TOP232-234

B

7/01

9

for line sensing by connecting a resistor from this pin to the

rectified DC high voltage bus to implement OV, UV and DC

MAX

reduction with line voltage functions. In this mode, the value

of the resistor determines the line OV/UV thresholds, and the

DC

starting from just above the UV threshold. In high efficiency

applications this pin can be used in the external current limit

mode instead, to reduce the current limit externally to a value

close to the operating peak current, by connecting the pin to the

SOURCE pin through a resistor. The same pin can also be used

as a remote on/off and a synchronization input in both modes.

Please refer to Table 2 for possible combinations of the functions

with example circuits shown in Figure 13 through Figure 23. A

description of specific functions in terms of the MULTI-

FUNCTION pin I/V characteristic is shown in Figure 8. The

horizontal axis represents MULTI-FUNCTION pin current

with positive polarity indicating currents flowing into the pin.

The meaning of the vertical axes varies with functions. For

those that control the on/off states of the output such as UV, OV

and remote ON/OFF, the vertical axis represents the enable/

disable states of the output. UV triggers at I

and OV triggers at I

+225

µA, the output is enabled. For external current limit and

line feed forward with DC

represents the magnitude of the I

Line feed

forward with DC

78% at I

current limit is available only with negative MULTI-FUNCTION

pin current. Please see graphs in the typical performance

characteristics section for the current limit programming range

and the selection of appropriate resistor value.

FREQUENCY (F) Pin Operation

The FREQUENCY pin is a digital input pin available in

TO-220 package only. Shorting the FREQUENCY pin to

SOURCE pin selects the nominal switching frequency of

132 kHz (Figure 10) which is suited for most applications. For

other cases that may benefit from lower switching frequency

such as noise sensitive video applications, a 66 kHz switching

frequency (half frequency) can be selected by shorting the

FREQUENCY pin to the CONTROL pin (Figure 11). In

addition, an example circuit shown in Figure 12 may be used to

lower the switching frequency from 132 kHz in normal operation

to 66 kHz in standby mode for very low standby power

consumption.

MULTI-FUNCTION (M) Pin Operation

When current is fed into the MULTI-FUNCTION pin, it works

as a voltage source of approximately 2.6 V up to a maximum

current of +400

µA (typical). At +400 µA, this pin turns into

a constant current sink. When current is drawn out of the

MULTI-FUNCTION pin, it works as a voltage source of

approximately 1.32 V up to a maximum current of –240

µA

(typical). At –240

µA, it turns into a constant current source.

Refer to Figure 9.

There are a total of five functions available through the use of

the MULTI-FUNCTION pin: OV, UV, line feed forward with

DC

A short circuit between the MULTI-FUNCTION pin and

SOURCE pin disables all five functions and forces

TOPSwitch-FX to operate in a simple three terminal mode like

TOPSwitch-II. The MULTI-FUNCTION pin is typically used

✔

✔✔

✔

✔✔

✔

✔✔

✔

✔✔

✔

✔

✔

✔✔✔

✔

Table 2. Typical MULTI-FUNCTION Pin Configurations.

MULTI-FUNCTION PIN TABLE*

13

14

15

16

17

18

19

20

21

22

23

*This table is only a partial list of many MULTI-FUNCTION pin configurations that are possible.

Using FREQUENCY and MULTI-

FUNCTION Pins

Figure Number

Three Terminal Operation

Under-Voltage

Overvoltage

Line Feed Forward (DC

Line Feed Forward (I

External Current Limit

Remote ON/OFF