LM3551 datasheet(14/23 Pages) TI1

14 / 23 page

fZC =

2SRCCC

2S(RC + RO)CC

fPC =

LM3551, LM3552

SNVS371D – AUGUST 2005 – REVISED MAY 2013

www.ti.com

Compensation Components

The LM3551 and LM3552 provide a compensation pin (VC) to customize the voltage loop feedback. It is

recommended that a series combination of RC and CC be used for the compensation network, as shown in the

typical application circuit. For any given application, there exists a unique combination of RC and CC that will

optimize the performance of the LM3551/2 circuit in terms of its transient response. The series combination of RC

and CC introduces a pole-zero pair. The frequency of the pole created is determined by the equation:

(19)

where RO is the output impedance of the error amplifier, approximately 900kΩ. Since RC is generally much less

than RO, it has little effect on the above equation and can be neglected until a value is chosen to set the zero fZC.

fZC is created to cancel the pole created by the output capacitor, fP1. The output capacitor pole will shift with

different load currents as shown by the equation, so setting the zero is not exact. Determine the range of fP1 over

the expected loads and then set the zero fZC to a point approximately in the middle. The frequency of this zero is

determined by:

(20)

Now RC can be chosen with the selected value for CC. Check to make sure that the pole fPC is still in the 10Hz to

500Hz range, and change each value slightly if needed to ensure both component values are in the

recommended range. For both typical applications circuits shown on the front page, the Recommended value

for CC is 4.7nF and RC = 10kΩ for Lumiled applications. 10nF and 27kΩ are recommended for Sharp

applications.

RECOMMENDED MINIMUM COMPONENT SPECIFICATIONS

Component

Value

Ratings

4.7µH

2.0A 30% ISAT Rating

4.7µF (Sharp)

CIN

10V X5R or X7R

10µF (Lumiled)

4.7µF (Sharp)

COUT

16V X5R or X7R

10uF (Lumiled)

CFTO

User Determined

6.3V X5R or X7R

4.7nF

(Lumiled)

6.3V X5R or X7R

10nF

(Sharp)

10k

Ω

(Lumiled)

27k

Ω

(Sharp)

RFLASH

User Determined

Application Specific

RTORCH

User Determined

Application Specific

CSS

User Determined

6.3V X5R or X7R

Torch and Flash Resistor ratings are dependent upon the current through each resistor. The minimum ratings will

vary depending upon the current selected on an applicaiton by application basis. Power Rating Minimum =

(Desired Current)2 × Resistor Value.See the CURRENT SET EQUATIONS section to determine torch and flash

currents.

Submit Documentation Feedback

Product Folder Links: LM3551 LM3552

LM3551 Datasheet(PDF) 14 Page - Texas Instruments

LM3551 Datasheet(HTML) 14 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL