Datasheet > ADT6780

ADT6780

更新时间： 2022-10-09 10:37:32

品牌	Logo	应用领域
ADTECH		驱动器
页数	文件大小	规格书
13页	490K
描述
Thermally enhanced Low VFB Step-Down LED Driver

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ADT6780 数据手册

Thermally enhanced Low V_FBStep-Down LED Driver

APPLICATION INFORMATION (continued)

ADT6780

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:

A_VDC= R₁×A_EA×G_CS

Where A_EAis the error amplifier voltage gain. G_CSis the current sense transconductance and R1 is the current sense

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_LED=△V_OUT/△I_LED) . These poles are expressed as:

G_EA

f_P1=

2π×C3×A_EA

f_P2=

2π×C_O×R_LED

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_P2out. This zero is:

f_Z1=

2π×C3×R2

If the output capacitor has a large capacitance and/or a high ESR value, unwanted zero is generated to the

location of:

f_Z2=

2π×C_O×ESR

In this case, third pole is needed to compensate f_Z2. This pole, f_P3, is made by the R2 and the selectively

added optional capacitor (C6) between COMP to GND. f_P3is expressed to:

f_P3=

2π×C6×R2

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.

* This specifications are subject to be changed without notice

http://www.ad-tech.co.kr

Jul. 12. 2012 / Preliminary

10/13

1...7 8 91011 12 13

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