LED7706 datasheet(19/31 Pages) STMICROELECTRONICS

19 / 31 page

LED7706

Operation description - boost section

19/31

damping will result in excessive ringing at the output and poor phase margin. Figure 10 a

and 10 b give an example of compensation adjustment for a typical application.

Figure 11.

Load transient response measurement set-up

6.6.2

Slope compensation

The constant frequency, Peak Current-Mode topology has the advantage of very easy loop

compensation with output ceramic caps (reduced cost and size of the application) and fast

transient response. In addition, the intrinsic peak-current measurement simplifies the

current limit protection, avoiding undesired saturation of the inductor.

On the other side, this topology has a drawback: there is an inherent open loop instability

when operating with a duty-ratio greater than 0.5. This phenomenon is known as “Sub-

Harmonic Instability" and can be avoided by adding an external ramp to the one coming

from the sensed current. This compensating technique, based on the additional ramp, is

called "Slope Compensation". In Figure 13, where the switching duty-cycle is higher than

0.5, the small perturbation ÄIL dies away in subsequent cycles thanks to the slope

compensation and the system reverts to a stable situation.

Figure 10.

Poor phase margin (a) and properly damped (b) load transient response

ROW1

ROW2

ROW3

ROW4

ROW5

ROW6

LDO5

BILIM

RILIM

SGND

VIN= 6V

VBST=30÷36V

COMP

+5V

Up to 10 WLEDs per row

LED7706

6.8

μH

4.7

μF

MLCC

50mA

500Hz

RL =

VBST

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