AN90026
LED drivers

Rev. 1.0 — 12 February 2021

application note

Document information
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Content

Keywords

LED driver, constant-current

Abstract

This application note presents the concept of constant-current LED drivers. The LED drivers
referred in this application note with the suffix NCR are available from Nexperia.

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LED drivers

1. Introduction
LEDs and LED applications are widely used across all market segment, having a significant impact
on everyday life. Although high power LEDs need complex driver schemes and dedicated thermal
concepts, the vast majority of applications use LEDs with currents far below 500 mA and can be
driven by a very simple and robust driver solution using constant-current LED drivers which operate
as a simple linear regulator. These are easy-to-use, reliable, cost efficient, and do not affect EMC.
In this application note the concept of constant-current LED drivers is presented, including how
LED drivers can be easily integrated in the lighting concept for modern electronics with respect topower considerations, thermal concept, and packaging aspects.
Drivers referred to in this application note with the suffix NCR are available from Nexperia. They
are used in automotive applications such as interior and exterior lighting (e.g. door handles,
dashboard, number plate light, indicators and rear lights).

2. Concept of LED Drivers
Fig. 1 shows the internal topology and application schematic of three different LED drivers.
Internally, such a driver consists of a BJT, two diodes, and two resistors. In Fig. 1 a) and Fig. 1 b)
NPN BJTs operate as low-side drivers, while in Fig. 1 c) a PNP BJT is used to operate as highside driver. One of the resistors defines the minimum output current and the other provides the bias
voltage and plays an important role for the enable characteristic. Both the low-side drivers and thehigh-side driver have an enable pin. Low-side drivers need a specific potential to be enabled.
NCRx20x series drivers require a voltage in the range of the supply voltage to enable them, as
shown in Fig. 1 a) . The NCRx21x driver can be enabled at a much lower voltage of 3.3 V asindicated in Fig. 1 b) . This part draws a current of about 1 - 2 mA at the enable pin so it can easily
be driven by the output pin of a microcontroller or a logic device as indicated in Fig. 1 b). This
provides convenient control of the LED's brightness by using a PWM output from a controller. The
high-side constant-current driver has an enable pin which is connected to ground, thus the LED,can be turned off by disconnecting this pin. In practice, this is performed using a resistor-equipped
transistor (RET), as illustrated in Fig. 1 c), or a MOSFET.
Low-side
VCC

High-side

VCC

VSUP

µC/
logic
IEN

VEN

Iout

IEN

IOUT

VEN

EN

Iout

VO

IOUT
EN

EN
GND

REXT

REXT
Rext

Rext

GND
a)

OUT

GND
b)

IN/
OUT

c)

IO

LED

GND
aaa-033101

Fig. 1.

LED driver topologies. Low-side using an NPN and high-side using a PNP

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3. Electrical performance
The output current of constant-current LED drivers is adjustable by an external resistor Rext.
Some types are tuned to commonly used fixed currents using an internal resistor. As they do notfeature an external resistor, they are available in 3-pin packages. In applications where an external
resistor can be connected, it will be in parallel to the internal resistor, thereby lowering the effective
resistance. The low-side constant-current drivers (NCRx2xx) using an NPN transistor have aninternal resistor of 95 Ω. An analysis of the measurement curves provides the following formulawhich enables the relationship between external resistor and output current for a given VEN = 3.3 V.

I out ≈ 0.5 mA ·

( (
R ext
Ω

−0.75

, Rext ≈ 1 Ω ·((I out −1.33
0.5 mA(1)

It should be mentioned that these formulas are derived by fitting measured data. They are not valid
for values of VEN other than 3.3 V. In those cases the formula should be adapted again using a
curve fitting procedure.
The value of Rext should not be too low to avoid excessive output current and hence damage of the
LED driver. In case the external resistor is smaller than the internal resistor, most of the current willflow through the external resistor. Given a specific current target, the value of the resistor, based on
Eq (1), can be chosen from Table. 1.
Table 1. LED current for a given resistor values (E24 series, 5%) calculated using Eq (1)Rext (Ω)
Iout (A)
Rext (Ω)
Iout (A)
1,000

0.50

4,300

0.167

1,100

0.466

4,700

0.157

1,300

0.411

5,100

0.154

1,500

0.369

5,600

0.137

1,600

0.351

6,200

0.127

1,800

0.322

6,800

0.119

2,000

0.279

7,500

0.110

2,200

0.277

8,200

0.103

2,400

0.259

9,100

0.095

2,700

0.237

10,000

0.089

3,000

0.219

11,000

0.083

3,300

0.204

11,100

0082

3,600

0.191

11,300

0.081

3,900

0.180

Based on the tolerance of the E24 series of 5% the tolerance of the output current will be ≤3.8%.
It is important to note that the overall tolerance of the of the LED driver is not only dependent on
the tolerance of Rext but also on the internal tolerances of the device (e.g., internal resistors hFE).
Those internal tolerances are always present and contribute to an overall tolerance of the LED
driver.
It should be mentioned that per definition the concept of constant-current LED drivers introduces
a current drift depending on the temperature. In practice, this current drift is below -0.3%/K which
would lead to a 6% current drift for a considerable large temperature drift of e.g., 20 °C. Typically,such temperature drifts happen very slowly and hence will not be very noticeable to the humaneye.
In addition, the output current drift depends on supply voltage. This is typically 0.8%/V. In practice,when the LED driver is placed behind a buck or boost stage there is no relevant voltage change
and hence no significant current drift is expected. When the LED driver is powered by the battery
voltage and having a typical variation of the battery voltage between 11 and 16 V, the resulting
current drift will be maximum 4%.

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LED drivers
The dimming capability of the LED drivers (NCR321Z, NCR4x1Z) can be assumed to be linear,which means that the mean of the output current is linearly dependent on the duty cycle. Thiscan be assumed for all typical dimming frequencies up to several kHz. However, the maximum
recommended dimming frequency should maximum 10 kHz. Even for frequencies above therecommended 10 kHz, this linear relationship is maintained. However, due to higher harmonics,
a switching frequency below 10 kHz should be preferred to stay compliant with the EMC
requirements.
The minimum voltage drop across a constant-current driver is about 1.4 V. Below this voltage,
linear regulation does not work properly. Above it, the voltage drop across the constantcurrent driver dynamically adjusts to enforce the desired output current. In low- and high-side
configurations, the output voltage is always calculated as Vout = VCC – VLED with VLED as thevoltage at the LEDs and VCC as the supply voltage. Constant-current LED drivers are used
when the brightness of an LED should be independent of the supply voltage to some degree.
For example, the 12 V power supply in a car fluctuates between 11 V and 15 V during regular
operation. If the desired drive current and the voltage drop across the LEDs are known, theminimum possible supply voltage can be calculated by adding the voltage drop across the driver
plus the voltage at the LEDs. The maximum allowed voltage is governed by the maximumpermitted voltage drop across the driver Vout or the total power dissipation which can be estimatedby Ptot ≈ Vout × Iout. Hence, the allowed operating range, if data sheet limits are not exceeded, canbe specified as:

VLED + 1.4 V < VCC < VLED +

Ptot
I out

(2)

Constant-current drivers in the SOT457 package usually have Ptot = 0.75 W. For an output currentof 50 mA, this relates to a margin of 15 V. Newly released devices in a SOT223 package from
Nexperia have an increased Ptot of 1.25 W which increases the voltage margin to 25 V.
Additionally, the voltage margin can be increased by reducing the output current. Placing two or
more constant-current drivers in parallel, as illustrated in Fig. 2, effectively doubles the current.
Using this method, current exceeding the capability of a single driver can be driven, or a smaller
current per driver can be used to increase the voltage margin. Using two constant-current,
SOT223-packaged drivers with a drive capability of 250 mA, enables the circuit to drive 500 mA
LEDs with a voltage margin of 5 V. When driving constant-current drivers in parallel, the accuracyof the external resistors is the most important factor for the symmetry of the output currents of theindividual drivers. Fig. 3 illustrates the voltage margins depending on the output current for single
and parallel drivers in SOT457, SOT223, and DFN2020D-6 packages using V = Ptot/Iout.
VCC

IEN

VEN

Iout

IEN

IOUT

VEN

REXT

Iout

VEN

REXT
Rext

GND

IEN

IOUT

Iout

IOUT

REXT
Rext

GND

Rext

GND

aaa-033102

Fig. 2.

Using two or more constant-current source LED drivers in parallel

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Voltageoverhead(V)

40
NCR42xU

35

NCR42xZ
2 x NCR42xU

30

2 x NCR42xZ
NCR32xU

25

NCR32xZ
2 x NCR32xU

20

2 x NCR32xZ

15

NCR32xPAS
2 x NCR32xPAS

10

5

0

0

50

100

150

200

250

300

350

400
450
Output current (mA)500

aaa-033103

Fig. 3.

Voltage margin of single and parallel drivers in SOT457 (NCR42xU, NCR32xU),
SOT223 (NCR42xZ, NCR32xZ), and DFN2020D-6 (NCR32xPAS)

4. EMC Behavior
In DC applications, where the EN pin is driven by a DC voltage, there is typically no emission
spectrum expected. However, in the presence of external electromagnetic (EM) waves the enable
pin can be disturbed which can then lead to significant deviation of the LED current. To prevent
this, a capacitor placed as close as possible to the enable pin can be used, see Fig. 4. The valueof this capacitor can be chosen according to the frequency where the disturbance is expected, seeFig. 5.
VCC

µC/
logic

IEN

Buffer capacitor
against short voltage
interrupts by e.g.
external EM fields.

VEN

Iout

IOUT

REXT
Rext

GND

aaa-033104

capacitor value depending on frequency at which the impact is occurringFig. 4.

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application note

Capacitor placement for protection against external EM fields

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aaa-033174

106

impedance

C
(pF)
105

10

104

1
w/o Ltrace

including Ltrace

103

10-1

102

10

Fig. 5.

aaa-033175

102

10-2

102

10

f (MHz)10-3103Capacitor value as a function of EMC frequency

102

10

103

f (MHz)for a 100 nF capacitor
Fig. 6.

Impedance as a function of frequency

In Fig. 4 the placement of the capacitor is indicated in the schematic and values for the capacitor
are calculated using:

C =

1

(3)

2

(2·π·f) ESL

400 pH is used as a typical value of equivalent series inductance (ESL) for an 0603 capacitor. In
some cases, it is not possible to connect the capacitor directly to the ground plane, so additional
traces and/or vias are used. All traces and vias add a significant parasitic inductance (~1 nH per
mm trace or via) to the circuit which impacts the effective operating frequency of the capacitor. To
consider this, the capacitance values are given for a parasitic inductance of Ltrace = 2 nH. It shouldbe mentioned that when adding this capacitor, the output of your logic should be checked to beable to charge and discharge the capacitor in the required time, otherwise a series resistor should
be added.
For PWM application where the EN pin is driven by a digital signal with a certain frequency, therecommendation is for the switching frequency not to exceed a switching frequency of 10 kHz.
However, having a rectangular signal means that higher harmonics should be also considered. As
shown in Fig. 7, the first 10 Fourier amplitudes for a fundamental 10 kHz. Only the 10th harmonic
appears close to the 125 kHz band.
log (|A|)125 kHz

100 k

150 k

LED driver in PWM mode
fo = 10 kHz

Other switching boost or
buck topologies
fo = 400 kHz, fo = 4 MHz
AM
0.5 M

1.72 M

DAB, TVIII
170 M 241 M

FM, TVII
76 M

10 kHz

100 kHz

1 MHz

10 MHz

108 M

100 MHzlog (f)aaa-033106

Fig. 7.

Fourier amplitudes (first 10) of current-source LED drivers in PWM mode

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The limits of this band are 41dB (µV/m) which is much higher compared to the 10th harmonics of
the LED Driver. In comparison, other LED driving solutions such as switched boost or buck solution
are shown. Their typically switching frequency is in the range of several hundred kHz to a few
MHz. From the emission point of view, this can be very critical because several very important
frequency bands such as the AM radio which is ranging from 0.5 to 1.73 MHz, can be violated.
Also, higher bands such as FM and DAB which are beyond 100 MHz can be impacted heavily byhigher harmonics, especially in presents of internal resonances.
Compared to conventional boost or buck topologies, the current source drivers show a significantly
better EMC performance even in the PWM mode.

5. Thermal performance
The LED drivers come in five different packages. From the perspective of the power dissipation
there are several differences between the packages. Table 2 shows the comparison of electricaland thermal parameters for the available packages. As can be seen, the S0T23 is suitable forlow-current and low-power application requiring also small area on the PCB. The SOT223 as thelargest package shown here, is well suited for up to 1250 mW. It should be mentioned that also
DFN packages have a very solid thermal performance with a maximum current up to 300 mA andpower up to 700 mW.
Table 2. Thermal parameter (maximum values) for the packages available for the LED drivers
SOT23
SOT223
SOT457
SOT353
DFN2020D-6

Ptot (mW) [1]

400

1,250

750

Imax (mA)

20

250

Rth(j-a) (K/W) [1]

465

100

Rth(j-a) (K/W) [2]

312

70

[1]
[2]

335

700

250

50

300

165

317

236

115

-

170

Device mounted on an FR4 PCB, 4-layer copper, tin-plated and standard footprint.
Device mounted on an FR4 PCB, 4-layer copper, tin-plated mounting pad for collector 1 cm².
IOUT

Thermal vias
(optional)

IOUT

Buffer C for
EMC robustnes

VEN

REXT

GND

GND

GND
Rext
VEN

Fig. 8.

aaa-033108

Layout suggestion for the SOD223 package for an optimal thermal design

For the IOUT pin it is recommended to use a sufficient pad size, with enough thermal vias toimprove the heat sinking. A similar approach can be used for other packages from Table 2.

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6. Revision history
Table 3. Revision history
Revision
Date
numberDescription

1.0

Initial version.

2021-02-12

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7. Legal information
Definitions
Draft — The document is a draft version only. The content is still underinternal review and subject to formal approval, which may result in
modifications or additions. Nexperia does not give any representations or
warranties as to the accuracy or completeness of information included hereinand shall have no liability for the consequences of use of such information.

Disclaimers
Limited warranty and liability — Information in this document is believed
to be accurate and reliable. However, Nexperia does not give any
representations or warranties, expressed or implied, as to the accuracy
or completeness of such information and shall have no liability for theconsequences of use of such information. Nexperia takes no responsibility
for the content in this document if provided by an information source outsideof Nexperia.
In no event shall Nexperia be liable for any indirect, incidental, punitive,
special or consequential damages (including - without limitation - lost
profits, lost savings, business interruption, costs related to the removalor replacement of any products or rework charges) whether or not suchdamages are based on tort (including negligence), warranty, breach ofcontract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
whatsoever, Nexperia’s aggregate and cumulative liability towards customer
for the products described herein shall be limited in accordance with theTerms and conditions of commercial sale of Nexperia.
Right to make changes — Nexperia reserves the right to make changes
to information published in this document, including without limitation
specifications and product descriptions, at any time and without notice. This
document supersedes and replaces all information supplied prior to the
publication hereof.
Suitability for use — Nexperia products are not designed, authorized or
warranted to be suitable for use in life support, life-critical or safety-criticalsystems or equipment, nor in applications where failure or malfunctionof an Nexperia product can reasonably be expected to result in personal
injury, death or severe property or environmental damage. Nexperia and its
suppliers accept no liability for inclusion and/or use of Nexperia products insuch equipment or applications and therefore such inclusion and/or use is at
the customer’s own risk.
Applications — Applications that are described herein for any of theseproducts are for illustrative purposes only. Nexperia makes no representation
or warranty that such applications will be suitable for the specified use
without further testing or modification.
Customers are responsible for the design and operation of their applications
and products using Nexperia products, and Nexperia accepts no liability forany assistance with applications or customer product design. It is customer’ssole responsibility to determine whether the Nexperia product is suitableand fit for the customer’s applications and products planned, as well as
for the planned application and use of customer’s third party customer(s).
Customers should provide appropriate design and operating safeguards tominimize the risks associated with their applications and products.
Nexperia does not accept any liability related to any default, damage, costs
or problem which is based on any weakness or default in the customer’s
applications or products, or the application or use by customer’s third party
customer(s). Customer is responsible for doing all necessary testing for thecustomer’s applications and products using Nexperia products in order toavoid a default of the applications and the products or of the application or
use by customer’s third party customer(s). Nexperia does not accept any
liability in this respect.
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may be subject to export control regulations. Export might require a priorauthorization from competent authorities.
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between the translated and English versions.

Trademarks
Notice: All referenced brands, product names, service names andtrademarks are the property of their respective owners.

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List of Tables
Table 1. LED current for a given resistor values (E24
series, 5%) calculated using Eq (1)..................................... 3
Table 2. Thermal parameter (maximum values) for the
packages available for the LED drivers............................... 7
Table 3. Revision history......................................................8

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List of Figures
Fig. 1. LED driver topologies. Low-side using an NPN
and high-side using a PNP.................................................. 2
Fig. 2. Using two or more constant-current source LED
drivers in parallel..................................................................4
Fig. 3. Voltage margin of single and parallel drivers in
SOT457 (NCR42xU, NCR32xU), SOT223 (NCR42xZ,
NCR32xZ), and DFN2020D-6 (NCR32xPAS)...................... 5
Fig. 4. Capacitor placement for protection against
external EM fields................................................................ 5
Fig. 5. Capacitor value as a function of EMC frequency...... 6
Fig. 6. Impedance as a function of frequency...................... 6
Fig. 7. Fourier amplitudes (first 10) of current-source
LED drivers in PWM mode.................................................. 6
Fig. 8. Layout suggestion for the SOD223 package foran optimal thermal design....................................................7

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Contents
1. Introduction................................................................... 2
2. Concept of LED Drivers............................................... 2
3. Electrical performance................................................. 3
4. EMC Behavior............................................................... 5
5. Thermal performance................................................... 7
6. Revision history............................................................8
7. Legal information......................................................... 9© Nexperia B.V. 2021. All rights reserved

For more information, please visit: http://www.nexperia.com
For sales office addresses, please send an email to: salesaddresses@nexperia.com
Date of release: 12 February 2021

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