LM3421 datasheet(16/72 Pages) TI1

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P =

r x

SNS

CSH

500V

620V

(

)

LIM

LED

(

)

LIM

HSP

1 Z

LM3421/23

VIN

nDIM

VIN

RPD

OVP

Enable

RUV2

RUV1

ROV2

ROV1

LM3421, LM3421-Q1, LM3423, LM3423-Q1

SNVS574F – JULY 2008 – REVISED JULY 2015

www.ti.com

Feature Description (continued)

7.3.6 Overcurrent Protection

The LM34xx-Q1 devices have a secondary method of overcurrent protection. Switching action is disabled

whenever the current in the LEDs is more than 30% above the regulation set point. The dimming MOSFET

switch driver (DDRV) is not disabled however as this would immediately remove the fault condition and cause

oscillatory behavior.

7.3.7 Zero Current Shutdown

The LM34xx-Q1 devices implement zero current shutdown through the EN and RPD pins. When pulled low, the

EN pin places the devices into near-zero current state, where only the leakage currents will be observed at the

pins (typical 0.1 µA). The applications circuits frequently have resistor dividers to set UVLO, OVLO, or other

similar functions. The RPD pin is an open-drain N-channel MOSFET that is enabled only when the device is

enabled. Tying the bottom of all resistor dividers to the RPD pin as shown in Figure 16 allows them to float

during shutdown, thus removing their current paths and providing true application-wide zero current shutdown.

Figure 16. Zero Current Shutdown Circuit

7.3.8 Control Loop Compensation

The LM34xx-Q1 control loop is modeled like any current mode controller. Using a first order approximation, the

uncompensated loop can be modeled as a single pole created by the output capacitor and, in the boost and

buck-boost topologies, a right half plane zero created by the inductor, where both have a dependence on the

LED string dynamic resistance. There is also a high-frequency pole in the model; however, it is near the

switching frequency and plays no part in the compensation design process. Therefore, it will be neglected.

Because ceramic capacitance is recommended for use with LED drivers, due to long lifetimes and high ripple

current rating, the ESR of the output capacitor can also be neglected in the loop analysis. Finally, there is a DC

gain of the uncompensated loop which is dependent on internal controller gains and the external sensing

network.

A buck-boost regulator will be used as an example case.

Use Equation 12 to calculate the uncompensated loop gain for a buck-boost regulator.

(12)

Where the uncompensated DC loop gain of the system is calculated using Equation 13.

(13)

And the output pole (

ωP1) is approximated using Equation 14.

(14)

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

LM3421 Datasheet(PDF) 16 Page - Texas Instruments

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