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TC2575

1.0A Step-Down Switching Regulator

TC2575-1 3/13/00

Design Recommendations

The same design rules as for the previous inverting

buck–boost converter can be applied. The output capacitor

COUT must be chosen larger than what would be required for

a standard buck converter. 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 negative boost regulator is the same as for

inverting converter design.

Another important point is that these negative boost

converters cannot provide any current limiting load protec-

tion in the event of a short in the output so some other

means, such as a fuse, may be necessary to provide the

load protection.

Delayed Startup

There are some applications, like the inverting regulator

already mentioned above, which require a higher amount of

start-up current. In such cases, if the input power source is

limited, this delayed start-up feature becomes very useful.

To provide a time delay between the time when the input

voltage is applied and the time when the output voltage

comes up, the circuit in Figure 13 can be used. As the input

voltage is applied, the capacitor C1 charges up, and the

voltage across the resistor R2 falls down. When the voltage

on the ON/OFF pin falls below the threshold value 1.4 V, the

regulator starts up. Resistor R1 is included to limit the

maximum voltage applied to the ON/OFF pin. It reduces the

power supply noise sensitivity, and also limits the capacitor

C1 discharge current, but its use is not mandatory.

When a high 50Hz or 60Hz (100Hz or 120Hz respec-

tively) ripple voltage exists, a long delay time can cause

some problems by coupling the ripple into the ON/OFF pin,

the regulator could be switched periodically on and off with

the line (or double) frequency.

Negative Boost Regulator

This example is a variation of the buck–boost topology

and it is called negative boost regulator. This regulator

experiences relatively high switch current, especially at low

input voltages. The internal switch current limiting results in

lower output load current capability.

The circuit in Figure 12 shows the negative boost

configuration. The input voltage in this application ranges

from –5.0 to –12V and provides a regulated –12V output. If

the input voltage is greater than –12V, the output will rise

above –12 V accordingly, but will not damage the regulator.

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 (1.4V

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 11. Inverting Buck-Boost Regulator Shutdown Circuit Using

a PNP Transistor

NOTE: This picture does not show the complete circuit.

5.6 k

2N3906

TC2575

GND

ON/OFF

12k

–VOUT

+VIN

Shutdown

Input

Off

+VIN

CIN

100

µF

Figure 12. Negative Boost Regulator

1N5819

Output

Feedback

VOUT = –12V

Load Current from

200mA for VIN = –5.2V

to 500mA for VIN –7.0V

VIN

COUT

1000

µF/16V

CIN

100

µF/

50V

TC2575

(12V)

GND

+VIN

Unregulated

DC Input

–VIN = –5.0V to –12V

ON/OFF

150

µH

Regulated

Output

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