Datasheet
Table Of Contents
- Figure 1. Typical application circuit
- 1 Pin settings
- 2 Maximum ratings
- 3 Electrical characteristics
- 4 Functional description
- 5 Application notes - buck conversion
- 5.1 Closing the loop
- 5.2 GCO(s) control to output transfer function
- 5.3 Error amplifier compensation network
- 5.4 LED small signal model
- 5.5 Total loop gain
- 5.6 Compensation network design
- 5.7 Example of system design
- 5.8 Dimming operation
- 5.9 Component selection
- 5.10 Layout considerations
- 5.11 Thermal considerations
- 5.12 Short-circuit protection
- 5.13 Application circuit
- 6 Application notes - alternative topologies
- 7 Package mechanical data
- 8 Ordering information
- 9 Revision history
LED5000 Application notes - alternative topologies
Doc ID 023951 Rev 1 39/51
Equation 61
This is due to the fact that the current flowing through the internal power switch is delivered
to the output only during the OFF phase.
The switch peak current must be lower than the minimum current limit of the overcurrent
protection (see
Section Table 5.: Electrical characteristics
for details) while the average
current must be lower than the rated DC current of the device.
As a consequence, the maximum output current is:
Equation 62
where I
SW MAX
represents the rated current of the device.
The current capability is reduced by the term (1-D
REAL
) and so, for example, with a duty
cycle of 0.5, and considering an average current through the switch of 3 A, the maximum
output current deliverable to the load is 1.5 A.
Figure 29
shows the circuit schematic for an LED current source based on positive buck-
boost topology. The input voltage ranges from 18 to 30 V and it can drive a string composed
of 7 LEDs with 0.7 A DC (V
FW_LED
= 3.75 V so V
OUT
=26.4 V).
The network D5, R4, R
S
implements an inexpensive overvoltage protection. R4 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
D5. In case the load is disconnected or in case of open LED:
Equation 63
R4 must be dimensioned to limit the D5 rated power so it is an inexpensive small signal
Zener diode.
Figure 29. LED current source based on positive BB+ topology
I
SW
I
LOAD
1D
REAL
–
---------------------------=
I
LOAD MAX
I
SW MAX
1D
REAL
–()⋅≅
V
OUT
V
FB
V
ZENER_DIODE
+=
I
ZENER_DIODE
V
FB
R
S
R
4
+
---------------------=
small signal
power plane
L1
XAL6060-22uH
D3STPS3L60U
2 1
D5
BZX384-C30
R4 10k
R3
10K
R1 10k
C9
1nF
TP1
DIM
RS
0.28R
C6 100nF
R5
NM
JP1
C10
NM
C7
NM
TP4INH
C5
15pF
C4
1.5nF
TP5
VLED-
TP6
GND
C1
10uF
C8
2.2uF
50V
D4
BZX384-C16
JP2
TP3
VLED+
R6
100
TP2
VIN
R8
10k
R7
NM
D6
NM
Q1
STN3NF06L
D1STPS3L60U
2
1
U1 LED5000
SW
8
VIN
7
BOOT
1
EP
EP
DIM
2
INH
3
GND
6
COMP
4
FB
5
TP7
GND
D2BZX84-4V7
R2
NM
C2
10uF
C3
100nF
AM13513v1