ST1CC40 datasheet(20/37 Pages) STMICROELECTRONICS

20 / 37 page

Application information

ST1CC40

20/37

DocID18279 Rev 5

Application information

7.1

Component selection

7.1.1

Sensing resistor

In closed loop operation the ST1CC40 feedback pin voltage is 100 mV so the sensing

resistor calculation is expressed as:

Equation 18

Since the main loop (see Section 6.1) regulates the sensing resistor voltage drop, the

average current is regulated into the LEDs. The integration period is at minimum 5 * TSW

since the system bandwidth can be dimensioned up to FSW/5 at maximum.

The system performs the output current regulation over a period which is at least five times

longer than the switching frequency. The output current regulation neglects the ripple

current contribution and its reliance on external parameters like input voltage and output

voltage variations (line transient and LED forward voltage spread). This performance can

not be achieved with simpler regulation loops like a hysteretic control.

For the same reason the switching frequency is constant over the application conditions,

that helps to tune the EMI filtering and to guarantee the maximum LED current ripple

specifications in the application range. This performance cannot be achieved using constant

on/off-time architecture.

7.1.2

Inductor and output capacitor selection

The output capacitor filters the inductor current ripple that, given the application conditions,

depends on the inductor value. As a consequence, the LED current ripple, that is the main

specification for a switching current source, depends on the inductor and output capacitor

selection.

Figure 12. Equivalent circuit

100 mV

ILED

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ST1CC40 Datasheet(PDF) 20 Page - STMicroelectronics