0.5A Step-Down Switching Regulator
TC2574
ADDITIONAL APPLICATIONS
Inverting Regulator
12 to 25V
Unregulated
DC Input
Feedback
1
+V
IN
L1
TC2574
(12V)
µH
68
Output
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.
C
5
IN
C1
F
22
/50 V
m
7
0.1µF
3
ON/OFF 4 Pwr
GND
2
Sig
GND
C
680
OUT
D1
MBR150
R1
47k
µF
R2
47k
/16V
–12V @ 100mA
In this example the TC2574 (12V) is used to generate a
–12Voutput.Themaximuminputvoltageinthiscasecannot
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.
Regulated
Output
Figure 9. Inverting Buck-Boost Regulator with Delayed Startup
Thefollowingformulaisusedtoobtainthepeakinductor
current:
ILOAD (VIN – IVOUTI) VIN x tON
Thiscircuitconfigurationisabletodeliverapproximately
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-
tionarehigherthaninthestandardbuckconvertertopology,
the available output current is lower.
This type of buck–boost inverting regulator can also
requirealargeramountofstartupinputcurrent,evenforlight
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.
IPEAK
≈
+
VIN
2L1
IVOUTI
IN + IVOUTI
1.0
fOSC
where tON
≈
, and fOSC = 52kHz.
V
Under normal continuous inductor current operating
conditions, the worst case occurs when VIN 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
(1.3Vapproximately)hastoberelatedtothenegativeoutput
voltage level. There are many different possible shutdown
methods, two of them are shown in Figures 10 and 11.
Thehighinputcurrentneededforstartupisnowpartially
supplied by the input capacitor CIN.
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
+V
IN
+V
IN
TC2574–XX
5
C
R1
47
IN
22µF
k
capacitor COUT
.
3
ON/OFF
2
GNDs
Shutdown
Input
and Pins
4
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.
5.0 V
0
Off
R3
470
On
R2
47 k
–V
OUT
MOC8101
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
16
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
