LM2574DW-3.3 View Datasheet(PDF) - Motorola => Freescale
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Description
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LM2574DW-3.3 Datasheet PDF : 24 Pages
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LM2574
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.
Figure 29. Inverting Buck–Boost Regulator Shutdown
Circuit Using an Optocoupler
+Vin
Shutdown
5.0 V
Input
Off
0 On
R3
470
+Vin
LM2574–XX
5
Cin R1
22 µF 47 k
3 ON/OFF 2 Gnds
and Pins
4
MOC8101
R2
47 k
–Vout
NOTE: This picture does not show the complete circuit.
Figure 30. Inverting Buck–Boost Regulator Shutdown
Circuit Using a PNP Transistor
+V
0
+Vin
Off
On
R2
5.6 k
Shutdown
Input
+Vin
Cin
22 µF
5
LM2574–XX
Q1
2N3906
3 ON/OFF 2 Gnds
and Pins
4
R1
12 k
–Vout
NOTE: This picture does not show the complete circuit.
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.
The circuit in Figure 31 shows the negative boost
configuration. The input voltage in this application ranges
from –5.0 to –12 V and provides a regulated –12 V output. If
the input voltage is greater than –12 V, the output will rise
above –12 V accordingly, but will not damage the regulator.
Figure 31. Negative Boost Regulator
Cin
22 µF
1
+Vin
Feedback
Cout
1000 µF
LM2574–12
5
Output D1
7
4 Pwr 2 Sig 3 ON/OFF
Gnd Gnd
1N5817
Vout = –12 V
Vin
–5.0 to –12 V
L1
330 µH
Load Current
60 mA for Vin = –5.2 V
120 mA for Vin = –7.0 V
Design Recommendations:
The same design rules as for the previous inverting
buck–boost converter can be applied. The output capacitor
Cout must be chosen larger than what would be required for a
standard buck converter. 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 negative boost regulator is the same as for
inverting converter design.
Another important point is that these negative boost
converters cannot provide any current limiting load protection
in the event of a short in the output so some other means,
such as a fuse, may be necessary to provide the load
protection.
Delayed Startup
There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
startup current. In such cases, if the input power source is
limited, this delayed startup feature becomes very useful.
To provide a time delay between the time when the input
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 32 can be used. As the input
voltage is applied, the capacitor C1 charges up, and the
voltage across the resistor R2 falls down. When the voltage
on the ON/OFF pin falls below the threshold value 1.3 V, the
regulator starts up. Resistor R1 is included to limit the
maximum voltage applied to the ON/OFF pin. It reduces the
power supply noise sensitivity, and also limits the capacitor
C1 discharge current, but its use is not mandatory.
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
respectively) ripple voltage exists, a long delay time can
cause some problems by coupling the ripple into the ON/OFF
pin, the regulator could be switched periodically on and off
with the line (or double) frequency.
MOTOROLA ANALOG IC DEVICE DATA
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