LM3421 datasheet(22/65 Pages) TI1

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RSNS

LM3421/23

DDRV

100 nF

100

10V

RSNS

LED+

10V

VIN

5 k:

LM3421/23

500:

DDRV

VCC

100 pF

100 nF

10:

tPULSE =

2 x ILED x VO X L1

VIN

LM3421, LM3421-Q1

LM3423, LM3423-Q1

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

www.ti.com

When using the PWM functionality in a boost regulator, the PWM signal can drive a ground referenced FET.

However, with buck-boost and buck topologies, level shifting circuitry is necessary to translate the PWM dim

signal to the floating dimFET as shown in Figure 30 and Figure 31. If high side dimming is necessary in a boost

regulator using the LM3423, level shifting can be added providing the polarity inverting DPOL pin is pulled low

(see LM3423 ONLY: DPOL, FLT, TIMR, and LRDY section) as shown in Figure 32.

When using a series dimFET to PWM dim the LED current, more output capacitance is always better. A general

rule of thumb is to use a minimum of 40 µF when PWM dimming. For most applications, this will provide

adequate energy storage at the output when the dimFET turns off and opens the LED load. Then when the

dimFET is turned back on, the capacitance helps source current into the load, improving the LED current rise

time.

A minimum on-time must be maintained in order for PWM dimming to operate in the linear region of its transfer

function. Because the controller is disabled during dimming, the PWM pulse must be long enough such that the

energy intercepted from the input is greater than or equal to the energy being put into the LEDs. For boost and

buck-boost regulators, the minimum dimming pulse length in seconds (tPULSE) is:

(32)

Even maintaining a dimming pulse greater than tPULSE, preserving linearity at low dimming duty cycles is difficult.

The second helpful modification is to remove the CFS capacitor and RFS resistor, eliminating the high frequency

compensation pole. This should not affect stability, but it will speed up the response of the CSH pin, specifically

at the rising edge of the LED current when PWM dimming, thus improving the achievable linearity at low dimming

duty cycles.

Figure 30. Buck-boost Level-Shifted PWM Circuit

Figure 31. Buck Level-Shifted PWM Circuit

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

LM3421 Datasheet(PDF) 22 Page - Texas Instruments

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