TLC5960 datasheet(27/33 Pages) TI

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REF

DC/DC

LED

Primary

Loop

Secondary

Loop

HVM1

TLC5960

iHVM

Process

Core

SINK

SOURCE

OUT

´ dv(iHVM)<

LED

OUT

´ dt(iHVM)

OUT

LED

10V/s

TLC5960

www.ti.com

SBVS147 – SEPTEMBER 2010

STABILITY ANALYSIS OF PROPOSED iHVM SYSTEM

Figure 31 shows the feedback system with the proposed iHVM control mechanism. The biggest difference (and

advantage) compared to a conventional system is that the iHVM processing core controls the secondary loop.

With this system, there are no limitations on the primary loop and secondary loop settings. The FETs drain node

voltage information is processed easily and can be automatically adjusted to fit the primary loop feedback

response, as long as the primary loop unity gain frequency is approximately 1kHz. See Equation 8 for the iHVM

output voltage setting calculation. The second term is the variable portion of VLED output voltage and is

well-regulated by the iHVM mechanism.

Figure 31. iHVM Loop Structure

Here, focusing on the appropriate minimal time range, the only condition required to maintain the total iHVM

feedback system stability is shown in Equation 19.

(19)

The left-hand term describes the value of the output voltage range by iHVM during the minimal time of dt(iHVM).

Therefore, in an iHVM system, the slowest response in the entire system can be designated by the values of ILED

and COUT, as long as the primary system unity gain frequency is set at a minimum of 1kHz. Here, the derivation

of the iHVM circuit response is already known as 10V/s. The resulting equation to keep the entire iHVM system

stable is shown in Equation 20.

Where the unity gain frequency of the primary loop > 1kHz.

(20)

In this case, ILED = 100mA, R1 = 400kΩ, and R3 = 40kΩ, COUT should be less than 1000µF to retain the transient

response to achieve 100% power saving by iHVM caused by LED VF variance. In a typical application, this value

is large enough. Therefore, we can conclude that the only condition required to keep iHVM stable is to set the

primary feedback loop unity-gain frequency greater than 1kHz with a minimum amount of COUT (typically 100µF

to 300µF, depending on the required LED current value) to keep the VLED line stable when the LED is switching.

Refer to the dc/dc controller manual for setting the appropriate minimum output capacitance value.

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