Many of today’s portable electronics require backlight
LED-driver solutions with the following features: direct
control of current, high efficiency, PWM dimming, overvoltage
protection, load disconnect, small size, and ease of
use. This article discusses each of these features and how
they are achieved, and concludes with a typical circuit
that implements each of these features.
Direct control of current
LEDs are current-driven devices whose brightness is proportional
to their forward current. Forward current can be
controlled in two ways. The first method is to use the LED
V-I curve to determine what voltage needs to be applied to
the LED to generate the desired forward current. This is
typically accomplished by applying a voltage source and
using a ballast resistor as shown in Figure 1. However, this
method has several drawbacks. Any change in LED forward
voltage creates a change in LED current. With a nominal
forward voltage of 3.6 V, the LED in Figure 1 has 20 mA of
current. If this voltage changes to 4.0 V, which is within
the specified voltage tolerance due to temperature or manufacturing
changes, the forward current drops to 14 mA.
This 11% change in forward voltage causes a much larger
30% change in forward current. Also, depending upon the
available input voltage, the voltage drop and power dissipation
across the ballast resistor waste power and reduce
battery life.
The second, preferred method of regulating LED current
is to drive the LED with a constant-current source. The
constant-current source eliminates changes in current due
to variations in forward voltage, which translates into a
constant LED brightness. Generating a constant-current
source is fairly simple. Rather than regulating the output
voltage, the input power supply regulates the voltage
across a current-sense resistor. Figure 2 shows this implementation.
The power-supply reference voltage and the
value of the current-sense resistor determine the LED
current. Multiple LEDs should be connected in a series
configuration to keep an identical current flowing in each
LED. Driving LEDs in parallel requires a ballast resistor in
each LED string, which leads to lower efficiency and
uneven current matching.
High efficiency
Battery life is critical in portable applications. For an LED
driver to be useful, it must be efficient. An efficiency measurement
of an LED driver differs from that of a typical
power supply. An efficiency measurement of a typical
power supply is defined as the output power divided by
the input power. With an LED driver, the output power is
not the parameter of interest. What is important is the
amount of input power required to generate the desired
LED brightness. This is easily determined by dividing the
power in the LEDs by the input power. Defining the efficiency
in this way means that the power dissipated in the
current-sense resistor contributes to the power lost in the
supply. The following equation shows that smaller currentsense
voltages contribute to higher-efficiency LED drivers.
Figure 3 shows that choosing a power supply with a 0.25-V
reference voltage versus a supply with a 1-V reference
voltage improves efficiency. A supply with a lower currentsense
voltage is more efficient regardless of input voltage
or LED current. With all else being equal, a lower reference
voltage significantly improves efficiency and extends
battery life.
PWM dimming
Many portable LED applications require dimming. In
applications such as LCD backlighting, dimming provides
brightness and contrast adjustment. Two types of dimming
are available: analog and PWM. With analog dimming, 50%
brightness is achieved by applying 50% of the maximum
current to the LED. Drawbacks to this method include
LED color shift and the need for an analog control signal,
which is not usually readily available. PWM dimming is
achieved by applying full current to the LED at a reduced
duty cycle. For 50% brightness, full current is applied at a
50% duty cycle. The frequency of the PWM signal must be
above 100 Hz to ensure that the PWM pulsing is not visible
to the human eye. The maximum PWM frequency depends
upon the power-supply startup and response times. To
provide the most flexibility and ease of integration, the
LED driver should be able to accept PWM frequencies as
high as 50 kHz.
Overvoltage protection
Operating a power supply in a constant-current mode
requires overvoltage protection. A constant-current supply
generates a constant output current regardless of load. If
the load resistance increases, the supply’s output voltage
also must increase to supply a constant current. If the
supply encounters an excessive load resistance, or if the
load is disconnected, the output voltage can increase above
the voltage rating of the IC or of the other discrete circuit
components. Several overvoltage protection schemes are
available for constant-current LED drivers. One scheme is
to place a zener diode in parallel with the LEDs. This limits
the output voltage to the zener’s breakdown voltage plus
the supply’s reference voltage. During an overvoltage condition,
the output voltage increases to the point where the
zener breaks down and begins to conduct. The output
current flows through the zener, then through the currentsense
resistor to ground. The supply continues to generate
the constant output current while the zener limits the
maximum output voltage. A more preferred method of
overvoltage protection is to monitor the output voltage
and shut down the supply when the overvoltage trip point
is reached. Shutting down the supply under an overvoltage
condition reduces power dissipation and extends battery
life in the event of a fault.
Load disconnect
An often overlooked feature in an LED-driver supply is
load disconnect. Load disconnect electrically removes the
LEDs from the power supply when the supply is disabled.
This is important in two situations: shutdown and PWM
dimming. As shown in Figure 2, during shutdown of a
boost converter, the load is still connected to the input
through the inductor and catch diode. Since the input
voltage is still connected to the LEDs, a small current
continues to flow, even when the supply is disabled. Even
small leakage currents significantly reduce battery life
during extended periods of off time. Load disconnect is
also important during PWM dimming. During the off time
of the dimming period, the supply is disabled; but the output
capacitor is still connected across the LEDs. Without
load disconnect, the output capacitor discharges through
the LEDs until the dimming pulse turns the supply on
again. Since the capacitor is partially discharged at the
beginning of each dimming cycle, the supply must charge
up the output capacitor at the beginning of each dimming
cycle. This creates a spike of inrush current during each
dimming cycle. The inrush current lowers system efficiency
and creates voltage transients on the input bus. With load
disconnect, the LEDs are removed from the circuit so
there is no leakage current when the supply is disabled,
and the output capacitor remains fully charged during
PWM dimming. A load-disconnect circuit is best implemented
by placing a MOSFET between the LEDs and the
current-sense resistor. Placing the MOSFET between the
current-sense resistor and ground creates an additional
voltage drop that manifests itself as an error in the outputcurrent
setpoint.
