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
Application notes - buck conversion LED5000
30/51 Doc ID 023951 Rev 1
Equation 45
where DCR
L
is the series resistance of the inductor and V
FWDIODE
is the forward voltage
drop across the external rectifying diode.
The pulse-by-pulse current limitation is effective to implement constant current protection
when:
Equation 46
From
Equation 44
and
Equation 45
we can gather that the implementation of the constant
current protection becomes more critical the lower is the V
OUT
and the higher is V
IN
.
In fact, in short-circuit condition the voltage applied to the inductor during the OFF time
becomes equal to the voltage drop across parasitic components (typically the DCR of the
inductor and the forward voltage of the diode) since V
OUT
is negligible, while during T
ON
the
voltage applied the inductor is maximized and it is approximately equal to V
IN
.
In general the worst case scenario is heavy short-circuit at the output with maximum input
voltage. The
Equation 44
and
Equation 45
in overcurrent conditions can be simplified to:
Equation 47
considering T
ON
that has been already reduced to its minimum.
Equation 48
where T
SW
=1/f
SW
considering the nominal f
SW
.
At high input voltage
ΔI
L TON
could be higher than ΔI
L TOFF
and so the inductor current could
escalate. As a consequence, the system typically meets the
Equation 46
at a current level
higher than the nominal value thanks to the increased voltage drop across stray
components. In most application conditions the pulse-by-pulse current limitation is effective
to limit the inductor current. Whenever the current escalates, a second level current
protection called “hiccup mode” is enabled. The hiccup protection offers an additional
protection against heavy short-circuit condition at very high input voltage even considering
the spread of the minimum conduction time of the power element. In case the hiccup current
level (6.2 A typical) is triggered the switching activity is prevented for 16 msec typ. (see
hiccup time in
Table 5: Electrical characteristics
).
Figure 19
shows the operation of the constant current protection when a short-circuit is
applied at the output at the maximum input voltage.
I
L TON
Δ
V
OUT
DCR
L
IV
FW DIODE
+⋅+()–
L
----------------------------------------------------------------------------------------
T
OFF
()=
I
L TON
Δ I
L TOFF
Δ=
I
L TON
Δ
V
IN
DCR
L
R
DSON HS
+()I⋅–
L
---------------------------------------------------------------------------
T
ON MIN
()
V
IN
L
-------- -
90ns()≅=
I
L TOFF
Δ
DCR
L
I⋅ V
FW DIODE
+()–
L
-------------------------------------------------------------------
T
SW
90ns–()
DCR
L
I⋅ V
FW DIODE
+()–
L
-------------------------------------------------------------------
1.18μs()≅=