TC2574 datasheet(16/25 Pages) TELCOM

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TC2574

TC2574-1 1/6/00

0.5A Step-Down Switching Regulator

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 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

capacitor COUT.

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 68

µH and 220 µH. To

select an inductor with an appropriate current rating, the

inductor peak current has to be calculated.

The following formula is used to obtain the peak inductor

current:

IPEAK ≈

ILOAD (VIN – IVOUTI)

VIN x tON

VIN

2L1

where tON ≈

IVOUTI

1.0

, and fOSC = 52kHz.

VIN + IVOUTI

fOSC

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

(1.3V 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.

Figure 9. Inverting Buck-Boost Regulator with Delayed Startup

MBR150

µH

Output

Feedback

12 to 25V

Unregulated

DC Input

CIN

22mF

/50 V

3ON/OFF

Pwr

GND

+VIN

–12V @ 100mA

Regulated

Output

COUT

680

µF

/16V

TC2574

(12V)

0.1

µF

47k

Sig

GND

Figure 10. Inverting Buck-Boost Regulator Shutdown Circuit

Using an Optocoupler

TC2574–XX

and

GNDs

Pins

ON/OFF

+VIN

47 k

CIN

µF

NOTE: This picture does not show the complete circuit.

47 k

470

Shutdown

Input

MOC8101

–VOUT

Off

5.0 V

+VIN

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