LM3421 datasheet(16/65 Pages) TI1

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P =

1+D

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

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

www.ti.com

OVER-CURRENT PROTECTION

The LM3421/23 devices have a secondary method of over-current 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.

ZERO CURRENT SHUTDOWN

The LM3421/23 devices implement "zero current" shutdown via 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 21 allows them to float during

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

Figure 21. Zero Current Shutdown Circuit

CONTROL LOOP COMPENSATION

The LM3421/23 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. Since 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. See the Design Guide section for compensation of all

topologies.

The uncompensated loop gain for a buck-boost regulator is given by the following equation:

(15)

Where the uncompensated DC loop gain of the system is described as:

(16)

And the output pole (

ωP1) is approximated:

(17)

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

LM3421 Datasheet(PDF) 16 Page - Texas Instruments

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