V

REF2

DC/DC

R1

R2

V

LED

I

=

LED

C

OUT

V

REF

R

S

V

R

REF

S

Error

Amp

V

REF2

DC/DC+LEDDriver

Primary

Loop

Secondary

Loop

TLC5960

www.ti.com

SBVS147 – SEPTEMBER 2010

iHVM: STABILITY ANALYSIS ON CONVENTIONAL SYSTEM

Figure 29 shows a dual loop, a conventional dc/dc output topology adjustment enabled by the information of the

drain node feedback mechanism. This system basically consists of two feedback loops. The primary feedback

loop is a voltage feedback loop for the dc/dc converter. The additional secondary feedback loop is established to

integrate the cathode node information of the LED string to optimize thermal dissipation regarding LED string

voltage change as a result of variance of LED component forward voltage.

Figure 29. Conventional Drain Information Feedback Loop Structure

In order to keep this total system stable, the circuit constants are appropriately set so that they do not conflict

with these feedback loops. The primary loop frequency response must be set faster than the secondary loop;

typically, a 10x faster setting is known to generate a stable system.

The conventional approach restricts total system performance on the following:

1. The LED drive dimming speed must be as slow as the secondary loop response; or,

2. If faster dimming LED switching is needed, then the dc/dc output capacitance must be larger (sometimes

maintain required thermal performance.

Copyright © 2010, Texas Instruments Incorporated

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