HPH Series

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MDC_HPH_A12 Page 7 of 12

the PWM will restart, causing the output voltage to begin ramping up to its ap-

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

will initiate. This on/off cycling is called “hiccup mode”. The hiccup cycling

reduces the average output current, thereby preventing excessive internal

temperatures. A short circuit can be tolerated indefinitely.

Remote Sense Input

Sense inputs compensate for output voltage inaccuracy delivered at the load.

This is done by correcting voltage drops along the output wiring such as mod-

erate IR drops and the current carrying capacity of PC board etch. Sense inputs

also improve the stability of the converter and load system by optimizing the

control loop phase margin.

Note: The Sense input and power Vout lines are internally connected through

low value resistors to their respective polarities so that the converter can

operate without external connection to the Sense. Nevertheless, if the Sense

function is not used for remote regulation, the user should connect +Sense to

+Vout and –Sense to –Vout at the converter pins.

The remote Sense lines carry very little current. They are also capacitively

coupled to the output lines and therefore are in the feedback control loop to

regulate and stabilize the output. As such, they are not low impedance inputs

and must be treated with care in PC board layouts. Sense lines on the PCB

should run adjacent to DC signals, preferably Ground. In cables and discrete

wiring, use twisted pair, shielded tubing or similar techniques

Please observe Sense inputs tolerance to avoid improper operation:

[Vout(+) –Vout(-)] – [ Sense(+) – Sense(-)] ≤ 10% of Vout

a single fixed resistor connected between the Trim input and either the +Sense

or –Sense terminals. (On some converters, an external user-supplied precision

DC voltage may also be used for trimming). Trimming resistors should have a

low temperature coefficient (±100 ppm/deg.C or less) and be mounted close

to the converter. Keep leads short. If the trim function is not used, leave the

trim unconnected. With no trim, the converter will exhibit its specified output

voltage accuracy.

There are two CAUTION’s to be aware for the Trim input:

CAUTION: To avoid unplanned power down cycles, do not exceed EITHER the

maximum output voltage OR the maximum output power when setting the trim.

Be particularly careful with a trimpot. If the output voltage is excessive, the

OVP circuit may inadvertantly shut down the converter. If the maximum power

is exceeded, the converter may enter current limiting. If the power is exceeded

for an extended period, the converter may overheat and encounter overtem-

perature shut down.

CAUTION: Be careful of external electrical noise. The Trim input is a senstive

input to the converter’s feedback control loop. Excessive electrical noise may

cause instability or oscillation. Keep external connections short to the Trim

input. Use shielding if needed. Also consider adding a small value ceramic

capacitor between the Trim and –Vout to bypass RF and electrical noise.

Figure 4 – Remote Sense Circuit Configuration

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

Sense pin. Therefore excessive voltage differences between Vout and Sense

together with trim adjustment of the output can cause the overvoltage protec-

tion circuit to activate and shut down the output.

Power derating of the converter is based on the combination of maximum out-

put current and the highest output voltage. Therefore the designer must insure:

(Vout at pins) x (Iout) ≤ (Max. rated output power)

Trimming the Output Voltage

The Trim input to the converter allows the user to adjust the output voltage

over the rated trim range (please refer to the Specifications). In the trim equa-

tions and circuit diagrams that follow, trim adjustments use either a trimpot or

LOAD

5

8

7

6

9

Contact and PCB resistance

losses due to IR drops

Contact and PCB resistance

losses due to IR drops

+OUTPUT

+SENSE

TRIM

-SENSE

-OUTPUT

-INPUT

ON/OFF

CONTROL

+INPUT

1

3

4

Sense Current

Sense Return

Figure 5 – Trim adjustments using a trimpot

LOAD

5

8

7

6

5-22

TURNS

1

3

4

9

+OUTPUT

+SENSE

TRIM

-SENSE

-OUTPUT

-INPUT

ON/OFF

CONTROL

+INPUT

Figure 6 – Trim adjustments to Increase Output Voltage using a Fixed Resistor

LOAD

5

8

7

6

1

3

4

9

+OUTPUT

+SENSE

TRIM

-SENSE

-OUTPUT

-INPUT

ON/OFF

CONTROL

+INPUT