Datasheet

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 (V
FW_LED
= 3.74 V so V
OUT
=41 V).
Figure 31. LED current source based on floating boost topology
The network D1, R1, R
S
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 V
IN
, low level is V
IN
-V
OUT
), the green trace the inductor current
(see
Figure 69 ) and the purple is the output voltage.
SMB FLAT
R3
100K
T1
13
2
D1 BZX384-C47
C1
100nF
RS1
0.28R
TP2
VLED+
TP1
VIN
TP3
VLED-
TP4
DIM
Q1
BC807-16
R5
47K
C4
2.2nF
L1 33uH
R4
10K
C3
2.2uF
50V
D2
STPS3L60U
C5
100nF
C6
NM
TP5
GND
R2
10K
U1 LED5000
SW
8
VIN
7
BOOT
1
EP
EP
DIM
2
INH
3
GND
6
COMP
4
FB
5
C2
15pF
R1 10K
AM13515v1
V
OUT
V
FB
V
ZENER_DIODE
+=
I
ZENER_DIODE
V
FB
R
S
R
1
+
---------------------=
V
LGND
V
IN
V
OUT
–=