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http://www.union-ic.com Rev.05 Jan.2015

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UM1770

Pin Function

SHDN

___________

: Shutdown, Active Low. This pin is used to put the UM1770 into shutdown. The SHDN

___________

pin cannot be left floating and must be tied to the input pin if not used.

GND: Ground and Heat Sink. Solder to a ground plane or large pad to maximize heat dissipation.

VIN: Power for UM1770 and Load. Power is supplied to the devices through the IN pin. The IN

pin should be locally bypassed to ground if the UM1770 series are more than a few inches away

from another source of bulk capacitance.

VOUT: Voltage Regulated Output. The OUT pin supplies power to the load. A minimum output

capacitor of 1μF is required to ensure stability. Larger output capacitors may be required for

applications with large transient loads to limit peak voltage transients.

NC: Not Connected.

FB: Output voltage feedback. This terminal is used to set the output voltage.

Output Voltage Setting

The output voltage of the UM1770 adjustable regulator is programmed using an external resistor

divider as shown in Figure 2. The output voltage is calculated using:

















2

1

1

R

R

V

V

FB

OUT

Resistors R1 and R2 should be chosen for approximately 3-5μA divider current. Lower value

resistors can be used but offer no inherent advantage and waste more power. Higher values should

be avoided, as leakage currents at FB increase the output voltage error. The recommended design

procedure is to choose R2 = 200kΩ to set the divider current at 5μA and then calculate R1 using:

2

1

1

R

V

V

R

FB

OUT



















Typical Application Circuit

VIN

VOUT

SHDN

GND

1

μF

Chip Enable

1

μF

Figure 1. Fixed Output Voltage Application Circuit

VIN

VOUT

SHDN

GND

FB

1

μF

Chip Enable

1

μF

R1

R2

Figure 2. Adjustable Output Voltage Application Circuit