LM3421 datasheet(13/65 Pages) TI1

13 / 65 page

ILED =

RSNS

1.24V

RSNS

VSNS

RCSH

RHSP

VSNS = 1.24V x

RCSH

RHSP

ICSH =

RHSP

VSNS

RSNS

ILED

RHSP

RHSN

HSN

HSP

High-Side

Sense Amplifier

CSH

1.24V

CCMP

RCSH

COMP

Error Amplifier

VSNS

To PWM

Comparator

LM3421/23

ICSH

LM3421/23

RCT

Start tON

VIN/25

Reset timer

RSNS

VIN

LED-

VSW

LM3421/23

RCT

Start tON

VIN/25

Reset timer

LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

Figure 16. Off-timer Circuitry for Boost and Buck-

Figure 17. Off-timer Circuitry for Buck Regulators

boost Regulators

Figure 18. LED Current Sense Circuitry

AVERAGE LED CURRENT

The LM3421/23 uses an external current sense resistor (RSNS) placed in series with the LED load to convert the

LED current (ILED) into a voltage (VSNS) as shown in Figure 18. 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 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, therefore ICSH

can be calculated:

(7)

This means VSNS will be regulated as follows:

(8)

ILED can then be calculated:

(9)

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