0.5A Step-Down Switching Regulator
TC2574
Another important point is that these negative boost
converters cannot provide any current limiting load protec-
tion 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.
+V
0
Shutdown
Input
Off
On
R2
5.6 k
+V
IN
+V
IN
5
TC2574
Delayed Startup
C
in
22µF
There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
start-up current. In such cases, if the input power source is
limited, this delayed start-up feature becomes very useful.
Toprovideatimedelaybetweenthetimewhentheinput
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 13 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.
Q1
2N3906
2
3
ON/OFF
GNDs
and Pins
4
R1
12 k
–V
OUT
NOTE: This picture does not show the complete circuit.
Figure 11. 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.
The circuit in Figure 12 shows the negative boost
configuration. The input voltage in this application ranges
from –5.0 to –12V and provides a regulated –12V output. If
the input voltage is greater than –12V, the output will rise
above –12 V accordingly, but will not damage the regulator.
When a high 50Hz or 60Hz (100Hz or 120Hz respec-
tively) 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.
+V
IN
+V
IN
TC2574
5
1
C1
0.1µF
C
OUT
1000µF
3
ON/OFF
2
and
4
GNDs
and Pins
Feedback
+V
IN
TC2574
(12V)
C
IN
22mF
5
D1
Output
7
R1
47 k
C
22µF
IN
R2
47 k
1N5817
4
2
3
Pwr
Sig
ON/OFF
GND
GND
V
OUT = –12V
Load Current
L1
330µH
NOTE: This picture does not show the complete circuit.
Figure 13. Delayed Startup Circuitry
60mA for V
120mA for V = –7.0V
IN = –5.2V
V
IN
IN
–5.0 to –12 V
Figure 12. Negative Boost Regulator
Undervoltage Lockout
Design Recommendations:
Some applications require the regulator to remain off
until the input voltage reaches a certain threshold level.
Figure 14 shows an undervoltage lockout circuit applied to
a buck regulator. A version of this circuit for buck–boost
converter is shown in Figure 15. Resistor R3 pulls the
ON/OFF pin high and keeps the regulator off until the input
voltage reaches an predetermined threshold level, which is
determined by the following expression:
The same design rules as for the previous inverting
buck–boost converter can be applied. The output capacitor
COUT must bechosenlargerthanwhat wouldberequiredfor
astandardbuckconverter.Lowinputvoltagesorhighoutput
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.
17
TC2574-1 1/6/00
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