PM6600TR datasheet(29/43 Pages) STMICROELECTRONICS

29 / 43 page

PM6600

Operation description

29/43

Figure 48.

Load transient response measurement set-up

7.4.2

Slope compensation

The Constant Frequency, Peak Current-Mode topology has the advantage of very easy loop

compensation with output ceramic capacitors (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 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 11, where the switching duty-cycle is higher than 0.5, the

small perturbation

∆IL dies away in subsequent cycles thanks to the slope compensation

and the system reverts to a stable situation.

Figure 49.

Main loop and current loop diagram

ROW1

ROW2

ROW3

ROW4

ROW5

ROW6

LDO5

BILIM

RILIM

SGND

VIN= 6V

VBST=30÷36V

COMP

+5V

Up to 10 WLEDs per row

PM6600

6.8

μH

4.7

μF

MLCC

50mA

500Hz

RL =

VBST

VIN

0.4V

PWM

Minimum voltage drop

selector

COMP

ROWx

SGND

RILIM

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