CAT4201: Three LED 300 mA MR16 Driver

DN06067/D
Three LED 300 mA
MR16 Driver
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DESIGN NOTE
Table 1. DEVICE DETAILS
Device
Application
Input Voltage
Output Power
Topology
I/O Isolation
CAT4201
MR-16 LED Lamp
12 Vac
2.7 W
Step-down
None
Table 2. OTHER SPECIFICATIONS
Output
Output Voltage
9.1 V
Nominal Input Power
3.4 W
Nominal Average Current
300 mA
Max Average Current
310 mA
Min Average Current
295 mA
Typical Efficiency
83%
Table 3. LED KEY SPECIFICATIONS
LED
Maximum Rating
Forward Voltage
@ 350 mA
Typical Color
Typical Luminous Flux
@ 350 mA
CREE Xlamp® XR-E
1 A/3.7 W
3.3 V
Cool White
107 lm
12 V AC
D3
MBR0520L
D2
MBR0520L
D4
MBR0520L
LED1
U1
1
C1
220 mF/25 V
D1
MBR0520L
12 V AC
CTRL
VBAT
5
C2
4.7 mF
CAT4201TD
2
D5
MBR0540
C3
10 mF
GND
LED2
L1
R1
3
4
RSET
SW
10 kW
R2
1 kW
22 mH/0.83 A
LED3
Figure 1. Schematic
Circuit Description
adequate voltage. The RSET pin determines the output DC
current; CTRL is for dimming signal input. SW is the output
of internal MOSFET.
The CAT4201 driver is strongly recommended for its
compact circuitry and high efficiency making it ideally
suited for the replacement of filament-based bulb, in
applications such as spot light.
The CAT4201 is a step-down, easy-to-configure,
dimmable LED driver. The package is a 5-pin SOT−23; with
fewer than 10 discrete components (most of them surface
mount), the whole PCB can be small enough to fit into the
base of an MR16 bulb. The voltage ratings on pins VBAT,
CTRL and SW are 40 V, so driving 12 LEDs is possible with
© Semiconductor Components Industries, LLC, 2014
August, 2014 − Rev. 1
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Publication Order Number:
DN06067/D
DN06067/D
Figure 2. MR-16 LED Cases. Miniature LED Driver Board can be Embedded into the Base
high frequency fluctuation, capacitance larger than 10 mF is
not necessary. The value of L is recommended to be 22 mH
in order to set a proper switching frequency about 150 kHz.
The LED current range is 0~350 mA, so the rms current
rating of inductor around 800 mA is adequate.
The total continuous current though the rectifiers are
always below 400 mA even though 350 mA output current
is supplied. Therefore a 0.5 A continuous current rating is
enough for rectifiers. Careful observation of voltage across
the freewheeling D5 diode shows that voltage spikes over
20 V can be found even with 12 Vac input. A higher input
voltage (such as 15 V) further stresses the diode, increasing
the risk of failure. So the voltage rating on D5 should be 30 V
or 40 V. ON Semiconductor’s Schottky rectifier MBR0540
was selected for D5.
RSET pin configures the value of the output current.
RSET voltage is regulated at 1.2 V. Adding a resistor
between RSET and GND determines the RSET current,
which has an approximately linear relationship with output
constant voltage:
Theory of Operation
The application circuit is a simple buck converter. In the
case of AC applications, the input is rectified through
a bridge rectifier. VBAT and GND are directly connected to
the two terminals of the bulk capacitor C1 which reduces the
current ripple. During the first switching phase, the internal
MOSFET will charge the inductor with linearly rising
current until the switching-off of MOSFET; during the
second phase, the MOSFET is cut off, and the current stored
in the inductor will discharge through the Schottky diode
(D5), and the current decays till next period of switching.
The output capacitor is used to reduce the current ripple in
the LED. RSET is voltage regulated at 1.2 V, thus resistor
connected to RSET determines the RSET current.
The RSET current is approximately proportional to the
output constant current of CAT4201.
Circuit Configuration
VBAT has a voltage rating of −0.3 to +40 V, therefore an
input voltage up to 24 Vac at the bridge is safe. A large C1
is necessary to keep a higher level of input voltage. Large
fluctuation of rectified AC current will pull the output
current to zero at double line frequency, thus reducing the
output current. 220 mF is adequate in this design. Voltage
rating of C1 should be 25 V for 12 Vac input. MBR0520L
was selected for the bridge rectifier because of its low
forward voltage.
It is recommended to use at least 4.7 mF for C2 as output
capacitor to reduce output ripple. Larger C2 will effectively
suppress the output ripple, and raise the output current by
several milliampers. However since its contribution to total
efficiency is insignificant and that human eye cannot sense
I LED + 2500 @ I RSET
(eq. 1)
It can be calculated that for 300 mA output,
IRSET = 0.12 mA. The RSET resistor value should be
10 kW.
A smaller RSET resistance will possibly increase the
output current, but it is recommended to use at least 8.0 kW
for stable operation. If the input voltage is high enough
(e.g. 15 Vac), one can set RSET to be 8.2k, which guarantees
at least 350 mA output.
CTRL derives voltage from the cathode of LED through
R2. Value of R2 is not critical.
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Performance and Characteristics
1. Input Voltage − Output Current − Efficiency Relationship*
90%
350
85%
250
200
80%
150
100
Output Current
Efficiency
Output Current (mA)
300
75%
Efficiency
50
0
70%
8
9
10
11
12
13
14
15
16
17
18
19
Input Voltage (Vrms)
*Equipment: Global Specialties 1506 AC isolated variable AC line supply; Voltech PM1000 AC Power analyzer; Tektronix TDS754D digital
phosphor oscilloscope; Tektronix TCP202 current probe; Agilent 34401A multimeter.
2. LED Current Waveforms*
VIN = 12 Vac 50 Hz, circuit is configured according to the
schematic in page 1 of this document.
LED Current
100 mA/div
LED Current
100 mA/div
5 ms/div
5 ms/div
The LED current has got very slight low frequency (at
twice the AC frequency, 2 × 50 Hz here) fluctuation due to
the AC line input.
Using larger C1 or higher VIN will further flatten the AC
related ripple current.
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3. Startup and Power Down Transient*
VIN = 12 Vac 50 Hz, circuit is configured according to the
schematic in page 1 of this document.
CH2: VBAT
Voltage
5 V/div
CH2: VBAT
Voltage
5 V/div
CH3: LED
Current
100 mA/div
CH3: LED
Current
100 mA/div
2 ms/div
5 ms/div
*Equipment: Global Specialties 1506 AC isolated variable AC line supply; Voltech PM1000 AC Power analyzer; Tektronix TDS754D digital
phosphor oscilloscope; Tektronix TCP202 current probe.
Conclusion
References
The information presented in this design note covers the
various factors required about designing a three-LED
MR−16 lamp using the CAT4201. The CAT4201 allows
smaller footprint, fewer components for a MR−16
compatible adapter.
Reducing capacitor and inductor values, or removing the
CTRL resistor is possible for further reduction of the cost
and PCB size. Using a small bulk capacitor will lead to large
drop of average output current, which is not recommended,
but suitable for applications with less brightness
(200~250 mA). Selecting high-quality LEDs with smaller
forward voltage is very important in this design to achieve
a higher output current.
[1] Data Sheet CAT4201: 350 mA High Efficiency Step
Down LED Driver
[2] Data Sheet MBR0520L: 0.5 A, 20 V Schottky
Rectifier
[3] Data Sheet MBR0540: 0.5 A, 40 V Schottky
Rectifier
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Table 4. BILL OF MATERIALS
Part
Specifications and Ratings
Manufacturer
CAT4201TD
SOT−23, 40 V, 350 mA
ON Semiconductor
MBR0520L
0.5 A, 20 V SMT Schottky Rectifier, Vf = 0.38 V
ON Semiconductor
MBR0540
0.5 A, 40 V SMT Schottky Rectifier, Vf = 0.51 V
ON Semiconductor
LEDs
CREE Xlamp 7090 XR−E, Cool White, Group Q5
Cree
L1
22 mH, 0.9 A, 0.83 A (Isat.) Shielded 4 × 4 × 1.8 mm (LPS4018−223)
Coilcraft
C1
220 mF, 25 V
Panasonic
C2
4.7 mF, 25 V 1206 Ceramic
TDK
C3
10 mF, 16 V Ceramic
TDK
R1
10 kW 1/8 W SMT−0805, 1%
−
R2
1 kW 1/8 W SMT−0805, 5%
−
XLamp is a registered trademark of Cree, Inc
ON Semiconductor and the
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