NSM4002MR6 Datasheet, PDF (3/7 Pages) ON Semiconductor – Dual NPN Transistors for Driving LEDs

NSM4002MR6
Application Section
Introduction
The NSM4002MR6 is designed to be used as a constant
current driver for LEDs. The two resistors in Figure 1 are
external from the NSM4002MR6 to allow for
customization. R_set controls the current through the load,
and R₁ controls the bias current.
based on the biasing current. To determine the R_set value
simply divide the V_BE voltage by the desired driving
current.
Selecting R₁
The R₁ resistor is used to set the biasing current. The
biasing current is split between the base of Q2 and the
collector of Q1. When desiring the lowest overhead voltage
R₁ should be set as high as possible. It is important to ensure
it is not set too high so that Q2 falls out of saturation.
However, a lower R₁ value will drive more current through
Q1. This will reduce the change in the driving current as
temperature is increased. It will also allow a higher driving
current to be achieved while maintaining good current
regulation. The side affect of a lower R₁ value is that it
reduces the overall efficiency because more power is being
used in the driving circuit.
Figure 1. Typical Application Schematic
Selecting R_set
The R_set resistor is used to set the driving current of the
load. It is connected across the Base−Emitter junction of Q1.
This V_BE voltage is what sets up the constant voltage across
the R_set resistor. Figure 5 gives the typical values of V_BE
Input Votlage, V_s
The maximum input voltage, V_s, is determined by the
load. No more than 45 V can be applied across Q2. This leads
to:
V_s(max) + V_Load ) 45 V
(eq. 1)
Overhead Voltage
The overhead voltage of this device to reach full current
regulation is the combination of the V_BE voltages of the two
transistors. Under typical conditions this overhead voltage
will typically be 1.4 V.
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