LM2576
Since the switch currents in this buck–boost configuration
are higher than in the standard buck converter topology, the
available output current is lower.
Figure 27. Inverting Buck–Boost Regulator
with Delayed start–up
This type of buck–boost inverting regulator can also
require a larger amount of start–up input current, even for
light loads. This may overload an input power source with a
current limit less than 5.0 A.
Such an amount of input start–up current is needed for at
least 2.0 ms or more. The actual time depends on the output
voltage and size of the output capacitor.
Because of the relatively high start–up currents required
by this inverting regulator topology, the use of a delayed
start–up or an undervoltage lockout circuit is recommended.
Using a delayed start–up arrangement, the input capacitor
can charge up to a higher voltage before the switch–mode
regulator begins to operate.
12 V to 25 V
Unregulated
DC Input
Feedback
+V
L1
68
in
4
LM2576–12
µH
Output
1
C
µF
in
C1
100
2
0.1 µF
/50 V
5
ON/OFF 3 Gnd
C
2200
/16 V
out
D1
1N5822
R1
47 k
µF
R2
47 k
–12 V @ 700 m A
Regulated
Output
The high input current needed for start–up is now partially
supplied by the input capacitor C .
in
It has been already mentioned above, that in some
situations, the delayed start–up or the undervoltage lockout
features could be very useful. A delayed start–up circuit
applied to a buck–boost converter is shown in Figure 27,
Figure 33 in the “Undervoltage Lockout” section describes an
undervoltage lockout feature for the same converter
topology.
Figure 28. Inverting Buck–Boost Regulator Shutdown
Circuit Using an Optocoupler
+V
in
+V
in
LM2576–XX
1
C
R1
F 47 k
in
100
µ
Design Recommendations:
The inverting regulator operates in a different manner than
the buck converter and so a different design procedure has to
5
ON/OFF
3
Gnd
Shutdown
Input
5.0 V
0
Off
be used to select the inductor L1 or the output capacitor C
.
R3
470
out
On
The output capacitor values must be larger than what is
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of µF).
R2
47 k
–V
out
MOC8101
The recommended range of inductor values for the
inverting converter design is between 68 µH and 220 µH. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
NOTE: This picture does not show the complete circuit.
The following formula is used to obtain the peak inductor
current:
With the inverting configuration, the use of the ON/OFF pin
requires some level shifting techniques. This is caused by the
fact, that the ground pin of the converter IC is no longer at
ground. Now, the ON/OFF pin threshold voltage (1.3 V
approximately) has to be related to the negative output
voltage level. There are many different possible shut down
methods, two of them are shown in Figures 28 and 29.
I
(V
|V |)
V
x t
on
Load in
O
in
2L
I
peak
on
V
1
in
|V |
O
1.0
where t
x
, and f
52 kHz.
osc
V
|V |
f
osc
in
O
Under normal continuous inductor current operating
conditions, the worst case occurs when V is minimal.
in
19
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
