LM2575D2T-005 Datasheet, PDF (20/28 Pages) ON Semiconductor – 1.0 A, Adjustable Output Voltage, Step-Down

LM2575, NCV2575
Using a delayed startup arrangement, the input capacitor
can charge up to a higher voltage before the switch−mode
regulator begins to operate.
The high input current needed for startup is now partially
supplied by the input capacitor C_in.
Design Recommendations:
The inverting regulator operates in a different manner
than the buck converter and so a different design procedure
has to be used to select the inductor L1 or the output
capacitor C_out.
The output capacitor values must be larger than 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 mF).
The recommended range of inductor values for the
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.
The following formula is used to obtain the peak inductor
current:
Ipeak
[
ILoad (Vin )
Vin
|VO|)
)
Vin x ton
2L1
where ton
+
|VO|
Vin ) |VO|
x
1
fosc
,
and
fosc
+
52 kHz.
Under normal continuous inductor current operating
conditions, the worst case occurs when V_in is minimal.
Note that the voltage appearing across the regulator is the
absolute sum of the input and output voltage, and must not
exceed 40 V.
Unregulated
DC Input
12 V to 25 V +V_in
C_in
100 mF
1
C1
/50 V ^{0.1 mF}
LM2575−12
Feedback
4
Output
L1
100 mH
2
5 ON/OFF 3 GND
R1
47 k _R2
47 k
D1
1N5819
C_out
1800 mF
/16 V
Regulated
Output
-12 V @ 0.35 A
Figure 26. Inverting Buck−Boost
Regulator with Delayed Startup
It has been already mentioned above, that in some
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.
+V_in
Shutdown
5.0 V
Input
Off
0 _On
R3
470
+V_in
LM2575−XX
1
C_in
R1
100 mF 47 k
5 ON/OFF 3 _GND
MOC8101
R2
47 k
-V_out
NOTE: This picture does not show the complete circuit.
Figure 27. Inverting Buck−Boost Regulator Shut Down
Circuit Using an Optocoupler
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
(1.4 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.
+V
0
+V_in
Off
On
R2
5.6 k
Shutdown
Input
+V_in
C_in
100 mF
1
LM2575−XX
Q1
2N3906 5 ON/OFF 3 GND
R1
12 k
-V_out
NOTE: This picture does not show the complete circuit.
Figure 28. Inverting Buck−Boost Regulator Shut Down
Circuit Using a PNP Transistor
Negative Boost Regulator
This example is a variation of the buck−boost topology
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.
The circuit in Figure 29 shows the negative boost
configuration. The input voltage in this application ranges
from −5.0 V to −12 V and provides a regulated −12 V output.
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