Datasheet

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 V
IN
, low level is -V
OUT
), 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
C5
1uF
50V
C7
15pF
TP3
DIM
TP5
VLED+
L1XAL6060-15uH
R1 100K
D2
STPS3L60U
C9
NM
C4
100nF
50V
RS1
0.2
C8
4.7uF
C3
10uF
50V
R3
10K
R2
100K
TP4
GND
T1
13
2
D1
BZX384-C39
R5
10K
Q1
BC807-16
U1 LED5000
SW
8
VIN
7
BOOT
1
EP
EP
DIM
2
INH
3
GND
6
COMP
4
FB
5
TP1
VIN
C2
10uF
50V
TP2
VLED-
C6
1.5nF
R4
47K
C1 100nF
AM13508v1
AM1
3509v1