UD05121

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

5 of 7

www.unisonic.com.tw

QW-R221-028.E



FUNCTION DESCRIPTION

Normal Operation

The UD05121 uses a user adjustable frequency, current mode, synchronous step-down architecture with internal

paver switch. During normal operation, the internal power switch is turned on each cycle when the oscillator sets the

SR latch, and turned off when the comparator resets the SR latch. The peak inductor current at which comparator

resets the SR latch is controlled by the output of error amplifier EA. While the high-side switch is off, the external

schottky diode turns on until either the inductor current starts to reverse or the beginning of the next switching cycle.

Setting the Output Voltage

Application circuit item shows the basic application circuit with adjustable output version. The external resistor

sets the output voltage according to the following equation:

×

OUT

FB

FB

R1

V

= V

(1+

), V = 0.6V, R2 suggest 60k ~ 300kΩ

R2

Table 1. Resistor select for output voltage setting

R1

R2

1.0V

100K

150K

1.2V

100K

100K

1.5V

150K

100K

1.8V

200K

100K

2.5V

470K

150K

3.3V

450K

100K

Dropout Operation

As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward

the maximum on-time. Further reduction of the supply voltage forces the high-side switch to remain on for more than

one cycle until it reaches 100% duty cycle.

The output voltage is dropped from the input supply for the voltage which across the high-side switch.

Over Temperature Protection

In most applications the UD05121 does not dissipate much heat due to high efficiency. But, in applications where

the UD05121 is running at high ambient temperature with low supply voltage and high duty cycles, such as in

dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature

reaches approximately 160°C, the internal high-side power switch will be turned off and the LX node will become

high impedance.

Over Current Protection

The UD05121 cycle-by-cycle limits the peak inductor current to protect embedded switch from damage. Hence

the maximum output current (the average of inductor current) is also limited. In case the load increases, the inductor

current is also increase. Whenever the current limit level is reached, the output voltage cannot be regulated and

starting to drop.

Soft-Start

The UD05121 employs internal soft-start circuitry to reduce supply inrush current during startup conditions. When

the device exits under-voltage lockout or shut-down mode, the soft-start circuitry will slowly ramp up the output

voltage.

Short-circuit Protection

Short-circuit protection will activate once the feedback voltage falls below, and the operating frequency will be

reducing normal switching frequency to reduce power delivered from input to output.