TC2575 Datasheet, PDF (17/25 Pages) TelCom Semiconductor, Inc – 1.0A Step-Down Switching Regulator

1.0A Step-Down Switching Regulator
TC2575
ADDITIONAL APPLICATIONS
Inverting Regulator
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
An inverting buck–boost regulator using the TC2575
(12V) is shown in Figure 8. This circuit converts a positive
input voltage to a negative output voltage with a common
ground by bootstrapping the regulators ground to the
negative output voltage. By grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
In this example the TC2575 (12V) is used to generate a
12V output. The maximum input voltage in this case cannot
exceed 28V because the maximum voltage appearing
across the regulator is the absolute sum of the input and
output voltages and this must be limited to a maximum of
40V.
This circuit configuration is able to deliver approximately
0.35A to the output when the input voltage is 12V or higher.
At lighter loads the minimum input voltage required drops to
approximately 4.7V, because the buck–boost regulator to-
pology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
Since the switch currents in this buck–boost configura-
tion are higher than in the standard buck converter topology,
the available output current is lower.
This type of buck–boost inverting regulator can also
require a larger amount of startup input current, even for light
loads. This may overload an input power source with a
current limit less than 1.5A.
Such an amount of input start-up current is needed for
at least 2.0msec or more. The actual time depends on the
output voltage and size of the output capacitor.
Because of the relatively high startup currents required
by this inverting regulator topology, the use of a delayed
startup or an undervoltage lockout circuit is recommended.
Using a delayed startup arrangement, the input capaci-
tor 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 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 68µH and 220 µH. To
The following formula is used to obtain the peak inductor
current:
I_PEAK ≈
I_LOAD (V_IN – IV_OUTI) ₊ V_IN x t_ON
V_IN
2L₁
where t_ON ≈
IV_OUTI
V_{IN +} IV_OUTI
x
1.0
f_OSC
,
and
f_OSC
=
52kHz.
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 40V.
12 to 25V
Unregulated
DC Input
C_IN
100µF
/50V
C1
0.1µF
Feedback
+VIN
1
5
R1
47k
TC2575
(12V)
ON/OFF 3 GND
R2
47k
4
L1
Output ^100µH
2
D1
1N5819
COUT
1800µF/16V
–12V @ 0.35A
Regulated
Output
Figure 9. Inverting Buck-Boost Regulator with Delayed Startup
It has been already mentioned above, that in some
situations, the delayed startup or the undervoltge lockout
features could be very useful. A delayed startup circuit
applied to a buck-boost converter is shown in Figure 9.
Figure 15 in the "Undervoltage Lockout" section describes
an undervoltage lockout feature for the same converter
topology.
+VIN
Shutdown
5.0V
Input
Off
0 _On
R3
470
+VIN
1
TC2575
CIN R1
100µF 47k
5 ON/OFF 3 GND
R2
47k
–VOUT
MOC8101
NOTE: This picture does not show the complete circuit.
Figure 10. Inverting Buck-Boost Regulator Shutdown
Circuit Using an Optocoupler
TC2575-1 3/13/00
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