LM2576-12 MOTOROLA [Motorola, Inc], LM2576-12 Datasheet - Page 19

• Manufacturer Part Number: LM2576-12
• Description: Easy Switcher 3.0A Step-Down Voltage Regulator
• Manufacturer: MOTOROLA [Motorola, Inc]

are higher than in the standard buck converter topology, the
available output current is lower.
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.
least 2.0 ms or more. The actual time depends on the output
voltage and size of the output capacitor.
by this inverting regulator topology, the use of a delayed
start–up or an undervoltage lockout circuit is recommended.
can charge up to a higher voltage before the switch–mode
regulator begins to operate.
supplied by the input capacitor C in .
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.
Design Recommendations:
the buck converter and so a different design procedure has to
be used to select the inductor L1 or the output capacitor C out .
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 F).
inverting converter design is between 68 H and 220 H. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
current:
conditions, the worst case occurs when V in is minimal.
where t on
MOTOROLA ANALOG IC DEVICE DATA
Since the switch currents in this buck–boost configuration
This type of buck–boost inverting regulator can also
Such an amount of input start–up current is needed for at
Because of the relatively high start–up currents required
Using a delayed start–up arrangement, the input capacitor
The high input current needed for start–up is now partially
It has been already mentioned above, that in some
The inverting regulator operates in a different manner than
The output capacitor values must be larger than what is
The recommended range of inductor values for the
The following formula is used to obtain the peak inductor
Under normal continuous inductor current operating
I
peak
+
[
V
in
I
Load
|V
)
O
|V
|
(V
O
V
in
|
in
x 1.0
)
f osc
|V
O
, and f osc
|)
)
V
in
2L 1
x t on
+
52 kHz.
LM2576
100 F
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.
12 V to 25 V
Unregulated
DC Input
5.0 V
Figure 28. Inverting Buck–Boost Regulator Shutdown
/50 V
With the inverting configuration, the use of the ON/OFF pin
C in
0
On
Figure 27. Inverting Buck–Boost Regulator
0.1 F
NOTE: This picture does not show the complete circuit.
C1
Off
+V in
Shutdown
Input
+V in
Circuit Using an Optocoupler
1
47 k
R1
470
R3
with Delayed start–up
5 ON/OFF
LM2576–12
R2
47 k
C in
100 F
3
MOC8101
Gnd
47 k
R1
Output
4
2
Feedback
+V in
1
D1
1N5822
68 H
5
L1
LM2576–XX
ON/OFF
R2
47 k
–12 V @ 700 m A
Regulated
Output
C out
2200 F
/16 V
3
19
–V out
Gnd