lm2575-5 ON Semiconductor, lm2575-5 Datasheet - Page 20

• Manufacturer Part Number: lm2575-5
• Description: 1.0 A, Adjustable Output Voltage, Step-down Switching Regulator
• Manufacturer: ON Semiconductor

can charge up to a higher voltage before the switch−mode
regulator begins to operate.
supplied by the input capacitor C
Design Recommendations:
than the buck converter and so a different design procedure
has to be used to select the inductor L1 or the output
capacitor C
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 mF).
inverting converter design is between 68 mH and 220 mH. 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
absolute sum of the input and output voltage, and must not
exceed 40 V.
Unregulated
DC Input
12 V to 25 V
situations, the delayed startup or the undervoltage lockout
features could be very useful. A delayed startup circuit
applied to a buck−boost converter is shown in Figure 26.
Figure 32 in the “Undervoltage Lockout” section describes
an undervoltage lockout feature for the same converter
topology.
where t on +
100 mF
/50 V
Using a delayed startup arrangement, the input capacitor
The high input current needed for startup is now partially
The inverting regulator operates in a different manner
The output capacitor values must be larger than 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
Note that the voltage appearing across the regulator is the
It has been already mentioned above, that in some
C
in
0.1 mF
I
peak
C1
out
Figure 26. Inverting Buck−Boost
+V
Regulator with Delayed Startup
.
[
V
in
1
47 k
R1
in
I
Load
|V
5 ON/OFF
) |V
LM2575−12
O
R2
47 k
|
(V
O
V
in
|
in
x
3
) |V
f osc
1
GND
in
4
Output
2
Feedback
.
O
, and f osc + 52 kHz.
|)
)
D1
1N5819
100 mH
in
V
L1
is minimal.
in
2L 1
x t on
-12 V @ 0.35 A
Regulated
Output
C
1800 mF
/16 V
http://onsemi.com
out
20
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
(1.4 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 27 and 28.
Negative Boost Regulator
and is called a negative boost regulator. This regulator
experiences relatively high switch current, especially at low
input voltages. The internal switch current limiting results in
lower output load current capability.
configuration. The input voltage in this application ranges
from −5.0 V to −12 V and provides a regulated −12 V output.
5.0 V
Figure 27. Inverting Buck−Boost Regulator Shut Down
Figure 28. Inverting Buck−Boost Regulator Shut Down
With the inverting configuration, the use of the ON/OFF
This example is a variation of the buck−boost topology
The circuit in Figure 29 shows the negative boost
0
NOTE: This picture does not show the complete circuit.
NOTE: This picture does not show the complete circuit.
On
+V
in
Off
+V
0
Shutdown
Input
Circuit Using a PNP Transistor
+V
Circuit Using an Optocoupler
On
C
100 mF
in
in
470
R3
Off
5.6 k
R2
Q1
2N3906
C
100 mF
Shutdown
Input
in
+V
MOC8101
1
in
47 k
R1
5
LM2575−XX
+V
ON/OFF
R1
12 k
in
1
5
LM2575−XX
3
ON/OFF
R2
47 k
GND
-V
out
3
GND
-V
out