TLC5960 Datasheet, PDF (25/33 Pages) Texas Instruments – Eight-Channel LED Driver with Intelligent Headroom Voltage Monitor (iHVM)

TLC5960
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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.
DC/DC + LED Driver
DC/DC
Primary
Loop
V_LED
R1
R2
C_OUT
Secondary
Loop
Error
Amp
V_REF2
V_REF2
V_REF
I_LED
=
V_REF
R_S
R_S
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
over 1000µF) in order to keep the V_LED line stable enough, with tight LED forward voltage selection criteria to
maintain required thermal performance.
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