Device description

ALED6001

14/26

DocID026965 Rev 2

Figure 5. Simplified output regulation circuitry

The boost controller regulates the output (LED) current by measuring the voltage across the

external sensing resistor. The internal circuitry related to the two pins connected to the

sensing resistor (VFBP and VFBN) has been designed to implement a high-side sensing

scheme and can sustain a relatively high voltage. The voltage drop across the sensing

resistor is the actual feedback voltage for the boost regulator control loop and it can be

linearly varied by means of the ADIM pin (see Section 7.3: LED current regulation and

brightness control on page 17 for details).

The COMP pin is the output of the transconductance amplifier involved in the regulation

loop and a simple RC series must be connected between this pin and SGND to ensure

proper loop stability.

7.2.3

Boost converter stability and slope compensation

As visible in Figure 5, the difference between the feedback voltage and the programmed is

converted into an error current by the transconductance amplifier. This current, provided at

the COMP pin, is turned into a voltage across the compensation network externally

connected to the same pin. This voltage, in turn, determines the trip current for the following

error amplifier.

When the boost converter operates in continuous conduction mode (CCM) and the

switching duty cycle is higher than 50%, sub-harmonic instability may occur.

In order to prevent this, the trip current has to be properly shaped by summing a negative

sawtooth ramp voltage (slope compensation) with the amplified error voltage.