LD1985

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

7 of 10

www.unisonic.com.tw

QW-R102-021,C

APPLICATION INFORMATION

EXTERNAL CAPACITORS

An external capacitor for regulator stability is required. This capacitor must be selected to meet the requirements

of minimum capacitance and equivalent series resistance. It is recommended to solder input and output capacitors

as close as possible to the relative pins.

INPUT CAPACITOR

A 1µF input capacitor (ceramic, tantalum or film capacitors can be used) is required with the UTC LD1985 (amount

of capacitance can be increased without limit). This capacitor must be located a distance less than 0.5" from the

input pin of the device and returned to a clean analog ground.

OUTPUT CAPACITOR

The output capacitor must meet both the requirement for minimum amount of capacitance and E.S.R. (equivalent

series resistance) value. The output capacitor must maintain its ESR in the stable region over the full operating

temperature to assure stability. Also, capacitor tolerance and variation with temperature must be considered to

assure the minimum amount of capacitance is provided over time. Due to the different loop gain and the stability

improves for higher output versions, if low E.S.R. ceramic type is used, the recommended minimum output capacitor

other versions. However, if an output capacitor lower than the suggestion, it's possible to make stable the regulator

adding a resistor in series to the capacitor. This capacitor should be located not more than 0.5" from the output pin of

the device and returned to a clean analog ground.

INHIBIT INPUT OPERATION

The inhibit pin can be used to turn OFF the regulator when pulled low, so drastically reducing the current

regulator output ON over time. To assure proper operation, the signal source used to drive the inhibit pin must be

Any slew rate can be used to drive the inhibit.

REVERSE CURRENT

The power transistor used in the UTC LD1985 has not an inherent diode connected between the regulator input

and output. If the output is forced above the input, no current will flow from the output to the input across the series

across the two feedback resistors. This current typical value is 160µA. R1 and R2 resistors are implanted type; and

typical values are respectively 42.6 KΩ and 51.15 KΩ.

2

Bypass

IN

1μF

+

INH

1

GND

R2

R1

OUT

4.7μF

+

A

Fig 1. REVERSE CURRENT TEST CIRCUIT