LM3424 Datasheet, PDF(11/50 Page) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424 Datasheet, PDF (11/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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FIGURE 2. Timing Circuitry

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AVERAGE LED CURRENT

To first understand how the LM3424 regulates LED current,

the thermal foldback functionality will be ignored. Figure 3

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 (ITF = 0A).

The LM3424 uses an external current sense resistor (RSNS)

placed in series with the LED load to convert the LED current

(ILED) into a voltage (VSNS). The HSP and HSN pins are the

inputs to the high-side sense amplifier which are forced to be

equal potential (VHSP=VHSN) through negative feedback. Be-

cause of this, the VSNS voltage is forced across RHSP which

generates a current that is summed with the thermal foldback

current (ITF) to generate the signal current (ICSH) which flows

out of the CSH pin and through the RCSH resistor. The error

amplifier will regulate the CSH pin to 1.24V and assuming

ITF = 0A, ICSH can be calculated:

This means VSNS will be regulated as follows:

ILED can then be calculated:

The selection of the three resistors (RSNS, RCSH, and RHSP) is

not arbitrary. For matching and noise performance, the sug-

gested signal current ICSH is approximately 100 ÂµA. This

current does not flow in the LEDs and will not affect either the

off-state LED current or the regulated LED current. ICSH 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 VSNS is suggested to be

50 mV. Finally, a resistor (RHSN = RHSP) should be placed in

series with the HSN pin to cancel out the effects of the input

bias current (~10 ÂµA) 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.

FIGURE 3. LED Current Sense Circuitry

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