LM3421 datasheet(12/65 Pages) TI1

12 / 65 page

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fSW =

25 x VIN - VO

RT x CT X VIN

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)

LM3421, LM3421-Q1

LM3423, LM3423-Q1

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

www.ti.com

PREDICTIVE OFF-TIME (PRO) CONTROL

PRO control is used by the LM3421/23 to control ILED. It is a combination of average peak current control and a

one-shot off-timer that varies with input voltage. The LM3421/23 uses peak current control to regulate the

average LED current through an array of HBLEDs. This method of control uses a series resistor in the LED path

to sense LED current and can use either a series resistor in the MosFET path or the MosFET RDS-ON for both

cycle-by-cycle current limit and input voltage feed forward. D is indirectly controlled by changes in both tOFF and

tON, which vary depending on the operating point.

Even though the off-time control is quasi-hysteretic, the input voltage proportionality in the off-timer creates an

essentially constant switching frequency over the entire operating range for boost and buck-boost topologies.

The buck topology can be designed to give constant ripple over either input voltage or output voltage, however

switching frequency is only constant at a specific operating point .

This type of control minimizes the control loop compensation necessary in many switching regulators, simplifying

the design process. The averaging mechanism in the peak detection control loop provides extremely accurate

LED current regulation over the entire operating range.

PRO control was designed to mitigate “current mode instability” (also called “sub-harmonic oscillation”) found in

standard peak current mode control when operating near or above 50% duty cycles. When using standard peak

current mode control with a fixed switching frequency, this condition is present, regardless of the topology.

However, using a constant off-time approach, current mode instability cannot occur, enabling easier design and

control.

Predictive off-time advantages:

•

There is no current mode instability at any duty cycle.

•

Higher duty cycles / voltage transformation ratios are possible, especially in the boost regulator.

The only disadvantage is that synchronization to an external reference frequency is generally not available.

SWITCHING FREQUENCY

An external resistor (RT) connected between the RCT pin and the switch node (where D1, Q1, and L1 connect),

in combination with a capacitor (CT) between the RCT and AGND pins, sets the off-time (tOFF) as shown in

Figure 16. For boost and buck-boost topologies, the VIN proportionality ensures a virtually constant switching

frequency (fSW).

For a buck topology, RT and CT are also used to set tOFF, however the VIN proportionality will not ensure a

constant switching frequency. Instead, constant ripple operation can be achieved. Changing the connection of RT

in Figure 16 from VSW to VIN will provide a constant ripple over varying VIN. Adding a PNP transistor as shown in

Figure 17 will provide constant ripple over varying VO.

The switching frequency is defined:

Buck (Constant Ripple vs. VIN)

(4)

Buck (Constant Ripple vs. VO)

(5)

Boost and Buck-boost

(6)

For all topologies, the CT capacitor is recommended to be 1 nF and should be located very close to the

LM3421/23.

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

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