Short Circuit Condition

When a converter is in current-limit mode, the output voltage will drop as

the output current demand increases. If the output voltage drops too low, the

magnetically coupled voltage used to develop primary side voltages will also

drop, thereby shutting down the PWM controller. Following a time-out period,

the PWM will restart causing the output voltage to begin ramping to their

appropriate value. If the short-circuit condition persists, another shutdown

cycle will be initiated. This on/off cycling is referred to as "hiccup" mode. The

hiccup cycling reduces the average output current, thereby preventing internal

temperatures from rising to excessive levels. The UHP Series is capable of

enduring an indefinite short circuit output condition.

Features and Options

Remote Sense

(=<10 ). Nevertheless, if the sense function is not used for remote regulation

DC converter pins.

UHP Series converters employ a sense feature to provide point of use regula-

tion, thereby overcoming moderate IR drops in pcb conductors or cabling.

The remote sense lines carry very little current and therefore require minimal

cross-sectional-area conductors. The sense lines, which are capacitively

coupled to their respective output lines, are used by the feedback control-loop

to regulate the output. As such, they are not low impedance points and must

be treated with care in layouts and cabling. Sense lines on a pcb should be

run adjacent to dc signals, preferably ground. In cables and discrete wiring

applications, twisted pair or other techniques should be implemented.

UHP Series converters will compensate for drops between the output voltage

at the DC/DC and the sense voltage at the DC/DC provided that:

Output overvoltage protection is monitored at the output voltage pin, not the

in conjunction with trim adjustment of the output voltage can cause the

overvoltage protection circuitry to activate (see Performance Specifications for

overvoltage limits). Power derating is based on maximum output current and

voltage at the converter's output pins. Use of trim and sense functions can

cause output voltages to increase, thereby increasing output power beyond

the conveter's specified rating, or cause output voltages to climb into the

output overvoltage region. Therefore, the designer must ensure:

On/Off Control

The primary-side, Remote On/Off Control function (pin 3) can be specified to

operate with either positive or negative polarity. Positive-polarity devices ("P"

suffix) are enabled when pin 3 is left open or is pulled high. Positive-polarity

devices are disabled when pin 3 is pulled low (0-0.8V with respect to –Input).

Negative-polarity devices are off when pin 3 is high/open and on when pin 3

is pulled low or grounded. See Figure 5.

Dynamic control of the remote on/off function is best accomplished with

a mechanical relay or an open-collector/open-drain drive circuit (optically

isolated if appropriate). The drive circuit should be able to sink appropri-

ate current (see Performance Specifications) when activated and withstand

appropriate voltage when deactivated.

Trimming Output Voltage

UHP converters have a trim capability (pin 7) that allows users to adjust the

output voltage within the specified range. Adjustments to the output voltages

can be accomplished via a trim pot (Figure 6) or a single fixed resistor, shown

in Figures 7 and 8. A single fixed resistor can increase or decrease the output

voltage depending on its connection. The resistor should be located close to

the converter and have a TCR less than 100ppm/°C to minimize sensitivity to

temperature changes. If the trim function is not used, leave the trim pin floating.

A single resistor connected from the Trim (pin 7) to the +Sense (pin 6), will

increase the output voltage (Figure 8). A resistor connected from the Trim (pin

9) to the –Sense (pin 8), will decrease the output voltage (Figure 9).

Trim adjustments greater than the specified range can have an adverse affect

on the converter's performance and are not recommended. Excessive voltage

the output voltage, can cause the overvoltage protection circuitry to activate

(see Performance Specifications for overvoltage limits). Power derating is

based on maximum output current and voltage at the converter's output

3

1

4

REF

+INPUT

EQUIVALENT CIRCUIT FOR

POSITIVE AND NEGATIVE

LOGIC MODELS

CONTROL

–INPUT

ON/OFF

CONTROL

+

LOAD

+OUTPUT

–INPUT

Sense Current

Contact and PCB resistance

losses due to IR drops

Contact and PCB resistance

losses due to IR drops

Sense Return

+INPUT

ON/OFF

CONTROL

TRIM

+SENSE

–OUTPUT

–SENSE

9

8

1

4

7

5

6

3

Figure 4. Remote Sense Circuit Configuration

Figure 5. Driving the On/Off Control Pin

UHP_28.2/12_D48.B02 Page 6 of 9

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