Datasheet

Application notes - alternative topologies LED5000
38/51 Doc ID 023951 Rev 1
6.2 Positive buck-boost
Positive buck-boost fits those applications that require a buck-boost topology (i.e. the input
voltage range crosses the output voltage value) and where the inverting buck-boost is not
suitable because of the main constraints for the final application (refer to
Chapter 6.1 ). As a
consequence the inverting buck-boost is the preferred option because it requires less
components and it has higher efficiency compared to the positive buck-boost topology.
Figure 28. Positive buck-boost
The positive buck-boost implementation (
Figure 28 ) requires one more diode and an
external power switch than inverting buck-boost. The device is not floating, referred to GND,
and it is supplied with the input voltage of the application (the input voltage in inverting buck-
boost topology is instead V
IN
-V
OUT
, refer to Chapter 6.1 for details). LED5000 does not see
the output voltage during the switching activity so V
OUT
can be higher than the maximum
input voltage.
The equations for the positive buck-boost are similar to those seen for the inverting.
Equation 58
where the ideal duty cycle D
IDEAL
for the buck-boost converter is:
Equation 59
However, due to power losses (mainly switching and conduction losses), the real duty cycle
is always higher than this. The real value (which can be measured in the application) should
be used in the following formulas.
The peak current flowing in the embedded switch is:
Equation 60
while its average current level is equal to:
V
IN
V
REF
I
SW
AM13512v1
V
OUT
V
IN
D
IDEAL
1D
IDEAL
–
----------------------------
⋅=
D
IDEAL
V
OUT
V
IN
V
OUT
+
------------------------------=
I
SW
I
LOAD
1D
REAL
–
---------------------------
I
RIPPLE
2
--------------------+
I
LOAD
1D
REAL
–
---------------------------
V
IN
2L⋅
-------------
D
f
SW
---------
⋅+==