ADT6780 ADTECH [ADTech], ADT6780 Datasheet - Page 10

• Manufacturer Part Number: ADT6780
• Description: Thermally enhanced Low VFB Step-Down LED Driver
• Manufacturer: ADTECH [ADTech]

Jul. 12. 2012 / Preliminary
APPLICATION INFORMATION
Loop Compensation
The ADT6780 uses a fixed frequency, peak current mode control scheme to provide easy compensation and
fast transient response. Peak current mode control eliminate the double pole effect of the output LC filter.
Therefore, the step-down converter can be simplified to be a one-pole system in frequency domain.
The goal of compensation design is to shape the converter transfer function to get the desired gain and
phase. System stability is provided with the addition of a simple series capacitor-resistor from COMP to
GND. This pole-zero combination serves to adjust the desired response of the closed-loop system.
The DC gain of the voltage feedback loop is given by:
Where A
resistor value.
The system has two dominant poles. One is made by the combination of both the output resistor of the error
amplifier and the compensation capacitor (C3). And the other is due to the output capacitor and the LED’s
AC resistor(R
where, GEA is the error amplifier transconductance.
For a stable one-pole converter system, one of two dominant poles needs to be eliminated by one zero. One
zero made by the series capacitor-resistor (R2-C3) cancels f
If the output capacitor has a large capacitance and/or a high ESR value, unwanted zero is generated to the
location of:
In this case, third pole is needed to compensate f
added optional capacitor (C6) between COMP to GND. f
The system crossover frequency (Fc), where the feedback loop has the unity gain, is important. The system
crossover frequency is called the converter bandwidth. Generally higher Fc means faster transient response
and load regulation. However, higher Fc could cause system unstable. A standard rule of thumb sets the
crossover frequency to be equal or less than 1/10 of switching frequency.
EA
is the error amplifier voltage gain. G
LED
=△V
OUT
Thermally enhanced Low V
* This specifications are subject to be changed without notice
/△I
LED
) . These poles are expressed as:
A
VDC
f
f
(continued)
f
CS
P1
P2
f
Z2
f
=
Z1
P3
is the current sense transconductance and R1 is the current sense
=
=
=
R
Z2
=
=
2π
2π
. This pole, f
1
2π
2π
2π
10/13
×
×
×
A
×
C3
G
C
×
×
C
EA
O
C3
C6
EA
1
FB
P3
O
1
1
×
1
×
×
×
is expressed to:
A
P2
×
Step-Down LED Driver
×
R
G
ESR
EA
R2
R2
LED
out. This zero is:
CS
P3
, is made by the R2 and the selectively
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ADT6780