ML4826 Datasheet, PDF (9/16 Pages) Fairchild Semiconductor – PFC and Dual Output PWM Controller Combo

PRODUCT SPECIFICATION
ML4826
nonlinearity such that under steady-state operating condi-
tions 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 devi-
ate 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 Charac-
teristics. 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 con-
ventional 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.
Main Oscillator (RTCT)
The oscillator frequency is determined by the values of RT
and CT, which determine the ramp and off-time of the
oscillator output clock:
f_OSC
=
-------------------------¹-------------------------
t_RAMP + t_DEADTIME
(2)
The deadtime of the oscillator is derived from the following
equation:
t_DEADTIME = ₅---²-_.--₁-^.-⁵-_m---^V---_A-- × C_T = 490 × C_T
(3)
at VREF = 7.5V:
f_OSC
=
200kHz
=
-------¹--------
t_RAMP
The ramp of the oscillator may be determined using:
t_RAMP = C_T × R_T × In_{V}-^V----^RR---^EE----^FF----––-----¹³---^..--²⁷---⁵⁵--_
(4)
The deadtime is so small (tRAMP >> tDEADTIME) that the
operating frequency can typically be approximated by:
f_OSC
=
-------¹--------
t_RAMP
(5)
For proper reset of internal circuits during dead time, values
of 1000pF or greater are suggested for CT.
EXAMPLE:
For the application circuit shown in the data sheet, with the
oscillator running at:
t_RAMP1
=
C_RAMP1
×
R_RAMP1
×
In_

-_V----_R--^V-_E----^RF---^E-–---^F--₅----_V--



= 1.1 × R_RAMP1 × C_RAMP1
t_RAMP = C_T × R_T × 0.51 = 1 × 10^–5
Solving for RT x CT yields 2 x 10^-4. Selecting standard com-
ponents values, CT = 1000pF, and RT = 8.63kΩ.
The deadtime of the oscillator adds to the Maximum PWM
Duty Cycle (it is an input to the Duty Cycle Limiter). With
zero oscillator deadtime, the Maximum PWM Duty Cycle is
typically 45%. In many applications, care should be taken
that CT not be made so large as to extend the Maximum
Duty Cycle beyond 50%.
PFC RAMP (RAMP1)
The intersection of RAMP1 and the boost current error
amplifier output controls the PFC pulse width. RAMP1 can
be generated in a similar fashion to the RTCT ramp.
The current error amplifier maximum output voltage has a
minimum of 6V. The peak value of RAMP1 should not
exceed that voltage. Assuming a maximum voltage of 5V for
RAMP1, Equation 6 describes the RAMP1 time. With a
100kHz PFC frequency, the resistor tied to VREF, and a
150pF capacitor, Equation 7 solves for the RAMP1 resistor.
t_RAMP1
=
C_RAMP1
×
R_RAMP1
×
I
n_

-_V----_R--^V-_E----^RF---^E-–---^F--₅----_V--



(6)
= 1.1 × R_RAMP1 × C_RAMP1
R_RAMP1 = -₁---_.--₁---^t-_×-^R----^A_C---^M--_R---^PA---¹-_M----_P----₁- = ₁----_.--₁----¹_×---⁰--₁--^µ-₅---^s-₀----_p---_F-- = 60kΩ (7)
VREF
60kΩ
150pF
ML4826
RAMP1
Figure 3.
PMW SECTION
Pulse Width Modulator
The PWM section of the ML4826 is straightforward, but
there are several points which should be noted. Foremost
among these is its inherent synchronization to the PFC
section of the device, from which it also derives its basic
timing (at twice the PFC frequency in the ML4826-2). The
PWM is capable of current-mode or voltage mode operation.
In current-mode applications, the PWM ramp (RAMP2) is
usually derived directly from a current sensing resistor or
REV. 1.0.5 2/14/02
9