LED5000 datasheet(42/52 Pages) STMICROELECTRONICS

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Application notes - alternative topologies

LED5000

42/51

Doc ID 023951 Rev 1

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

topology. The input voltage ranges from 12 to 36 V and it can drive a string composed of 11

LEDs with 0.7 A DC (VFW_LED = 3.74 V so VOUT=41 V).

Figure 31.

LED current source based on floating boost topology

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

be neglected during the 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:

Equation 72

R1 must be dimensioned to limit the D1 rated power so it is an inexpensive small signal

Zener diode.

The circuitry Q1, R3, R4, R5 implements a level shifter to convert the dimming signal

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

floating. The LED5000 local GND level is:

Equation 73

where VLGND represent the local GND value.

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 VIN-VOUT), the green trace the inductor current

(see

Figure 69) and the purple is the output voltage.

SMB FLAT

100K

BZX384-C47

100nF

RS1

0.28R

TP2

VLED+

TP1

VIN

TP3

VLED-

TP4

DIM

BC807-16

47K

2.2nF

L1 33uH

10K

2.2uF

50V

STPS3L60U

100nF

TP5

GND

10K

LED5000

VIN

BOOT

DIM

INH

GND

COMP

15pF

10K

AM13515v1

OUT

ZENER_DIODE

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

LGND

OUT

–

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