TOP250 datasheet(10/54 Pages) POWERINT

10 / 54 page

Rev. R 10/20

TOP242-250

www.power.com

Table 2. Typical LINE-SENSE and EXTERNAL CURRENT LIMIT Pin Configurations.

resumes providing automatic recovery. A large hysteresis of

°C (typical) is provided to prevent overheating of the PC

board due to a continuous fault condition. V

C is regulated

in hysteretic mode and a 4.8 V to 5.8 V (typical) sawtooth

waveform is present on the CONTROL pin while in thermal

shutdown.

Bandgap Reference

All critical TOPSwitch-GX internal voltages are derived

from a temperature-compensated bandgap reference.

This reference is also used to generate a temperature-

compensated current reference, which is trimmed to

accurately set the switching frequency, MOSFET gate

drive current, current limit, and the line OV/UV thresholds.

TOPSwitch-GX has improved circuitry to maintain all of the

above critical parameters within very tight absolute and

temperature tolerances.

High-Voltage Bias Current Source

This current source biases TOPSwitch-GX from the DRAIN

pin and charges the CONTROL pin external capacitance

during start-up or hysteretic operation. Hysteretic operation

occurs during auto-restart, remote OFF and over-temp-

erature shutdown. In this mode of operation, the current

source is switched on and off with an effective duty cycle of

approximately 35%. This duty cycle is determined by the

ratio of CONTROL pin charge (I

C) and discharge currents

CD1 and ICD2). This current source is turned off during normal

operation when the output MOSFET is switching. The effect

of the current source switching will be seen on the DRAIN

voltage waveform as small disturbances and is normal.

Using Feature Pins

FREQUENCY (F) Pin Operation

The FREQUENCY pin is a digital input pin available in the

Y, R or F package only. Shorting the FREQUENCY pin to

SOURCE pin selects the nominal switching frequency of

132 kHz (Figure 13), 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 14). In addition, an example circuit shown in

Figure 15 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.

LINE-SENSE (L) Pin Operation (Y, R and F Packages)

When current is fed into the LINE-SENSE 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. Refer to Figure 12a. In addition, a

comparator with a threshold of 1 V is connected at the

pin and is used to detect when the pin is shorted to the

SOURCE pin.

There are a total of four functions available through the

use of the LINE-SENSE pin: OV, UV, line feed-forward

with DC

MAX reduction, and remote ON/OFF. Connecting

the LINE-SENSE pin to the SOURCE pin disables all

four functions. The LINE-SENSE pin is typically used for

line sensing by connecting a resistor from this pin to the

rectified DC high voltage bus to implement OV, UV and

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

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

the DC

MAX is reduced linearly with rectified DC high voltage

starting from just above the UV threshold. The pin can

also be used as a remote ON/OFF and a synchronization

input. Refer to Table 2 for possible combinations of the

functions with example circuits shown in Figure 16 through

Figure 40. A description of specific functions in terms of

the LINE-SENSE pin I/V characteristic is shown in Figure

11 (right hand side). The horizontal axis represents LINE-

SENSE pin current with positive polarity indicating currents

LINE-SENSE AND EXTERNAL CURRENT LIMIT PIN TABLE*

Figure Number

Three Terminal Operation

Undervoltage

Overvoltage

Line Feed-Forward (DC

MAX)

Overload Power Limiting

External Current Limit

Remote ON/OFF

*This table is only a partial list of many LINE-SENSE and EXTERNAL CURRENT LIMIT pin configurations that are possible.

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