LM3421 datasheet(15/72 Pages) TI1

15 / 72 page

ILIM =

245 mV

RLIM

LM3421/23

PWM

COMP

RLIM

GATE

LEB

PGND

0.245V

0.8V

RDS-ON

Sensing

RLIM

Sensing

ILED = (ICSH - IADD) x

RHSP

RSNS

RADJ x VREF

IADD =

RADJ + RMAX

- VBE-Q6

RBIAS

LM3421, LM3421-Q1, LM3423, LM3423-Q1

www.ti.com

SNVS574F – JULY 2008 – REVISED JULY 2015

Feature Description (continued)

Method 2 provides a complete dimming range and better noise performance, though it is more complex. It

consists of a PNP current mirror and a bias network consisting of an NPN, 2 resistors and a potentiometer

(RADJ), where RADJ controls the amount of current sourced into the CSH pin. A higher resistance value will source

more current into the CSH pin causing less regulated signal current through RHSP, effectively dimming the LEDs.

VREF should be a precise external voltage reference, while Q7 and Q8 should be a dual pair PNP for best

matching and performance. The additional current (IADD) sourced into the CSH pin can be calculated using

Equation 9.

(9)

The corresponding ILED for a specific IADD is:

(10)

7.3.5 Current Sense and Current Limit

The LM34xx-Q1 achieves peak current mode control using a comparator that monitors the main MOSFET (Q1)

transistor current, comparing it with the COMP pin voltage as shown in Figure 15. Further, it incorporates a

cycle-by-cycle overcurrent protection function. Current limit is accomplished by a redundant internal current

sense comparator. If the voltage at the current sense comparator input (IS) exceeds 245 mV (typical), the on

cycle is immediately terminated. The IS input pin has an internal N-channel MOSFET which pulls it down at the

conclusion of every cycle. The discharge device remains on an additional 210 ns (typical) after the beginning of a

new cycle to blank the leading edge spike on the current sense signal. The leading edge blanking (LEB)

determines the minimum achievable on-time (tON-MIN).

Figure 15. Current Sense / Current Limit Circuitry

There are two possible methods to sense the transistor current. The RDS-ON of the main power MOSFET can be

used as the current sense resistance because the IS pin was designed to withstand the high voltages present on

the drain when the MOSFET is in the off state. Alternatively, a sense resistor located in the source of the

MOSFET may be used for current sensing; however, TI suggests a low inductance (ESL) type. The cycle-by-

cycle current limit (ILIM) can be calculated using either method as the limiting resistance (RLIM):

(11)

Submit Documentation Feedback

Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1

LM3421 Datasheet(PDF) 15 Page - Texas Instruments

LM3421 Datasheet(HTML) 15 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

Datasheet Download

Link URL