APPLICATION NOTES

POWER SUPPLIES

The output voltage of the BBF2800S may be adjusted from

90% to 110% of nominal value by the use of a 50KΩ potenti-

ometer as shown. Adjustment beyond this range is possible

however certain characteristics of the converter such as but

not limited to input voltage range, efficiency, ripple and tem-

perature performance will change. Characterization by the user

is recommended in such applications.

Adjust/comp (pin 4) may be driven by external circuitry refer-

enced to pin 8 (-output sense) if desired. Grounding pin 4

causes output voltage to increase (25% typically) while driving

pin 4 above 1.3V causes output voltage to decrease. Pin 4

may be driven through 10KΩ or more if connection of the comp

function is also required.

The comp function of pin 4 allows transient response and

phase margin to be tailored to suit a specific application. This

feature may be utilized by connecting a small (1-500nF) capaci-

tor between pins 4 and 6 or 7. This is generally recommended

when very large low ESR load capacitances are used.

7

SHUTDOWN PLUS

Pin 2 is used for remote shutdown, output fault detection,

and/or setting the input voltage point at which the converter

will turn on as shown in the typical application diagram. No

connection to pin 2 is necessary for normal operation of the

converter. Pin 2 is referenced to pin 12 (-input).

Shutdown may be implemented by simply connecting pin 2

to an open collector logic output or switch rated at 2.5mA,

25Vdc or higher.

Input voltage turn on point is programmed with a single resis-

tor from pin 2 to 12. An input turn on/off hysteresis (typically

3.5% of Vin) will be observed. This should be considered when

making or verifying set point adjustment. The value of the

setpoint resistor may be determined by the following:

R=

(±10% accuracy at 25°C)

Set point temperature coefficient is typically +400ppm/°C.

Output fault monitoring is accomplished by observing pin 2 with

a high impedance monitoring circuit. Pin 2 voltage drops from

over 10V to below 1V when a load fault causes the converters

fault protection circuitry to activate. It will remain low for at

least 100mS and return high. If the load fault is still present pin

2 will return low and the cycle will repeat. If there is no input

setpoint programming resistor already in place a resistor

>400KΩ from pin 2 to 12 will provide pin 2 pull down.

210·10³

MULTIPLE CONVERTER SYNCHRONIZATION

Synchronized operation of up to three BBF2800S series con-

verters may be accomplished without external components. One

unit selected to be the master is connected normally with pin

10 grounded. The clock output pin 3 will provide the sync sig-

nal for up to two slave units. The slave units have pin 10 not

connected and receive the clock signal into pin 11. The clock

signal lines may be shielded to prevent radiation. A separate 50

ohm coax to each slave unit is recommended in order to pre-

serve signal integrity. Shield ends should be connected to pin

12 of the nearest converter.

BBF2800S converters may also be synchronized to an exter-

nal 500 KHz (+/- 5%) frequency source driving pin 11 of the

converters. Pin 10 is not connected. The converters synchronize

to the positive edge of the frequency source allowing a variety

of wave forms (pulse, square, spike) to be used. Pin 11 is

internally capacitively coupled allowing unipolar or bipolar fre-

quency sources to be used. The source should have a waveform

rise time of 20 ns or less and be capable of driving at least 4 volts

peak into each 50 ohm pin 11 input (50 volts peak max).

An externally synchronized unit may be used to synchronize

other (slave) units. The slave units may be used in turn to

synchronize other slave units from their pin 3 outputs. Each

“generation” of slave units has a delay (100-200ns typical) from

the unit that it is synchronizing to.

8548-34 Rev. H 4/12