DN06018/D
12V or 24V DC, Constant
Current LED Driver
http://onsemi.com
DESIGN NOTE
Table 1. DEVICE DETAILS
Device
Application
Input Voltage
Output Power
Topology
I/O Isolation
CS51411
NCV51411
Constant Current
LED Driver
12 V or 24 V DC
Up to 4 W
Buck
None
Table 2. OTHER SPECIFICATIONS
Output 1
Output 2
Output 3
Output 4
Output Voltage
3.6 V nom
N/A
N/A
N/A
Ripple
20 mV
N/A
N/A
N/A
Nominal Current
700 mV
N/A
N/A
N/A
Max Current
1A
N/A
N/A
N/A
Min Current
N/A
N/A
N/A
N/A
PFC (Yes/No)
No
Cooling Method/Supply Orientation
Convection
Circuit Description
Design Notes
ON Semiconductor’s latest monolithic NCV51411
(CS51411) converter is to be used in a buck topology
optimized to drive a single LED at a constant current
between 350 mA to 1 A.
A high side, low drop, current sensing scheme has been
implemented, targeted for automotive and other high
efficiency applications.
DCR Inductor current sensing is used to generate the
control ramp required for the V2 controller.
This design note targets a constant current (350 mA to
1 A) driver suitable for driving a single LED (1 W or 3 W)
from a nominal 12 V or 24 V dc source. The output voltage
range assumes a single White/Blue/Green LED with
a forward voltage of 3.6 ± 35%. The converters used in the
design are from ON Semiconductor’s CS5141x family;
the CS51411 in a SOIC−8 and is offered in two ambient
temperature ranges (0−70°C or −40−85°C) while the
NCV51411 is specifically intended for automotive
applications and is specified for junction temperatures up to
125°C. Figure 1 shows the pin out of the SOIC−8. Refer to
the data sheet at the ON Semiconductor web site for the pin
out for other package options such as the NCV51411 DFN
package.
Key Features
• Constant Current Output with Voltage Clamp
• Low Drop High Side Current Sensing
• High Frequency (260 kHz/520 kHz*) Operation to
•
•
Enable Cost Effective Magnetic and Capacitive
(e.g. MLCC) Filter Components
Minimal Ripple Current through LED
High Side Sensing Allows LED Cathode to be Directly
Connected to System Ground
*CS51413 Supports 520 kHz Operation
© Semiconductor Components Industries, LLC, 2014
October, 2014 − Rev. 1
1
Publication Order Number:
DN06018/D
DN06018/D
D2
2
C3
R4
0.2 W
2
3
4
Boost VCOMP
Vin
VFB
VSW
GND
SHDN
SYNC
3
1
LED
TBD
2
2
Q1
MBT3906WT1
1
C6
47 nF
0
1
3
R6
1.27 kW
RTN
D3
MMSZ5V6ET1
2
C5
47 nF
1
1
2
0
RTN
R3
133 W
C9
10 mF/10 V
R1
10 kW
C8
10 mF/10 V
1 mF/50 V
C7
1 mF
1 mF/50 V
C4
47 nF
2
C2
1
C1
D1
MBRA340
L1
100 mH
VOUT
R2
10 W
MMSD914T1G
4.7 mF/10 V
VIN
Q2
MBT3906WT1
1
8
0
7
6
R5
1.27 kW
5
U1
CS51411/NCV51411
0
0
Figure 1. Schematic
Theory of Operation
Boost Diode D2
For low ripple current in the inductor and through the
LED, this design is based around Continuous Conduction
Mode (CCM) operating mode. The switch within the
controller turns on for time D ⋅ TS (D duty cycle, TS
switching period) charging inductor L1 through the voltage
differential (VIN–VOUT). When the switch is turned off by
the feedback signal, diode D1 conducts and delivers the
energy stored in the inductor to the output VOUT.
For the inductor flux (volt microsecond) to remain in
equilibrium each switching cycle, (VIN−VOUT) ⋅ D ⋅ TS
must equal VOUT ⋅ (1−D) ⋅ TS neglecting circuit losses.
Hence the voltage gain of buck is given by the expression
VOUT = D ⋅ VIN.
Diode D2 and MLCC C3, across the inductor L1, form
a simple boost circuit to supply base current to drive the high
side BJT in the controller. C3 is charged to VOUT during each
switching period (1−D) ⋅ TS, when the freewheeling diode
D1 is conducting.
Input/Output Capacitors
The input/output capacitors used for the application are
MLCC capacitors in a 1206 or a 0805 SMT package. Low
value MLCC capacitors (10 mF) have very small esr (2 mW)
and esl (100 nH) values. When combined in parallel
combinations they form the “perfect” capacitor.
Consequently the ripple voltage across them is due only to
charging and discharging of the capacitor by the inductor
ripple current.
The ripple voltage across the input capacitor
= 0.5 ⋅ D ⋅ TS ⋅ dI (L1) / CIN. For CIN = 2 ⋅ 1 mF, input
voltage ripple = 60 mV p/p.
The ripple developed across the output capacitors
= 0.5 ⋅ (1−D) ⋅ TS ⋅ dI (L1) / COUT. For COUT = 2 ⋅ 10 mF,
output ripple = 15 mV p/p.
Note the actual value of a MLCC decreases with dc
voltage applied. Therefore it is recommended to have
a voltage stress de-rating factor of 50% on each component.
Hence a 50 V rating is suggested for the 24 V application
and a 6.3 V is recommended across the 3.6 V output.
Depending on the maximum Vf of the LED, this output
capacitor rating should be increased to 10 V.
Power Components
The NCV/CS51411 has a switching frequency of 260 kHz
equivalent to a switching period TS = 3.85 ms For a nominal
12 V input to 3.6 V output, the duty cycle D = 3.6/12 = 0.3.
Output Inductor Selection
Ripple current in the inductor is obtained from the
expression dI (L1) = VIN ⋅ TS ⋅ D ⋅ (1−D) / L1.
A value for L1 of 47 mH will maintain ±15% ripple current
in the 700 mA application (3 W LED) discussed below.
Freewheel Diode D1
The MBRA340 Schottky diode has a forward drop of
300 mV at a forward current of 0.7 A. Power loss is
(1−D) ⋅ I (L1) ⋅ VD1. This equates to a power loss of
150 mW in this application.
http://onsemi.com
2
DN06018/D
Current Sensing Circuitry
In this application, this control ramp is generated from
indirectly sensing the current flowing in the inductor’s DCR
winding resistance. When an integrating network consisting
of R1, C4 is placed across the output inductor L1, the voltage
developed across the integrating capacitor C4 is given by the
equation below.
Driving a single LED will produce a voltage VOUT at the
converter’s output of approximately 3.6 volts. This voltage
will vary with device and temperature effects. If a sensing
scheme using a 0.6 V (BJT base emitter junction) or higher
voltage reference is used, the converter’s conversion
efficiency can be seriously degraded. For example if a sense
resistor is placed across a Vbe junction for current sensing,
the efficiency will be degrade by 17%. Also in automotive
applications, high side current sensing is preferred because
in an automobile the chassis is used for ground returns.
In this design, low drop, cost effective, high side current
sensing is achieved by the transistor pair of Q1 and Q2 and
resistors R2, R3, R4, R5 and R6. The feedback pin under
normal operation is maintained at 1.27 volts, equal to the
controller U1’s internal reference. Consequently a constant
current of 1.27 V/1.27 k or 1 mA flows through R2, R3, Q1
and R5. The voltage across R2 + R3 = (R2 + R3) ⋅ 1 mA or
140 mV. The output LED current is sensed by sense resistor
R4, which in turn develops a voltage ILED ⋅ R4 across it.
The current regulation point is determined when the
equation ILED = (1.27/R5) ⋅ {(R2 + R4) / R5} is satisfied.
For the values chosen ILED = 1 mA ⋅ (140 / 0.2) or 700 mA.
Above 700 mA, the current mirror, consisting of Q2 and R6
will cause additional current to flow in Q1. The increase in
voltage at the feedback pin VFB will cause the duty cycle to
reduce to limit the current at the designed set point. It is
worth noting that even though the ripple current in the
inductor is 200 mA, this is diverted into the output capacitor
bank. The ripple current in the LED itself is an order of
magnitude less determined by the ratio of the LED’s
dynamic impedance to the output capacitor’s impedance at
the 260 kHz switching frequency.
The LED current can be varied from 350 mA to 1 A by
scaling the value of either R3 or R4.
dV (C4) +
V IN @ T S @ D @ (1 * D)
R1 @ C4
(eq. 1)
Assuming the inductor winding resistance is dcr, the
voltage across this dcr resistance dV (dcr) is given by the
following equation.
dV (dcr) +
V IN @ T S @ D @ (1 * D) @ dcr
L1
(eq. 2)
It is apparent the two expressions are equal if the
integrator’s time constant R1 ⋅ C4 is matched to the
inductor’s time constant L / dcr. At this point in the design,
we can select the output inductor L1 to be a TDK
SLF10145T-470M1R4. This is a 47 mH inductor with a dcr
of 0.1 W and a saturation current of 1.4 A. Its time constant
is 470 ms. If we select R1 as 10 kW and C4 equal to 47 nF we
match the 470 ms time constant. Our control ramp is the
inductor current. Its amplitude is calculated from the dV
(C4) equation as 21 mV. Alternatively a Coilcraft inductor
DO3316P-473 having a larger 0.14 W dcr could be selected.
In order not to degrade this ramp with switching ripple from
the output, the filter network R2, C6 is recommended.
Finally the capacitor C5 is used to ac couple the current
control ramp to the feedback pin VFB.
In the event of an open circuit output condition, such as the
case if the output LED failed open, zener diode D3 conducts
to limit the output voltage to Vz + 1.27 V. In the application,
the voltage clamp is designed to operate at 6.9 V.
Control Circuitry
The error amplifier in the V2 controller U1 is
a trans-conductance amplifier having several mega ohms of
output impedance. Adding a small capacitor C5 to ground at
its output VCOMP will provide a low frequency pole at
20 Hz. This pole will filter the feedback signal providing
a dc error signal to one input of the PWM inside the
controller. The V2 control architecture requires a control
ramp to be included with the dc feedback information on the
feedback pin VFB. This signal is passed directly to the other
input of the PWM. When the dc error signal and dc feedback
plus ramp intersect, the switch cycle is terminated, thereby
allowing modulation of the duty cycle D to occur.
http://onsemi.com
3
DN06018/D
Table 3. BILL OF MATERIALS
Ref. Design
Description
Package
Manufacturer
Manufacturer
Part Number
U1
Buck Controller
SO−8
ON Semiconductor
CS51411
Buck Controller
18 Lead DFN
ON Semiconductor
NCV51411
Schottky (350 mA)
SOD123
ON Semiconductor
MBR140SFT1G
Schottky (700 mA)
SMA
ON Semiconductor
MBRA340
D2
Diode, 0.2 A, 100 V
SOD123
ON Semiconductor
MMSD914T1G
D3
Zener, 5.6 V
SOD123
ON Semiconductor
MMSZ5V6ET1
L1
Output Inductor,
47 mH, 0.14 W, 1.6 A Isat
Coilcraft
DO3316P-473
Output Inductor,
47 mH, 0.10 W, 1.4 A Isat
TDK
SLF10145T-470M1R4
D1
Q1, Q2
−0.2 A, −40 V, Dual PNP Array
SOT363
ON Semiconductor
MBT3906WT1
C1, C2
1 mF 50 V
1206 X7R
Murata
GRM31MR71H105K
C3
4.7 mF 10 V
0805 MLCC
TDK
C2012X%R1A475M
C4, C5, C6
47 nF
0603 MLCC
Vishay
VJ0603Y473KXXA
C7
1 mF, 16 V
0603 MLCC
TDK
C1608X5R1C105M
C8, C9
10 mF, 6.3 V
0805 MLCC
Taiyo Yuden
JMK316BJ106ML-T
R1
10 kW
0603
Vishay
CRCW06031002F
R2
10 W
0603
Vishay
CRCW060310R0F
R3
133 W
0603
Vishay
CRCW05031330F
R4
0.2 W
1206
TT Electronics
IRC LRC-LR1206-01-R200-F
R5, R6
1.27 kW
0603
Vishay
CRCW06031271F
ON Semiconductor and the
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC) or its subsidiaries in the United States and/or other countries.
SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed
at www.onsemi.com/site/pdf/Patent−Marking.pdf. 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 intended to support or sustain life, or for any other application in which
the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or
unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and
expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim
alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable
copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
Literature Distribution Center for ON Semiconductor
P.O. Box 5163, Denver, Colorado 80217 USA
Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada
Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada
Email: [email protected]
N. American Technical Support: 800−282−9855 Toll Free
USA/Canada
Europe, Middle East and Africa Technical Support:
Phone: 421 33 790 2910
Japan Customer Focus Center
Phone: 81−3−5817−1050
http://onsemi.com
4
ON Semiconductor Website: www.onsemi.com
Order Literature: http://www.onsemi.com/orderlit
For additional information, please contact your local
Sales Representative
DN06018/D