LM3421 datasheet(13/72 Pages) TI1

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VSNS = 1.24V x

RCSH

RHSP

ICSH =

RHSP

VSNS

RSNS

ILED

RHSP

RHSN

HSN

HSP

High-Side

Sense Amplifier

CSH

1.24V

CCMP

RCSH

COMP

Error Amplifier

VSNS

To PWM

Comparator

LM3421/23

ICSH

LM3421, LM3421-Q1, LM3423, LM3423-Q1

www.ti.com

SNVS574F – JULY 2008 – REVISED JULY 2015

Feature Description (continued)

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.

7.3.3 Average LED Current

Figure 13. LED Current Sense Circuitry

The LM34xx/Q1 uses an external current sense resistor (RSNS) placed in series with the LED load to convert the

LED current (ILED) into a voltage (VSNS) as shown in Figure 13. The HSP and HSN pins are the inputs to the

high-side sense amplifier which are forced to be equal potential (VHSP=VHSN) through negative feedback.

Because of this, the VSNS voltage is forced across RHSP to generate the signal current (ICSH) which flows out of

the CSH pin and through the RCSH resistor. The error amplifier will regulate the CSH pin to 1.24 V, therefore ICSH

can be calculated using Equation 4.

(4)

This means VSNS will be regulated as follows:

(5)

ILED can then be calculated using Equation 6.

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

LM3421 Datasheet(PDF) 13 Page - Texas Instruments

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