unr-3.3 C&D Technologies., unr-3.3 Datasheet - Page 5

• Manufacturer Part Number: unr-3.3
• Description: 5vin, 1.5-3.3vout Dc/dc Converters
• Manufacturer: C&D Technologies.

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When the output current of a thermally stabilized converter exceeds the
maximum rating (15A models by 20% typ., 30% max.; 20A models by 60%
typ., 73% max.), the internal overcurrent detection circuit automatically shuts
down the converter by discharging the soft-start circuit of the pulse width
modulator (PWM). In this off state, which is similar to that achieved by pulling
the On/Off Control low, the output voltage quickly drops as the output capaci-
tors discharge into the load. Since there is no longer any output current,
the overcurrent detection circuit is released, allowing the soft-start circuit
to recharge and the converter to turn on again. If the faulty load condition
persists, the overcurrent detection circuit will again discharge the soft-start
circuit and shut down the converter. This continuous on/off cycling of the
converter is referred to as “hiccup mode.” Once the overload condition is
removed, the converter remains on, and the output voltage is quickly restored
to its regulated value.
The components used to sense the output current have large temperature
coefficients. Consequently, in a “cold-start” situation, the Overcurrent Detec-
tion Point may temporarily move to 160% to 195% for 15A models and 120%
to 145% for 20A models above the rated current specification. The device
quickly heats up, particularly if an overload situation exists, and restores the
normal Overcurrent Detection Point.
The overcurrent detection circuitry helps keep internal current levels and
operating temperatures within safe operating limits. Nevertheless, sustained
operation at current levels above the rated output current but below the Over-
current Detection Point may result in permanent damage to the converter
(unless active cooling keeps internal temperatures below safe limits).
Output Voltage Trimming
Trimming is accomplished with either a trimpot or a single fixed resistor. The
trimpot should be connected between +Output and Output Return with its
wiper connected to the Trim pin as shown in Figure 3 below.
A trimpot can be used to determine the value of a single fixed resistor
which should be connected, as shown in Figure 4, between the Trim pin and
+Output to trim down the output voltage, or between the Trim pin and Output
Return to trim up the output voltage. Fixed resistors should be metal-film
types with absolute TCR’s less than 100ppm/°C to ensure stability.
The equations and look-up table below can be used as starting points for
selecting specific trim-resistor values. Recall that untrimmed devices are
guaranteed to be ±1% accurate.
INPUT
RETURN
+INPUT
Figure 3. Trim connections Using a Trimpot
+OUTPUT
OUTPUT
RETURN
TRIM
20kΩ
5-10
Turns
LOAD
Temperature Derating
The outstanding electrical efficiency of the UNR 30-66W Series' synchro-
nous-rectifier design and the excellent thermal conductivity of its aluminum
base PCB/Heatsink construction combine to eliminate the need for supple-
mental forced-air cooling in the majority of routine applications. See Derat-
ing Curves.
Note: Install either a fixed trim-up resistor or a fixed trim-down resistor
Note: Resistor values are in kΩ. Accuracy of adjustment is subject to
tolerances of resistors and factory-adjusted, initial output accuracy.
depending upon desired output voltage.
UNr-2.5/15-D5 & UNr-2.5/20-D5 Trim Equations
UNr-3.3/15-D5 & UNr-3.3/20-D5 Trim Equations
Figure 4. Trim connections Using Fixed resistors
UNR Open-Frame Series
+INPUT
INPUT
RETURN
Technical enquiries email: [e-mail], tel:
Single Output, Non-Isolated, 15-20 Amp
UNr-1.5/20-D5 Trim Equations
UNr-1.8/20-D5 Trim Equations
5V
V
O
= desired output voltage.
IN,
+OUTPUT
1.5-3.3V
OUTPUT
RETURN
TRIM
OUT
Trim
Down
Trim Up
DC/DC Converters
UNR30-66W.B01 Page 5 of 8
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