LM3421_10 Datasheet, PDF (12/48 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

AVERAGE LED CURRENT
The LM3421/23 uses an external current sense resistor
(R_SNS) placed in series with the LED load to convert the LED
current (I_LED) into a voltage (V_SNS) as shown in Figure 4. The
HSP and HSN pins are the inputs to the high-side sense am-
plifier which are forced to be equal potential (V_HSP=V_HSN)
through negative feedback. Because of this, the V_SNS voltage
is forced across R_HSP to generate the signal current (I_CSH)
which flows out of the CSH pin and through the R_CSH resistor.
The error amplifier will regulate the CSH pin to 1.24V, there-
fore I_CSH can be calculated:
ANALOG DIMMING
The CSH pin can be used to analog dim the LED current by
adjusting the current sense voltage (V_SNS). There are several
different methods to adjust V_SNS using the CSH pin:
1. External variable resistance : Adjust a potentiometer
placed in series with R_CSH to vary V_SNS.
2. External variable current source: Source current (0 µA to
I_CSH) into the CSH pin to adjust V_SNS.
This means V_SNS will be regulated as follows:
I_LED can then be calculated:
The selection of the three resistors (R_SNS, R_CSH, and R_HSP) is
not arbitrary. For matching and noise performance, the sug-
gested signal current I_CSH is approximately 100 µA. This
current does not flow in the LEDs and will not affect either the
off-state LED current or the regulated LED current. I_CSH can
be above or below this value, but the high-side amplifier offset
characteristics may be affected slightly. In addition, to mini-
mize the effect of the high-side amplifier voltage offset on LED
current accuracy, the minimum V_SNS is suggested to be
50 mV. Finally, a resistor (R_HSN = R_HSP) should be placed in
series with the HSN pin to cancel out the effects of the input
bias current (~10 µA) of both inputs of the high-side sense
amplifier.
The sense resistor (R_SNS) can be placed anywhere in the se-
ries string of LEDs as long as the voltage at the HSN and HSP
pins (V_HSP and V_HSN) satisfies the following conditions.
Typically, for a buck-boost configuration, R_SNS is placed at the
bottom of the string (LED-) which allows for greater flexibility
of input and output voltage. However, if there is substantial
input voltage ripple allowed, it can help to place R_SNS at the
top of the string (LED+) which limits the output voltage of the
string to:
FIGURE 5. Analog Dimming Circuitry
300673k3
In general, analog dimming applications require a lower
switching frequency to minimize the effect of the leading edge
blanking circuit. As the LED current is reduced, the output
voltage and the duty cycle decreases. Eventually, the mini-
mum on-time is reached. The lower the switching frequency,
the wider the linear dimming range. Figure 5 shows how both
CSH methods are physically implemented.
Method 1 uses an external potentiometer in the CSH path
which is a simple addition to the existing circuitry. However,
the LEDs cannot dim completely because there is always
some resistance causing signal current to flow. This method
is also susceptible to noise coupling at the CSH pin since the
potentiometer increases the size of the signal current loop.
Method 2 provides a complete dimming range and better
noise performance, though it is more complex. It consists of
a PNP current mirror and a bias network consisting of an NPN,
2 resistors and a potentiometer (R_ADJ), where R_ADJ controls
the amount of current sourced into the CSH pin. A higher re-
sistance value will source more current into the CSH pin
causing less regulated signal current through R_HSP, effective-
ly dimming the LEDs. V_REF should be a precise external
voltage reference, while Q7 and Q8 should be a dual pair PNP
for best matching and performance. The additional current
(I_ADD) sourced into the CSH pin can be calculated:
Note that he CSH pin can also be used as a low-side current
sense input regulated to 1.24V. The high-side sense amplifier
is disabled if HSP and HSN are tied to AGND (or V_HSN >
V_HSP) .
The corresponding I_LED for a specific I_ADD is:
www.national.com
12