LED5000 datasheet(36/52 Pages) STMICROELECTRONICS

36 / 52 page

Application notes - alternative topologies

LED5000

36/51

Doc ID 023951 Rev 1

Figure 24.

LED current source based on inverting BB topology

The circuitry Q1, R2, R3, R4 implements a level shifter to convert the dimming signal

voltage levels (referred to GND) to the device rails, since the LED5000 local ground is

referred to the negative output voltage (given by the voltage drop across the LEDs and the

sensing resistor).

Figure 25 shows the dimming operation: the light blue trace represents the

DIM pin, the yellow the SW (high level is VIN, low level is -VOUT), the green trace the inductor

current (see

Figure 54) and the purple is the output voltage.

Figure 25.

Inverting BB dimming operation

The network D1, R1, RS implements an inexpensive overvoltage protection. R1 effect can

be neglected during normal operation since the FB biasing current is negligible (tens of nA,

see

Table 5: Electrical characteristics) but it limits the current flowing in the Zener diode D1.

In case the load is disconnected or in case of open LED:

small signal

JUMPER

-Vout

1uF

50V

15pF

TP3

DIM

TP5

VLED+

100K

100nF

50V

RS1

0.2

4.7uF

10uF

50V

10K

100K

TP4

GND

10K

BC807-16

LED5000

VIN

BOOT

DIM

INH

GND

COMP

TP1

VIN

10uF

50V

TP2

VLED-

1.5nF

47K

100nF

AM13508v1

AM13509v1

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