Oscillator
The internal oscillator linearly charges and discharges the
internal capacitance between two voltage levels to create a
sawtooth waveform for the pulse width modulator. The oscillator
sets the pulse width modulator/current limit latch at the beginning
of each cycle. The nominal frequency of 100 kHz was chosen
to minimize EMI and maximize efficiency in power supply
applications. Trimming of the current reference improves
oscillator frequency accuracy.
Pulse Width Modulator
The pulse width modulator implements a voltage-mode control
loop by driving the output MOSFET with a duty cycle inversely
proportional to the current into the CONTROL pin which
generates a voltage error signal across RE. The error signal
across RE is filtered by an RC network with a typical corner
frequency of 7 kHz to reduce the effect of switching noise. The
filtered error signal is compared with the internal oscillator
sawtooth waveform to generate the duty cycle waveform. As
the control current increases, the duty cycle decreases. A clock
signal from the oscillator sets a latch which turns on the output
MOSFET. The pulse width modulator resets the latch, turning
off the output MOSFET. The maximum duty cycle is set by the
symmetry of the internal oscillator. The modulator has a
minimum ON-time to keep the current consumption of the
TOPSwitch independent of the error signal. Note that a minimum
current must be driven into the CONTROL pin before the duty
cycle begins to change.
楼主
发表于 2014-2-2 23:17:51
