UU28121

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

4 of 6

www.unisonic.com.tw

QW-R135-001.C



FUNCTION DESCRIPTION

Setting the Output Voltage

Application circuit item shows the basic application circuit with UTC UU28121 adjustable output version. The

external resistor sets the output voltage according to the following equation:

1

R

2

R

)

Table 1. Resistor Selection for Common Output Voltages

R1(kΩ)

R2(kΩ)

12

54

470

16

39

470

24

39

720

27

36

750

For most applications, R2 is a suggested a value by 300K~850KΩ. Place the resistor-divider as close to the IC as

possible to reduce the noise sensitivity.

Under Voltage Lockout (UVLO)

To avoid mis-operation of the device at low input voltages an under voltage lockout is included that disables the

device, if the input voltage falls below (2.25V~200mV).

Input Capacitor Selection

The input capacitor reduces the surge current drawn from the input and switching noise from the device. The input

capacitor impedance at the switching frequency shall be less than input source impedance to prevent high frequency

switching current passing to the input. A low ESR input capacitor sized for maximum RMS current must be used.

Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small

temperature coefficients. A 4.7µF ceramic capacitor for most applications is sufficient. For a lower output power

requirement application, this value can be decreased.

Output Capacitor Selection

The output capacitor is required to keep the output voltage ripple small and to ensure regulation loop stability. The

output capacitor must have low impedance at the switching frequency. Ceramic capacitors with X5R or X7R

dielectrics are recommended due to their low ESR and high ripple current. A 2.2uF ceramic capacitors works for

most of the applications. Higher capacitor values can be used to improve the load transient response.