TC2574 Datasheet, PDF (16/25 Pages) TelCom Semiconductor, Inc – 0.5A Step-Down Switching Regulator

0.5A Step-Down Switching Regulator
TC2574
ADDITIONAL APPLICATIONS
Inverting Regulator
An inverting buck–boost regulator using the TC2574
(12V) 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 nega-
tive output voltage. By grounding the feedback pin, the
regulator senses the inverted output voltage and regulates
it.
In this example the TC2574 (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
0.1 A to the output when the input voltage is 8.0 V 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 0.6A.
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.
While using a delayed startup arrangement, the input
capacitor 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:
12 to 25V
Unregulated
DC Input +VIN
CIN
5
22mF
C1
/50 V ^0.1µF
3
R1
47k
TC2574
(12V)
ON/OFF 4 Pwr 2
GND
R2
47k
Feedback
1
Output
L1
68µH
7
Sig
GND _D1
MBR150
COUT
680 µF
/16V
–12V @ 100mA
Regulated
Output
Figure 9. Inverting Buck-Boost Regulator with Delayed Startup
The following formula is used to obtain the peak inductor
current:
I_PEAK
≈
I_LOAD
(V_IN –
V_IN
IV_OUTI)
+
V_IN x t_ON
2L₁
where t_ON ≈
IV_OUTI
V_{IN +} IV_OUTI
1.0
f_OSC
, and f_OSC = 52kHz.
Under normal continuous inductor current operating
conditions, the worst case occurs when V_IN 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 9.
Figure 15 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
(1.3V 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.
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
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
+VIN
Shutdown
5.0 V
Input
Off
0 _On
R3
470
+VIN
5 ^{TC2574–XX}
CIN R1
22µF 47k
3 ON/OFF 2 GNDs
and Pins
4
MOC8101
R2
47 k
–VOUT
NOTE: This picture does not show the complete circuit.
Figure 10. Inverting Buck-Boost Regulator Shutdown Circuit
Using an Optocoupler
TC2574-1 1/6/00
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