMD
1
R12
680 mW, $1%
1206
SMD
1
U1
DC−DC Controller
SOIC8
SMD
Package
Mtg
1206
SMD
G, 10x10.2
SMD
0805
SMD
NCP3065
ON Semiconductor
SMD
Table 3. 12 VDC INPUT 1 W LED DRIVER WITH OUTPUT CAPACITOR BILL OF MATERIALS
Qty
Reference
Part Description
Mfg P/N
2
C1, C4
100 nF, Ceramic Capacitor
1
C2
220 mF/50 V, Electrolytic Capacitor
1
C3
1.8 nF, Ceramic Capacitor
1
C5
100 pF, Ceramic Capacitor
1
C6
100 mF/50 V, Electrolytic Capacitor
1
D1
1
D2
1
1
1
1
1
EEEVFK1H221P
Mfg
Panasonic
0805
SMD
F, 8x10.2
SMD
ON Semiconductor
SMB
SMD
ON Semiconductor
SOD123
SMD
EEEVFK1H101P
Panasonic
1 A, 40 V Schottky Rectifier
MBRS140LT3G
Switching Diode
MMSD4148T1G
L1
Surface Mount Power Inductor
DO3316P−473MLD
Coilcraft
Q4
Power MOSFET, P−Channel
NTF2955T1G
ON Semiconductor
SOT223
SMD
Q5
General Purpose Transistor
MMBT3904LT1G
ON Semiconductor
SOT23
SMD
R1
100 mW, 0.5 W
2010
SMD
R8
12k, Resistor
0805
SMD
1
R9
10 kW, Resistor
0805
SMD
2
R10, R15
1 kW, Resistor
0805
SMD
1
R11
1.2 kW Resistor
0805
SMD
1
R12
680 mW, $1%
1206
SMD
1
U1
DC−DC Controller
SOIC8
SMD
NCP3065
Table 4. 12 VDC Input 1 W LED Drivers Test Results
Test
Efficiency
With Output Cap
Without Output Cap
Line regulation
Output Current Ripple
With Output Cap
Without Output Cap
Result
74%
72%
$3%
< 50 mA
< 100 mA
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8
ON Semiconductor
SMD
AND8298
Table 5. 12 VDC INPUT 3 W LED DRIVER WITHOUT OUTPUT CAPACITOR BILL OF MATERIALS
Qty
Reference
Part Description
2
C1, C4
100 nF, Ceramic Capacitor
Mfg P/N
1
C2
220 mF/50 V, Electrolytic Capacitor
1
C3
1.8 nF, Ceramic Capacitor
1
C5
100 pF, Ceramic Capacitor
1
D1
2 A, 40 V Schottky Rectifier
1
D2
1
L1
1
Q4
1
1
1
1
2
1
1
R12
330 mW, $1%
1
U1
DC−DC Controller
EEEVFK1H221P
Mfg
Panasonic
Package
Mtg
1206
SMD
G, 10x10.2
SMD
0805
SMD
0805
SMD
MBRS240LT3G
ON Semiconductor
SMB
SMD
Switching Diode
MMSD4148T1G
ON Semiconductor
SOD123
SMD
Surface Mount Power Inductor
DO3340P−154MLD
Coilcraft
Power MOSFET, P−Channel
NTF2955T1G
ON Semiconductor
SOT223
SMD
Q5
General Purpose Transistor
MMBT3904LT1G
ON Semiconductor
SOT23
SMD
R1
100 mW, 0.5 W
2010
SMD
R8
12k, Resistor
0805
SMD
R9
10 kW, Resistor
0805
SMD
R10, R15
1 kW, Resistor
0805
SMD
R11
1.2 kW, Resistor
0805
SMD
1206
SMD
SOIC8
SMD
NCP3065
ON Semiconductor
SMD
Table 6. 12 VDC INPUT 3 W LED DRIVER WITH OUTPUT CAPACITOR BILL OF MATERIALS
Qty
Reference
Part Description
2
C1, C4
100 nF, Ceramic Capacitor
Mfg P/N
1
C2
220 mF/50 V, Electrolytic Capacitor
1
C3
1.8 nF, Ceramic Capacitor,
1
C5
100 pF, Ceramic Capacitor,
1
C6
100 mF/50 V, Electrolytic Capacitor
1
D1
1
D2
1
EEEVFK1H221P
Mfg
Panasonic
Package
Mtg
1206
SMD
G, 10x10.2
SMD
0805
SMD
0805
SMD
F, 8x10.2
SMD
ON Semiconductor
SMB
SMD
ON Semiconductor
SOD123
SMD
EEEVFK1H101P
Panasonic
2 A, 40 V Schottky Rectifier
MBRS240LT3G
Switching Diode
MMSD4148T1G
L1
Surface Mount Power Inductor
DO3316P−473MLD
Coilcraft
1
Q4
Power MOSFET, P−Channel
NTF2955T1G
ON Semiconductor
SOT223
SMD
1
Q5
General Purpose Transistor
MMBT3904LT1G
ON Semiconductor
SOT23
SMD
1
R1
100 mW, 0.5 W
2010
SMD
1
R8
16k, Resistor
0805
SMD
1
R9
10 kW, Resistor
0805
SMD
2
R10, R15
1 kW, Resistor
0805
SMD
1
R11
1.2 kW, Resistor
0805
SMD
1
R12
330 mW, $1%
1206
SMD
1
U1
DC−DC Controller
SOIC8
SMD
NCP3065
Table 7. 12 VDC Input 3 W LED Drivers Test Results
Test
Efficiency
With Output Cap
Without Output Cap
Line regulation
Output Current Ripple
With Output Cap
Without Output Cap
Result
76%
76%
$5%
< 50 mA
< 90 mA
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9
ON Semiconductor
SMD
AND8298
Table 8. 12 VDC INPUT 5 W LED DRIVER WITHOUT OUTPUT CAPACITOR BILL OF MATERIALS
Qty
Reference
Part Description
2
C1, C4
100 nF, Ceramic Capacitor
Mfg P/N
1
C2
220 mF/50 V, Electrolytic Capacitor
Package
Mtg
1206
SMD
G, 10x10.2
SMD
1
C3
1.8 nF, Ceramic Capacitor
0805
SMD
1
C5
100 pF, Ceramic Capacitor
0805
SMD
1
D1
2 A, 40 V Schottky Rectifier
1
D2
1
L1
1
Q4
1
EEEVFK1H221P
Mfg
Panasonic
MBRS240LT3G
ON Semiconductor
SMB
SMD
Switching Diode
MMSD4148T1G
ON Semiconductor
SOD123
SMD
Surface Mount Power Inductor
DO3340P−154MLD
Coilcraft
Power MOSFET, P−Channel
NTF2955T1G
ON Semiconductor
SOT223
SMD
Q5
General Purpose Transistor
MMBT3904LT1G
ON Semiconductor
SOT23
SMD
1
R1
100 mW, 0.5 W
2010
SMD
1
R8
12k, Resistor
0805
SMD
1
R9
10 kW, Resistor
0805
SMD
2
R10, R15
1 kW, Resistor
0805
SMD
1
R11
1.2 kW, Resistor
0805
SMD
1
R12
220 mW, $1%
1206
SMD
1
U1
DC−DC Controller
SOIC8
SMD
Package
Mtg
1206
SMD
G, 10x10.2
SMD
NCP3065
ON Semiconductor
SMD
Table 9. 12 VDC INPUT 5 W LED DRIVER WITH OUTPUT CAPACITOR BILL OF MATERIALS
Qty
Reference
Part Description
Mfg P/N
2
C1, C4
100 nF, Ceramic Capacitor
1
C2
220 mF/50 V, Electrolytic Capacitor
1
C3
1.8n F, Ceramic Capacitor,
0805
SMD
1
C5
100 pF, Ceramic Capacitor,
0805
SMD
1
C6
100 mF/50 V, Electrolytic Capacitor
EEEVFK1H101P
Panasonic
F, 8x10.2
SMD
1
D1
2 A, 40 V Schottky Rectifier
MBRS240LT3G
ON Semiconductor
SMB
SMD
1
D2
Switching Diode
MMSD4148T1G
ON Semiconductor
SOD123
SMD
1
L1
Surface Mount Power Inductor
DO3316P−473MLD
Coilcraft
1
Q4
Power MOSFET, P Channel
NTF2955T1G
ON Semiconductor
SOT223
SMD
1
Q5
General Purpose Transistor
MMBT3904LT1G
ON Semiconductor
SOT23
SMD
1
R1
100 mW, 0.5 W
2010
SMD
1
R8
15k, resistor
0805
SMD
1
R9
10 kW, resistor
0805
SMD
2
R10, R15
1 kW, resistor
0805
SMD
1
R11
1.2 kW, resistor
0805
SMD
1
R12
220 mW, $1%
1206
SMD
1
U1
DC−DC controller
SOIC8
SMD
EEEVFK1H221P
NCP3065
Table 10. 12 VDC Input 5 W LED Drivers Test Results
Test
Result
Efficiency
75%
Line regulation
$4%
Output Current Ripple
With Output Cap
Without Output Cap
< 50mA
< 110mA
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10
Mfg
Panasonic
ON Semiconductor
SMD
400
800
390
780
380
760
370
740
360
720
IOUT (mA)
IOUT (mA)
AND8298
350
340
680
330
660
320
640
310
620
300
600
9
10
11
12
13
14
15
14
16
17
18
19
20
VIN (V)
Figure 10. Current Regulation, 12 VDC Input
1 W LED Driver
Figure 11. Current Regulation, 12 VAC Input
3 W LED Driver
21
95
1100
90
EFFICIENCY (%)
1050
1000
950
85
80
75
900
850
10
15
VIN (V)
1150
IOUT (mA)
700
11
12
VIN (V)
13
14
70
14
Figure 12. Current Regulation, 12 VDC Input
5 W LED Driver
15
16
17
VIN (V)
18
19
Figure 13. 12 VAC Input 5 W LED Driver
Efficiency
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11
20
AND8298
Figure 14. 12 VDC, IOUT = 350 mA Input Inductor Ripple Without Output Capacitor, C1 Inductor Input, C4 Inductor
Current
Table 11. BUCK EFFICIENCY RESULTS FOR DIFFERENT RIPPLE WITH NO OUTPUT CAPACITOR
Efficiency
1 LED, Vf = 3.6 V
2 LEDs, Vf = 3.6 V
4 LED, Vf = 14.4 V
IOUT = 350 mA
> 74%
> 83%
−
IOUT = 700 mA
> 76%
> 83%
−
IOUT = 1000 mA
> 75%
−
−
IOUT = 350 mA
> 70%
> 80%
> 87%
IOUT = 700 mA
> 72%
> 82%
−
IOUT = 1000 mA
> 70%
−
−
IOUT = 350 mA
−
−
> 82%
IOUT = 700 mA
−
−
> 86%
IOUT = 1000 mA
−
−
> 87%
VIN = 12 VDC
VIN = 12 VAC
VIN = 24 VDC
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AND8298
Figure 15. NCP3065 Behavior with Dimming,
Frequency is 200 Hz, Duty Cycle 50%
Figure 16. NCP3065 Dimming Behavior,
Frequency 1 kHz, Duty Cycle 50%
800
24 VIN,
VF 3.6 V
700
ILED (mA)
600
500
400
12 VIN,
VF 3.6 V
300
200
24 VIN,
VF 7.2 V
100
0
0
10
20
30
40
50
60
70
80
90
100
DUTY CYCLE (%)
Figure 17. Output Current Dependency on the Dimming Duty Cycle
Pulse feedback design
The NCP3065 is a burst−mode architecture product which
is similar but not exactly the same as a hysteretic
architecture. The output switching frequency is dependent
on the input and output conditions. The NCP3065 oscillator
generates a constant frequency that is set by an external
capacitor. This output signal is then gated by the peak
current comparator and the oscillator. When the output
current is above the threshold voltage the switch turns off.
When the output current is below the threshold voltage the
switch is turned on and gated with the oscillator. A
simplified schematic is shown in Figure 18. This may cause
possible overshoots on the output. Using the pulse feedback
circuit will reduce this overshoot. This will result in a
stabilized switching frequency and reduce the overshoot and
output ripple. The pulse feedback circuit is implemented by
adding an external resistor R8 between the CT pin and
inductor input as shown in the buck schematic Figure 8.
The resistor value is dependent on the input/output
conditions and switching frequency. The typical range is 3k
to 200k. Table 1 contains a list of typical applications and the
recommended value for the pulse feedback resistor. Using
an adjustable resistor in place of R8 when evaluating an
application will allow the designer to optimize the value and
make a final selection.
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AND8298
Oscillator
Output from Peak
Current Comparator
LED
Vref
+
−
VSENSE
Figure 18. Burst−Mode Architecture
Figures 19 and 20 show the effect of the pulse feedback
resistor on the switching waveforms and load current ripple.
This results in a fixed frequency switching with constant
duty cycle, which is only dependent upon the input and
output voltage ratio. When the ratio (VOUT/VIN) is near 1
(high duty cycle) over the entire input voltage range, the
pulse feedback is not needed.
Boost Converter Demo Board
Figure 19. Switching Waveform Without Pulse
Feedback
Figure 21. Boost Demo Board
Boost Converter Topology
The Boost converter schematic is illustrated in Figure 22.
When the low side power switch is turned on, current drawn
from the input begins to flow through the inductor and the
current Iton rises up. When the low side switch is turned off,
the current Itoff circulates through diode D1 to the output
capacitor and load. At the same time the inductor voltage is
added with the input power supply voltage and as long as this
is higher than the output voltage, the current continues to
flow through the diode. Provided that the current through the
inductor is always positive, the converter is operating in
continuous conduction mode (CCM). On the next switching
cycle, the process is repeated.
Figure 20. Switching Waveform With Pulse Feedback
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15
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Figure 22. 12 VDC Input LED Driver Schematic
R10
R10
ON/OFF 1k2
J6
ON/OFF 1k2
J6
+VAUX
J4
GND
J3
+VIN
J2
Q1
R6
NU
R11
Q2
BC817−LT1G
22mF/50V
0.1mF
R5
C3
+
R3 R4
C5
R2
BC807−LGT1G
0R15
R1
6x 1R0 $1%
R7
SWC
SWE
1k0
R8
NCP3065
VCC TCAP
COMPGND
IPK
NC
U1
L1
R9
Rsense
MM3Z36VT1G
D2
2.2nF
C2
D1
MBRS140LT3G
0.1mF
C2
C1
100mF/
50V
+
−LED
J5
GND
J3
+LED
J1
AND8298
AND8298
transistor is off. You could connect an external PWM signal
to pin ON/OFF and a power source to pin +VAUX to realize
the PWM dimming function. When the dimming signal
exceeds the turn on threshold of the external PNP or NPN
transistor, the comp pin will be pulled up. A TTL level input
can also be used for dimming control. The range of the
dimming frequency is from 100 Hz to 1 kHz, but it is
recommended to use frequency around 200 Hz as this is
safely above the frequency where the human eye can detect
the pulsed behavior, in addition this value is convenient to
minimize EMI. There are two options to determine the
dimming polarity. The first one uses the NPN switching
transistor and the second uses a PNP switching transistor.
The switch on/off level is dependent upon the chosen
dimming topology. The external voltage source (VAUX)
should have a voltage ranging from +5 VDC to +VIN.
Figure 17 illustrates average LEDs current dependency on
the dimming input signal duty cycle.
For cycle by cycle switch current limiting a second
comparator is used which has a nominal 200 mV threshold.
The value of resistor R1 determines the current limit value
and is configured according to the following equation.
When operating in CCM the output voltage is equal to
V OUT + V IN @
1
1*D
The duty cycle is defined as
D+
t ON
t ON ) t OFF
+
t ON
T
The input ripple current is defined as
DI + V IN
D
f*L
The load voltage must always be higher than the input
voltage. This voltage is defined as
V load + V sense ) n * V f
where Vf = LED forward voltage, Vsense is the converter
reference voltage, and n = number of LED’s in cluster.
Since the converter needs to regulate current independent
of load voltage variation, a sense resistor is placed across the
feedback voltage. This drop is calculated as
V sense + I load ) n * R sense
The Vsense corresponds to the internal voltage reference or
feedback comparator threshold.
I pk(SW) +
The maximum output voltage is clamped with an external
zener diode, D2 with a value of 36 V which protects the
NCP3065 output from an open LED fault.
The demo board has a few options to configure it to your
needs. You can use one 150 mW (R1) or a combination of
parallel resistors such as six 1 W resistors (R2 − R7) for
current sense.
To evaluate the functionality of the board, high power
LEDs with a typical Vf = 3.42 V @ 350 mA were connected
in several serial combinations (4, 6, 8 LED’s string) and
4 chip and 6 chip LEDs with Vf =14V respectively Vf =
20.8 V @ 700 mA.
Simple Boost 350 mA LED driver
The NCP3065 boost converter is configured as a LED
driver is shown in Figure 22. It is well suited to automotive
or industrial applications where limited board space and a
high voltage and high ambient temperature range might be
found. The NCP3065 also incorporates safety features such
as peak switch current and thermal shutdown protection.
The schematic has an external high side current sense
resistor that is used to detect if the peak current is exceeded.
In the constant current configuration, protection is also
required in the event of an open LED fault since current will
continue to charge the output capacitor causing the output
voltage to rise. An external zener diode is used to clamp the
output voltage in this fault mode. Although the NCP3065 is
designed to operate up to 40 V additional input transient
protections might be required in certain automotive
applications due to inductive load dump.
The main operational frequency is determined by the
external capacitor C4. The ton time is controlled by the
internal feedback comparator, peak current comparator and
main oscillator. The output current is configured by an
internal feedback comparator with negative feedback input.
The positive input is connected to an internal voltage
reference of 0.235 V with 10% precision over temperature.
The nominal LED current is setup by a feedback resistor.
This current is defined as:
I OUT +
0.2
+ 1.33 A
0.15
Number of LEDs
String Forward Voltage at 255C
Min
Typ
Max
4
11.16
13.68
15.96
6
16.74
20.52
23.94
8
22.32
27.36
31.92
The efficiency was calculated by measuring the input
voltage and input current and LED current and LED voltage
drop. The output current is dependent on the peak current,
inductor value, input voltage and voltage drop value and of
course on the switching frequency.
I OUT + (D * D 2) *
0.235
R sense
There are two approaches to implement LED dimming.
Both use the negative comparator input as a shutdown input.
When the pin voltage is higher than 0.235 V the switch
D+
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16
ǒ
I pk(SW)
D
V OUT ) V F * V IN
V OUT ) V F * V SWCE
*
Ǔ
V IN * V SWCE
[*]
2*L*f
[A]
AND8298
is flat and represents normal operation, Region 3 occurs
when VIN is greater than VOUT and there is no longer
constant current regulation. Region 3 and 1 are included here
for illustrative purposes as this is not a normal mode of
operation.
Figure 9 illustrates the additional circuitry required to
support 12 VAC input signal which includes the addition of
a bridge rectifier and input filter capacitor. The rectified dc
voltage is
VOUT
Output Voltage
VIN
Input Voltage
VF
Schottky Diode Forward Voltage
VSWCE
Switch Voltage Drop
Ipk(SW)
Peak Switch Current
D
Duty Cycle
L
Inductor Value
f
Switching Frequency
Line regulation curve in Figure 24 illustrates three distinct
regions; in the first region, the peak current to the switch is
exceeded tripping the overcurrent protection and causing
the regulated current to drop, Region 2 is where the current
V INDC + Ǹ2 * V AC [ 17 V DC
95
400
390
90 Boost 4LED 350 mA
380
EFFICIENCY (%)
Boost 6LED 350 mA
370
ILOAD (mA)
85
80
Boost 6LED 350 mA
Boost 4LED 350 mA
360
350
340
330
320
75
310
70
6
8
10
12
14
16
18
20
300
22
8
6
10
12
14
16
18
20
22
VIN (V)
VIN (V)
Figure 23. Boost Converter Efficiency for 4 or
6 LEDs and Output Current 350 mA
Figure 24. Line Regulation for 4 or 6 LEDs and
Output Current 350 mA
Table 12. NCP3065 BOOST BILL OF MATERIALS
Qty
Reference
Part Description
Mfg P/N
Mfg
Package
Mtg
1
C1
100 mF/50 V, Electrolytic Capacitor
EEEVFK1H101P
Panasonic
F, 8x10.2
SMD
2
C2,C5
100 nF, Ceramic Capacitor
1206
SMD
1
C3
220 mF/50 V, Electrolytic Capacitor
G, 10x10.2
SMD
1
C4
2.2 nF, Ceramic Capacitor
0805
SMD
1
D1
1 A, 40 V Schottky Rectifier
MBRS140LT3G
ON Semiconductor
SMB
SMD
1
D2
Zener Diode, 36 V
MM3Z36VT1G
ON Semiconductor
SOD123
SMD
1
L1
Surface mount power inductor
DO3340P−104MLD
Coilcraft
1
Q2
General purpose transistor
BC817−LT1G
ON Semiconductor
1
R1
1
R8
1
EEEVFK1H221P
Panasonic
SMD
SOT23
SMD
150 mW, 0.5 W
2010
SMD
1k, Resistor
0805
SMD
R9
680 mW, $1%
1206
SMD
2
R10
1.2 kW, Resistor
0805
SMD
1
U1
DC−DC Controller
SOIC8
SMD
NCP3065
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ON Semiconductor
AND8298
Conclusion
for a variety of constant current buck and boost LED driver
applications. In addition there is an EXCEL tool at the
ON Semiconductor website for calculating inductor and
other passive components if the design requirements differ
from the specific application voltages and currents
illustrated in these example.
LEDs are replacing traditional incandescent and halogen
lighting sources in architectural, industrial, residential and
the transportation lighting. The key challenge in powering
LED’s is providing a constant current source. The demo
board for the NCP3065/NCV3065 can be easily configured
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
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