datasheetbank_Logo
Integrated circuits, Transistor, Semiconductors Search and Datasheet PDF Download Site

LM2574N-005 View Datasheet(PDF) - ON Semiconductor

Part Name
Description
View to exact match
LM2574N-005 Datasheet PDF : 28 Pages
First Prev 11 12 13 14 15 16 17 18 19 20 Next Last
LM2574
The high input current needed for startup is now partially
supplied by the input capacitor Cin.
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 Cout.
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).
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.
12 to 25 V
Unregulated
Feedback
DC Input
Cin
22 µF
+Vin
(3)
LM2574–12
5 (12)
C1
(14)
1
Output
7
L1
68 µH
/50 V 0.1 µF 3 ON/OFF 4 Pwr 2 Sig
(5)
R1
47 k R2
Gnd Gnd D1
(6)
(4) MBR150
47 k
Cout
680 µF
/16 V
-12 V @ 100 mA
Regulated
Output
Figure 28. Inverting Buck–Boost Regulator with
Delayed Startup
The following formula is used to obtain the peak inductor
current:
ǒ Ǔ Ipeak
[ ILoad
Vin ) |VO|
Vin
)
Vin x ton
2L1
where
ton
+
|VO|
Vin ) |VO|
x
1.0 ,
fosc
and
fosc
=
52
kHz.
Under normal continuous inductor current operating
conditions, the worst case occurs when Vin is minimal.
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 28.
Figure 34 in the “Undervoltage Lockout” section describes
an undervoltage lockout feature for the same converter
topology.
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
shutdown methods, two of them are shown in Figures 29
and 30.
+Vin
Shutdown
5.0 V
Input
Off
0 On
R3
470
+Vin LM2574–XX
5 (12)
Cin
R1
22 µF 47 k
3 ON/OFF 2 Gnds
(5) and Pins
4 (4)
and
(6)
R2
47 k
-Vout
MOC8101
NOTE: This picture does not show the complete circuit.
Figure 29. Inverting Buck–Boost Regulator Shutdown
Circuit Using an Optocoupler
+V
0
+Vin
Off
On
R2
5.6 k
Cin
22 µF
Shutdown
Input
+Vin
5 (12)
LM2574–XX
Q1
2N3906
3 ON/OFF 2 Gnds (4)
(5) and Pins and
4
(6)
R1
12 k
-Vout
NOTE: This picture does not show the complete circuit.
Figure 30. Inverting Buck–Boost Regulator Shutdown
Circuit Using a PNP Transistor
Negative Boost Regulator
This example is a variation of the buck–boost topology
and it is called 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.
http://onsemi.com
19
 

Share Link: 

datasheetbank.com [ Privacy Policy ] [ Request Datasheet ] [ Contact Us ]