Whitepaper

Constant current LED drivers
Cost efficient, reliable, and easy to use
By Dr.-Ing. Jan Preibisch, Application Marketing Manager, Nexperia

LEDs are widely used across all market segments. Although high power LEDs
need complex driver schemes, the vast majority of applications use LEDs
with currents below 500 mA and can be driven by linear constant-current LEDdrivers. These are easy-to-use, reliable, cost efficient, and do not affect EMC.
This article gives a short overview on the design and power considerations
necessary to use constant-current LED drivers in practical applications.

To drive high performance LEDs, there are various highend solutions using boost-buck topologies with complex
controllers that require a deep understanding of thetopology to create reliable and EMC-compliant designs. For
low and medium power LEDs, however, there is the very
simple and robust driver solution using constant-current
LED drivers which operate as a simple linear regulator. Froma power loss perspective, constant-current LED drivers are
not ideal, but their excellent EMC performance, reliability,
simple implementation, and - in the end - significantly lower
system costs make them a preferred solution for driving LEDs
up to about 500 mA. Drivers referred to in this article withthe suffix NCR are available from Nexperia. They are used inconstant current source and automotive applications such as
interior and exterior lighting (e.g. door handles, dashboard,number plate light, indicators or rear lights).

VCC

VCC

VS

μC /Logic
IEN
VEN

IEN

Iout
IOUT

VCC
Rext

VEN

Iout

REXT

IOUT

IOUT

REXT

GND

REXT

Iout

OUT
Rext
GND

Rext
GND

IN/OUT

LED

GND

Figure 1: Low- and high-side constant current LED driver topologies.

Figure 1 shows the basic circuits for driving LEDs using one
constant-current driver. Internally, such a driver consistsof a BJT, two diodes, and two resistors. Constant-current
drivers with PNP BJTs operate as high-side drivers, while theones with NPN BJTs operate as low-side parts. One resistor
defines the minimum output current and the other tunes
the bias voltage and plays an important role for the enablecharacteristic. The high-side constant-current driver has anenable pin which is connected to ground; both driver, andthus the LED, can be turned off by disconnecting this pin. In
practice, this is performed using a resistor-equipped transistor(RET), as illustrated in the Figure 1, or MOSFET. Low-sidedrivers, however, need a certain potential to be enabled.
NCRx20x series drivers require a voltage in the range of thesupply voltage to enable them. The NCRx21x driver enables
at a much lower voltage of 3.3 V. This part draws a current ofabout 1 - 2 mA at the enable pin so it can easily be driven by theoutput pin of a microcontroller or a logic device. This makes itvery convenient to turn the LED on and off, and, using a PWM
controller, dimming can also be simply achieved.

Mean of output current [mA]

The dimming function is illustrated in Figure 2, which shows
the mean of the output current as a function of the duty
cycle for a NCR321Z for different switching frequencies
with an external resistor of 6 Ω. The graph displays a linear
relationship between the duty cycle and the measured
average output current. Even for frequencies above therecommended 10 kHz, this linear relationship is maintained.
However, a frequency not exceeding 10 kHz should be
preferred to prevent EMC issues.

180160140120100806040

10 kHz100 kHz500 kHz

2000

1020304050607080 90 100
Duty Cycle [%]The output current of most constant-current LED drivers isadjustable by an external resistor. Some types are tuned tocommonly-used currents. As they do not feature an externalresistor, they are available in 3-pin packages. In applications
where an external resistor can be connected, it will bein parallel to the internal resistor, thereby lowering theeffective resistance. The low-side constant-current drivers(NCRx2xx) using a NPN transistor have an internal resistorof 95 Ω. An analysis of the measurement curves provides thefollowing formula which enables the relationship between
external resistor and output current to be estimated:

The value of the external resistor must not be too low,such that the maximum output current is not exceeded.
The majority of the output current will flow through theexternal resistor if it is smaller than the internal resistor;
this is significant when the external resistor is small and theoutput current high. Nevertheless, a ¼ W resistor will be
sufficient, since the power losses will not exceed 170 mW,
even for a 250 mA output current.
Constant-current drivers show a temperature dependence.
At elevated temperatures, the output current decreases
slightly. However, the output current remains independent
from the voltage drop across the driver. As the output
current decreases with temperature, there is no risk ofthermal runaway.
The minimum voltage drop across a constant-current driveris about 1.4 V. Below this voltage, linear regulation does
not work properly. Above it, the voltage drop across theconstant current driver dynamically adjusts to enforce thedesired output current. In low- and high-side configurations,the output voltage is always calculated as Vout = VCC – VLED
with the voltage at the LEDs VLED and the supply voltage VCC .
Constant-current LED drivers are used when the brightness ofan LED should be independent of the supply voltage to somedegree. 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,
the minimum possible supply voltage can be calculated byadding the voltage drop across the driver plus the voltage at
the LEDs. The maximum allowed voltage is governed by themaximum-permitted voltage drop across the driver Vout or thetotal power dissipation which can be estimated by Ptot ≈Vout Iout .
Hence, the allowed operating range, if datasheet limits are notexceeded, can be specified as:

Figure 2: Mean of the output current as a function of the duty cycle
for a NCR321Z for different switching frequencies with an external
resistor of 6 Ω.

Constant-current drivers in the SOT457 package usually have
Ptot = 0.75 W. For an output current of 50 mA, this 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.
2

Whitepaper, Constant current LED drivers

Voltage overhead (V)

VCC

IEN
VENIENIout
IOUT

VENIoutIOUT

40
NCR42xU
NCR32xU
NCR42xZ
NCR32xZ
2 x NCR42xU
2 x NCR32xU
2 x NCR42xZ
2 x NCR32xZ

35
30
2520REXT

REXT
Rext

GND

Rext
GND

1510
50Figure 3: Two low-side drivers in parallel operation.

Additionally, the voltage margin can be increased byreducing the output current. Placing two or moreconstant-current drivers in parallel, as illustrated in Figure
3, effectively doubles the current. Using this method, acurrent exceeding the capability of a single driver canbe driven, or a smaller current per driver can be used toincrease 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 inparallel, the accuracy of the external resistors is the mostimportant factor for the symmetry of the output currentsof the individual drivers. Figure 4 illustrates the voltage
margins depending on the output current for single andparallel drivers in SOT457 and SOT223 packages.

050 100 150 200 250 300 350 400 450 500
Output current (mA)

Figure 4: Voltage margin of single and parallel drivers in SOT457
and SOT223 packages.

Figure 5: Nexperia LED drivers (NCRx2xx) in SOT457 and SOT223
packages

In conclusion, constant-current LED drivers offer a very
cost-efficient and easy-to-implement solution for driving
low and medium power LEDs. Using packages with ahigher Ptot directly translates into a higher margin in
the supply voltage and parallel drivers can be used toincrease the current capability.
nexperia.com/led-driver© 2019 Nexperia B.V.
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nexperia.com

Date of release:
November 2019