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30085799
FIGURE 2. Timing Circuitry
AVERAGE LED CURRENT
To first understand how the LM3424 regulates LED current,
the thermal foldback functionality will be ignored.
shows the physical implementation of the LED current sense
circuitry assuming the thermal foldback circuitry is a simple
current source which, for now, will be set to zero (I
TF
= 0A).
The LM3424 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
). The HSP and HSN pins are the
inputs to the high-side sense amplifier which are forced to be
equal potential (V
HSP
=V
HSN
) through negative feedback. Be-
cause of this, the V
SNS
voltage is forced across R
HSP
which
generates a current that is summed with the thermal foldback
current (I
TF
) 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 and assuming
I
TF
= 0A, I
CSH
can be calculated:
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.
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 GND.
30085757
FIGURE 3. LED Current Sense Circuitry
11
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LM3424