LED5000 datasheet(23/52 Pages) STMICROELECTRONICS

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LED5000

Application notes - buck conversion

Doc ID 023951 Rev 1

23/51

the external power components and the compensation network are selected, a direct

measurement to determine TRISE, TFALL (see Equation 24) is necessary to certify the

achieved dimming performance.

5.9

Component selection

5.9.1

Sensing resistor

In closed loop operation the LED5000 feedback pin voltage is 200 mV, so the sensing

resistor calculation is expressed as:

Equation 26

Since the main loop (see

Chapter 5.1) 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.

A system loop based on a peak current mode architecture features consistent advantages in

comparison with simpler closed loop regulation schemes like the hysteretic or the constant

ON/OFF control.

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 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

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

ON/OFF time architectures.

5.9.2

Inductor and output capacitor selection

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

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 16.

Equivalent circuit

200 mV

LED

---------------------

DCR

COUT

VIN

ESR

Rd1

VIN

ESR

Dled1

COUT

Rdn

Dledn

AM13500v1

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