LM3421 datasheet(11/24 Pages) NSC

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PROGRAMMING AVERAGE LED CURRENT

30067357

FIGURE 2. LED Current Sense Circuitry

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 (VSENSE). This

is because V

SENSE and RSENSE 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

HSN. In other words, the amplifier’s output P-MOSFET tran-

sistor pulls current through R

HSP until VHSP=VHSN, and this

occurs when the voltage (|V

RHSP| - |VRHSN|) is equal to

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 RHSP. As stated previously, the control

system’s error amplifier regulates the CSH voltage (V

CSH) to

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,

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

SENSE) is directly proportional to the sense voltage and the

sense resistor value: P

SENSE = ILED

2 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

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

SENSE, the value of the resistor in series with the HSP pin

HSP) can be calculated. The signal current set up by RHSP

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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