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UC3666 датащи(PDF) 6 Page - Unisonic Technologies

номер детали UC3666
подробное описание детали  DUAL CHANNEL 800mA HIGH EFFICIENCY STEP-DOWN DC/DC CONVERTER
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производитель  UTC [Unisonic Technologies]
домашняя страница  http://www.utc-ic.com
Logo UTC - Unisonic Technologies

UC3666 датащи(HTML) 6 Page - Unisonic Technologies

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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:
D=TON×fOSC×100% V
OUT/VIN×100%
(Where TON is the main switch on time and fOSC is the oscillator frequency (1.6MHz)).
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.


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