LM3421_09 Datasheet, PDF (11/24 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

PROGRAMMING AVERAGE LED CURRENT
FIGURE 2. LED Current Sense Circuitry
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This section serves to explain how the LM3421/LM3423 con-
trollers use the high-side sense amplifier to regulate average
LED current. Instructions for calculation of component values
are also covered.
The voltage at the CSH pin is regulated by the error amplifier
to be 1.235V. Understanding how average LED current is
regulated requires understanding the relationship between
the CSH voltage (V_CSH) and the sense voltage (V_SENSE). This
is because V_SENSE and R_SENSE directly set the average LED
current, I_LED.
The high side amplifier in Figure 2 forces its input terminals
to equal potential. Because of this, the V_SENSE voltage is
forced across the differential voltages across R_HSP and
R_HSN. In other words, the amplifier’s output P-MOSFET tran-
sistor pulls current through R_HSP until V_HSP=V_HSN, and this
occurs when the voltage (|V_RHSP| - |V_RHSN|) is equal to
V_SENSE. So the current flowing down to the CSH pin is given
by:
And the voltage at the CSH pin is then given by:
So, the CSH voltage is the sense voltage (V_SENSE) gained up
by the ratio of R_CSH to R_HSP. As stated previously, the control
system’s error amplifier regulates the CSH voltage (V_CSH) to
V_REF. So, using the above equation with some slight substi-
tution and rearranging, we can conclude the following:
This leads to the final equation that can be used to calculate
average LED current given any combination of resistor val-
ues:
The equation above shows how current in the LED relates to
the regulated voltage V_REF, which is 1.235V for the LM3421/
LM3423.
The selection of resistors is not arbitrary; for matching and
noise performance we suggest that the CSH current is 100µA.
This current does not flow in the LEDs and will not affect either
the off state LED current or the regulated LED current. The
CSH current can be above or below this value, but the high
side amplifier offset characteristics may be affected slightly.
In addition, to hold an initial 5% tolerance on the LED current,
R_SENSE should be selected to have at least 50mV across it at
the desired LED current (R_SENSE greater than or equal to
50mV / I_LED). The power dissipated in the sense resistor
(P_SENSE) is directly proportional to the sense voltage and the
sense resistor value: P_SENSE = I_LED2 x R_SENSE.
Design Example: The user desires 1A of average LED cur-
rent. 100mV is a typical starting point for V_SENSE, providing an
R_SENSE of 100mV/1A = 100mΩ. This will limit the power dis-
sipation in R_SENSE to 100mW while providing good regulation.
Once a standard component value has been selected for
R_SENSE, the value of the resistor in series with the HSP pin
(R_HSP) can be calculated. The signal current set up by R_HSP
should be set for approximately 100µA at the desired LED
current.
A resistor of equal value should be placed in series with the
HSN pin to cancel out the effects of the input bias current
(~10µA) of both inputs of the high side sense amplifier. The
signal current (100µA) set up by the HSP resistor flows into
the HSP pin and is translated down to appear as a source
current from the CSH pin. The resistor from the CSH pin to
ground (R_CSH) would nominally be 12.4kΩ. This value is cho-
sen to convert the 100µA signal current representing the
average LED current to a voltage very close to 1.235V, the
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