LM3424 datasheet(13/69 Pages) TI1

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RSNS

ILED

RHSP

RHSN

HSN

HSP

High-Side

Sense Amplifier

CSH

1.24V

CCMP

RCSH

COMP

Error Amplifier

VSNS

To PWM

Comparator

LM3424

ITF

Thermal Foldback Current

ICSH

ILED =

RSNS

1.24V

RSNS

VSNS

RCSH

RHSP

VSNS = 1.24V x

RCSH

RHSP

ICSH =

RHSP

VSNS

LM3424, LM3424-Q1

www.ti.com

SNVS603C – AUGUST 2009 – REVISED AUGUST 2015

Feature Description (continued)

7.3.3 Average LED Current

To first understand how the LM3424 regulates LED current, the thermal foldback functionality will be ignored.

Figure 18 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. Because 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:

(4)

This means VSNS will be regulated as follows:

(5)

ILED can then be calculated:

(6)

The selection of the three resistors (RSNS, RCSH, and RHSP) is not arbitrary. For matching and noise performance,

the suggested signal current ICSH 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. ICSH can be above or below this value, but

the high-side amplifier offset characteristics may be affected slightly. In addition, to minimize 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 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.

Figure 18. LED Current Sense Circuitry

7.3.4 Thermal Foldback and Analog Dimming

Thermal foldback is necessary in many applications due to the extreme temperatures created in LED

environments. In general, two functions are necessary: a temperature breakpoint (TBK) after which the nominal

operating current needs to be reduced, and a slope corresponding to the amount of LED current decrease per

temperature increase as shown in Figure 19. The LM3424 allows the user to program both the breakpoint and

slope of the thermal foldback profile.

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