LED6000PHTR datasheet(42/49 Pages) STMICROELECTRONICS

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Application notes - alternative topologies

LED6000

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DocID027777 Rev 2

The switch peak current must be lower than the minimum current limit of the overcurrent

protection and the average current must be lower than the rated DC current of the device.

In addition to these constraints, the thermal considerations summarized in Section 5.4 on

page 30 must also be evaluated.

Figure 31. Floating-boost reference schematic

Figure 31 shows the schematic circuit for an LED current source based on floating-boost

topology. The expected input voltage range is from 18 V to 36 V and it can drive a string

composed of 15 LEDs with 0.5 A DC.

This schematic also includes two additional circuitry:



A level shifter network for the proper dimming operation, since the LED6000 local

ground is different from the board GND. This function is implemented by Q1, R15, R16

and R17. However, due to limited control loop bandwidth, the dimming operation in

floating-boost topology is quite limited in terms of the minimum achievable DIM pulse.



An external overvoltage protection which avoids the IC damage in case of the LED

open row fault. This function is implemented by a Zener diode, D2, R7 and the sensing

resistor, RSNS = R13.

In case the LED string is disconnected the maximum output voltage is given by:

Equation 38

Equation 38 must be verified in order avoid the LED6000 to be exposed to electrical stress

outside the allowed range (see Table 2 on page 7 for details).

The series resistor, R7, must be selected in order to limit the maximum current flowing

through the D2, as shown in Equation 32 on page 37.

The compensation network design strategy for floating-boost topology is described in

Section 6.4.

MAX

OUT





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