NUD4301
http://onsemi.com
5
Figure 7. Dimming Operation Curves
(Graph obtained from SPICE simulations)
Analog
Operation
Digital
Operation
time
time
V
DIM
3.3 V
I
Limit
I
LED
I
avg
Theory of Operation
This device contains two LED current sources. Each
channel is comprised of a lateral N-channel FET controlled
by a current limit circuit that senses the voltage drop across
the R
sense
resistor and compares it with an internal voltage
reference to provide the current regulation. For dimming
applications, the current limit circuit operates in
combination with the PWM signal applied to the dim pin of
the device for control purposes.
Current Limit Circuit
With a DC voltage of 3.3 V applied to the Dim pin of the
device, the internal reference voltage of the current limit
circuit is set to 94 mV. The R
sense
resistor is then selected
through a very simple formula: R
sense
= 94 mV / I
LED
. This
allows the user to set different LED currents (between 1 mA
and 30 mA). If different V
dim
voltage is used, then the I
LED
current will change according to the following formula:
I
LED
+
(V
dim
ń35)
R
sense
Dim Circuit
For dimming control, a PWM signal may be applied to the
dim pin of the device. This PWM signal can be used to
perform digital dimming.
For digital dimming, the amplitude of the PWM signal
must be 3.3 V or higher. The LED current will be
proportional to the duty cycle of the PWM signal.
For analog dimming, the input signal to the Dim pin must
be between 0 V and 3.3 V. The resulting output current will
be given by the previous I
LED
formula. It is important to
mention that variations on the turn on point from
part-to-part are expected due to the offset of the internal
amplifiers. That is, some devices may turn on right after
V
dim
= 50 mV and others until V
dim
is around 0.3 V.
If a PWM signal is beyond the input frequency range for
the Dim pin, a RC filter may be used to convert it to an analog
signal.
The RC filter generates an analog voltage signal, which is
proportional to the duty cycle of the PWM signal applied.
This analog signal is then used as the new reference voltage
for the current limit circuit, which compares it with the
voltage signal generated across R
sense
to provide the current
regulation.
Enable
The enable circuit turns the device on when a positive
signal is applied to the enable pin. The circuit is designed to
allow low current consumption (0.1 mA typical) when the
device is disabled.
LCD Backlighting Applications
The voltage and current capability of the NUD4301
device can be increased by using external FETs so that the
circuit can be used in high voltage backlighting applications
such as TV. Figure 8 shows the schematic diagram of this
concept.
