ML4841 Просмотр технического описания (PDF) - Micro Linear Corporation
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ML4841 Datasheet PDF : 15 Pages
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FUNCTIONAL DESCRIPTION (Continued)
the voltage and current error amplifiers, along with their
respective return points. The current loop compensation
is returned to VREF to produce a soft-start characteristic on
the PFC: as the reference voltage comes up from zero
volts, it creates a differentiated voltage on IEAO which
prevents the PFC from immediately demanding a full duty
cycle on its boost converter.
There are two major concerns when compensating the
voltage loop error amplifier; stability and transient
response. Optimizing interaction between transient
response and stability requires that the error amplifier’s
open-loop crossover frequency should be 1/2 that of the
line frequency, or 23Hz for a 47Hz line (lowest
anticipated international power frequency). The gain vs.
input voltage of the ML4841’s voltage error amplifier has a
specially shaped nonlinearity such that under steady-state
operating conditions the transconductance of the error
amplifier is at a local minimum. Rapid perturbations in
line or load conditions will cause the input to the voltage
error amplifier (VFB) to deviate from its 2.5V (nominal)
value. If this happens, the transconductance of the voltage
error amplifier will increase significantly, as shown in the
Typical Performance Characteristics. This increases the
gain-bandwidth product of the voltage loop, resulting in a
much more rapid voltage loop response to such
perturbations than would occur with a conventional linear
gain characteristic.
The current amplifier compensation is similar to that of the
voltage error amplifier with the exception of the choice of
crossover frequency. The crossover frequency of the
current amplifier should be at least 10 times that of the
voltage amplifier, to prevent interaction with the voltage
loop. It should also be limited to less than 1/6th that of the
switching frequency, e.g. 16.7kHz for a 100kHz switching
frequency.
For more information on compensating the current and
voltage control loops, see Application Notes 33 and 34.
Application Note 16 also contains valuable information for
the design of this class of PFC.
Oscillator (RT/CT)
The oscillator frequency is determined by the values of RT
and CT, which determine the ramp and off-time of the
oscillator output clock:
fOSC
=
tRAMP
+
1
tDISCHARGE
(2)
The ramp-charge time of the oscillator is derived from the
following equation:
tRAMP
=
CT
× RT
×
In
VREF
VREF
−
−
13..2755
(3)
at VREF = 7.5V:
tRAMP = CT × RT × 0.51
ML4841
The discharge time of the oscillator may be determined
using:
tDISCHARGE
=
2.5V
5.1mA
×
CT
=
490
×
CT
(4)
The deadtime is so small (tRAMP >> tDEADTIME) that the
operating frequency can typically be approximated by:
fOSC
=
1
tRAMP
(5)
EXAMPLE:
For the application circuit shown in the data sheet, with
the oscillator running at:
fOSC
=
200kHz
=
1
tRAMP
tRAMP = 0.51× RT × CT = 5 × 10−6
Solving for RT x CT yields 1 x 10-5. Selecting standard
components values, CT = 390pF, and RT = 24.9kΩ.
RAMP 1
The ramp voltage on this pin serves as a reference to
which the PFC’s current error amp output is compared in
order to set the duty cycle of the PFC switch. The external
ramp voltage is derived from a RC network similar to the
oscillator’s. The PWM’s oscillator sends a synchronous
pulse every other cycle to reset this ramp.
The ramp voltage should be limited to no more than the
output high voltage (6V) of the current error amplifier. The
timing resistor value should be selected such that the
capacitor will not charge past this point before being reset.
In order to ensure the linearity of the PFC loop’s transfer
function and improve noise immunity, the charging
resistor should be connected to the 13.5V VCC rather than
the 7.5V reference. This will keep the charging voltage
across the timing cap in the “linear” region of the charging
curve.
The component value selection is similar to oscillator RC
component selection.
fOSC
=
tCHARGE
1
+ tDISCHARGE
(6)
The charge time of Ramp 1 is derived from the following
equations:
tCHARGE
=
2
fOSC
(7)
tCHARGE
=
CT
× RT
× In
VCC − Ramp
VCC − Ramp
Valley
Peak
(8)
9
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