TC2575 Datasheet, PDF (18/25 Pages) TelCom Semiconductor, Inc – 1.0A Step-Down Switching Regulator

1.0A Step-Down Switching Regulator
TC2575
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 (1.4V
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 10
and 11.
+V
0
+VIN
Off
On
R2
5.6 k
Shutdown
Input
+VIN
CIN
100µF
1
TC2575
Q1
2N3906 5 ON/OFF 3 GND
R1
12k
–VOUT
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.
CIN
100µF/
50V
+VIN
1
3
TC2575
(12V)
GND
5
4
Feedback
Output ^D1
2
ON/OFF
1N5819
COUT
1000µF/16V
Regulated
Output
VOUT _{= –12V}
VIN
Unregulated
DC Input
–VIN = –5.0V to –12V
L1
150µH
Load Current from
200mA for VIN = –5.2V
to 500mA for VIN –7.0V
Figure 12. Negative Boost Regulator
Design Recommendations
The same design rules as for the previous inverting
buck–boost converter can be applied. The output capacitor
C_OUT 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 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.
Delayed Startup
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.
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 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 1.4 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 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.
TC2575-1 3/13/00
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