UC3666

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 8

www.unisonic.com.tw

QW-R502-489.a

FUNCTION DESCRIPTION

The UC3666 is a dual channel constant frequency current mode PWM step-down converter. It is designed for

low voltage, Li-Ion battery powered applications requiring high efficiency and small size. The output voltage can be

adjusted from 0.6V to 5.5V by means of an external resistor divider. A main switch and a synchronous rectifier are

integrated to provide high efficiency, thus an external Schottky diode is saved. This device can achieve 100% duty

cycle.

The duty cycle D of a step-down converter can be calculated as follows:

Current Mode PWM Control

Slope compensated current mode PWM control provides stable switching and cycle-by-cycle current limit for

superior load and line response and protection of the internal main switch and synchronous rectifier.

The UC3666 has a constant switching frequency of 1.6MHz and can adjust the output voltage. During each

cycle, the PWM comparator modulates the power transferred to the load by changing the inductor peak current

based on the feedback error voltage.

The main switch is switched on for a certain time to ramp the inductor current at each rising edge of the internal

oscillator, and switched off when the peak inductor current exceeds the error voltage during normal operation.

When the main switch is turned off, the synchronous rectifier will be turned on as soon as possible, and then

stay on until either the inductor current drops to zero or the next cycle starts. To get a high efficiency, the device is

skipped pulses at light load.

Dropout Operation

The UC3666 allows the main switch to keep for more than one switching cycle and increases the duty cycle

while the input voltage decreases until nearly equal to the output voltage. When the duty cycle reaches 100%, the

main switch is held on continuously to drive the output with current until it is equal to the P-MOSFET current limit.

Thus, the output voltage can be calculated as the input voltage minus the voltage drop across the main switch and

the inductor.

Short Circuit Protection

Short circuit protection is incorporated in UC3666. When the output is shorted to ground, the oscillator frequency

is reduced to protect inductor current from being damaged due to it increasing beyond the P-MOSFET current limit.

In order to make the short circuit current lower, the P-MOSFET current limit is also reduced. The values of

frequency and current limit are restored until the short circuit condition is removed as well as the feedback voltage

reaches 0.6V.

Maximum Load Current

The UC3666’s input voltage can be of as low as 2.5V, however, at lower input voltage, the maximum load

current decreases because of the large IR drop on the main switch and synchronous rectifier.

The slope compensation signal reduces the peak inductor current as a function of the duty cycle to prevent

sub-harmonic oscillations as duty cycles become higher than 50%. Conversely the current limit increases as the duty

cycle decreases match other diagrams in this datasheet.