TC2575 datasheet(17/25 Pages) TELCOM

17 / 25 page

TC2575

TC2575-1 3/13/00

1.0A Step-Down Switching Regulator

ADDITIONAL APPLICATIONS

Inverting Regulator

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 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.

Note that the voltage appearing across the regulator is

the absolute sum of the input and output voltage, and must

not exceed 40V.

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.

Figure 9. Inverting Buck-Boost Regulator with Delayed Startup

1N5819

100

µH

Output

GND

Feedback

12 to 25V

Unregulated

DC Input

0.1

µF

47k

ON/OFF

+VIN

–12V @ 0.35A

Regulated

Output

TC2575

(12V)

COUT

1800

µF/16V

CIN

100

µF

/50V

TC2575

GND

ON/OFF

+VIN

47k

CIN

100

µF

NOTE: This picture does not show the complete circuit.

47 k

470

Shutdown

Input

MOC8101

–VOUT

Off

5.0V

+VIN

Figure 10. Inverting Buck-Boost Regulator Shutdown

Circuit Using an Optocoupler

TC2575 数据表(PDF) 17 Page - TelCom Semiconductor, Inc

TC2575 数据表(HTML) 17 Page - TelCom Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

数据表下载

链接网址

TC2575 数据表(PDF) 17 Page - TelCom Semiconductor, Inc

TC2575 数据表(HTML) 17 Page - TelCom Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

数据表 下载

链接网址

数据表下载