Datel 20A Negative-Trim Units D Suffix, Model Trim Up, Floating Outputs, Remote Sense, Trim Down

2 0 A , S I N G L E O U T P U T D C / D C C O N V E R T E R S USQ Models

Negative-Trim Units ("D" Suffix)

Standard USQ's have a "positive-trim" function, consistent with the industry standard footprints and functionality. DATEL also offers "negative-trim" USQ's designated with a "D" suffix to the part number. The negative-trim devices trim up with a single resistor tied from the Output Trim (pin 6) to the –Sense (pin 5) to increase the output voltage. A resistor connected from the Output Trim (pin 6) to the +Sense (pin 7) will decrease the ouput voltage.

The "negative-trim" formula values for USQ 1.2/1.5/1.8 Volt devices with a

48 Volt input and negative logic reads:

RTRIM = A – Bx ∆V
		∆V
Model	Trim Up		Trim Down
	A	B	A	B
USQ-1.8/20-D48ND	0.57	1	0.2711	1.4676
USQ-1.5/20-D48ND	0.283	0.121	0.065	0.352
USQ-1.2/20-D48ND	0.5928	3.01	0.5686	3.96

where ∆V is the absolute value of the output voltage change desired.

Floating Outputs

Since these are isolated DC/DC converters, their outputs are "floating" with respect to their input. Designers will normally use the –Output (pin 4) as the ground/return of the load circuit. You can, however, use the +Output (pin 8) as ground/return to effectively reverse the output polarity.

Remote Sense

Note: The Sense and VOUT lines are not internally connected to each other. Therefore, if the sense function is not used for remote regulation, the user must connect the +Sense to +VOUT and –Sense to –VOUTat the DC/DC converter pins.

USQ series converters employ a sense feature to provide point-of-use regu- lation, thereby overcoming moderate IR drops in pcb conductors or cabling. The remote sense lines carry very little current and therefore require a mini- mal cross-sectional area conductor. 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.

USQ DC/DC converters will compensate for drops between the output voltage at the DC/DC and the sense voltage at the DC/DC:

[VOUT(+) –VOUT(–)] – [Sense(+) –Sense (–)] ≤ 10% VOUT

			Contact and PCB resistance
				losses due to IR drops
1	–INPUT	+OUTPUT	8
			7	IOUT
		+SENSE
2				Sense Current
	ON/OFF	TRIM	6	LOAD
	CONTROL
			5	Sense Return
		–SENSE
3				IOUT Return
	+INPUT		4
		–OUTPUT
			Contact and PCB resistance
				losses due to IR drops

Figure 19. 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, in conjunction with trim adjustment of the output voltage, can cause the overvoltage protection circuitry to activate (see Performance Specifica- tions 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 USQ’s specified rating, or cause output voltages to climb into the output overvoltage region. Therefore:

(VOUT at pins) ⋅ (IOUT) ≤ rated output power

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