ml4801 Fairchild Semiconductor, ml4801 Datasheet - Page 9

• Manufacturer Part Number: ml4801
• Description: Variable Feedforward Pfc/pwm Controller Combo
• Manufacturer: Fairchild Semiconductor

FUNCTIONAL DESCRIPTION
operating frequency can typically be approximated by:
operating frequency can typically be approximated by:
EXAMPLE:
EXAMPLE:
For the application circuit shown in the data sheet, with
For the application circuit shown in the data sheet, with
the oscillator running at:
the oscillator running at:
Solving for R
Solving for R
components values, C
components values, C
PWM SECTION
PWM SECTION
The PWM section of the ML4801 is straightforward, but
The PWM section of the ML4801 is straightforward, but
there are several points which should be noted. Foremost
there are several points which should be noted. Foremost
among these is its inherent synchronization to the PFC
among these is its inherent synchronization to the PFC
section of the device, and that the PWM stage is
section of the device, and that the PWM stage is
optimized for current-mode operation. In the ML4801, the
optimized for current-mode operation. In the ML4801, the
operating frequency of the PFC section is fixed at 1/2 of
operating frequency of the PFC section is fixed at 1/2 of
the PWM's operating frequency. This is done through the
the PWM's operating frequency. This is done through the
use of a 2:1 digital frequency divider ("T" flip-flop) linking
use of a 2:1 digital frequency divider ("T" flip-flop) linking
the two functional sections of the IC.
the two functional sections of the IC.
No voltage error amplifier is included in the PWM stage
No voltage error amplifier is included in the PWM stage
of the ML4801, as this function is generally performed on
of the ML4801, as this function is generally performed on
the output side of the PWM’s isolation boundary. To
the output side of the PWM’s isolation boundary. To
facilitate the design of optocoupler feedback circuitry, an
facilitate the design of optocoupler feedback circuitry, an
offset has been built into the PWM’s RAMP 2 input which
offset has been built into the PWM’s RAMP 2 input which
allows V
allows V
input voltages below 1.25V.
input voltages below 1.25V.
PWM Current Limit
PWM Current Limit
The RAMP 2 pin provides a direct input to the cycle-by-
The RAMP 2 pin provides a direct input to the cycle-by-
cycle current limiter for the PWM section. Should the
cycle current limiter for the PWM section. Should the
input voltage at this pin ever exceed 1.5V, the output of
input voltage at this pin ever exceed 1.5V, the output of
the PWM will be disabled until the output flip-flop is reset
the PWM will be disabled until the output flip-flop is reset
by the clock pulse at the start of the next PWM power
by the clock pulse at the start of the next PWM power
cycle.
cycle.
V
V
The V
The V
PFC and inhibits the PWM if this voltage on V
PFC and inhibits the PWM if this voltage on V
than its nominal 2.5V. Once this voltage reaches 2.5V,
than its nominal 2.5V. Once this voltage reaches 2.5V,
which corresponds to the PFC output capacitor being
which corresponds to the PFC output capacitor being
charged to its rated boost voltage, the soft-start
charged to its rated boost voltage, the soft-start
commences.
commences.
PWM Control (RAMP 2)
PWM Control (RAMP 2)
In addition to its PWM current limit function, RAMP 2 is
In addition to its PWM current limit function, RAMP 2 is
used as the sampling point for a voltage representing the
used as the sampling point for a voltage representing the
current in the primary of the PWM’s output transformer.
current in the primary of the PWM’s output transformer.
REV. 1.1 3/9/2001
IN
IN
f
f
t
OK Comparator
OK Comparator
OSC
OSC
RAMP
IN
IN
OK comparator monitors the DC output of the
OK comparator monitors the DC output of the
DC
DC
=
=
=
100
t
to command a zero percent duty cycle for
RAMP
to command a zero percent duty cycle for
0 51
T
T
1
.
kHz
x C
x C
´
T
T
R
=
yields 2 x 10
yields 2 x 10
T
t
´
T
T
RAMP
1
C
= 270pF, and R
= 270pF, and R
T
= ´
1 10
-4
-4
. Selecting standard
. Selecting standard
-
5
T
T
= 36.5kΩ.
= 36.5kΩ.
(Continued)
FB
FB
is less
is less
(5)
(5)
This voltage may be derived either by a current sensing
This voltage may be derived either by a current sensing
resistor or a current transformer.
resistor or a current transformer.
Soft Start
Soft Start
Start-up of the PWM is controlled by the selection of the
Start-up of the PWM is controlled by the selection of the
external capacitor at SS. A current source of 25µA
external capacitor at SS. A current source of 25µA
supplies the charging current for the capacitor, and start-
supplies the charging current for the capacitor, and start-
up of the PWM begins at 1.25V. Start-up delay can be
up of the PWM begins at 1.25V. Start-up delay can be
programmed by the following equation:
programmed by the following equation:
where C
where C
t
t
It is important that the time constant of the PWM soft-start
It is important that the time constant of the PWM soft-start
allow the PFC time to generate sufficient output power for
allow the PFC time to generate sufficient output power for
the PWM section. The PWM start-up delay should be at
the PWM section. The PWM start-up delay should be at
least 5ms.
least 5ms.
Solving for the minimum value of C
Solving for the minimum value of C
Generating V
Generating V
The ML4801 is a voltage-fed part. It requires an external
The ML4801 is a voltage-fed part. It requires an external
15V±10% or better Zener shunt voltage regulator, or some
15V±10% or better Zener shunt voltage regulator, or some
other V
other V
the part at 15V nominal. This allows a low power
the part at 15V nominal. This allows a low power
dissipation while at the same time delivering 13V
dissipation while at the same time delivering 13V
nominal of gate drive at the PWM OUT and PFC OUT
nominal of gate drive at the PWM OUT and PFC OUT
outputs. If using a Zener diode, it is important to limit the
outputs. If using a Zener diode, it is important to limit the
current through the Zener to avoid overheating or
current through the Zener to avoid overheating or
destroying it. This can be easily done with a single resistor
destroying it. This can be easily done with a single resistor
in series with the Vcc pin, returned to a bias supply of
in series with the Vcc pin, returned to a bias supply of
typically 18V to 20V. The resistor’s value must be chosen
typically 18V to 20V. The resistor’s value must be chosen
to meet the operating current requirement of the ML4801
to meet the operating current requirement of the ML4801
itself (8.5mA max.) plus the current required by the two
itself (8.5mA max.) plus the current required by the two
gate driver outputs.
gate driver outputs.
EXAMPLE:
EXAMPLE:
With a V
With a V
driving a total gate charge of 110nC at 100kHz (1 IRF840
driving a total gate charge of 110nC at 100kHz (1 IRF840
MOSFET and 2 IRF830 MOSFETs), the gate driver current
MOSFET and 2 IRF830 MOSFETs), the gate driver current
required is:
required is:
The ML4801 should be locally bypassed with a 10nF and
The ML4801 should be locally bypassed with a 10nF and
a 1µF ceramic capacitor. In most applications, an
a 1µF ceramic capacitor. In most applications, an
electrolytic capacitor of between 33µF and 100µF is also
electrolytic capacitor of between 33µF and 100µF is also
required across the part, both for filtering and as part of
required across the part, both for filtering and as part of
the start-up bootstrap circuitry.
the start-up bootstrap circuitry.
DELAY
DELAY
I
R
C
C
GATEDRIVE
BIAS
SS
SS
is the desired start-up delay.
is the desired start-up delay.
CC
CC
=
SS
SS
=
BIAS
BIAS
=
5
t
regulator, to maintain the voltage supplied to
regulator, to maintain the voltage supplied to
is the required soft start capacitance, and
is the required soft start capacitance, and
DELAY
ms
7 5
20
.
of 20V, a V
CC
CC
of 20V, a V
=
mA
×
V
100
25
125
×
.
-
+
125
16 5
25
µ
kHz
.
11
A
V
.
µ
mA
V
=
A
V
´
CC
100
CC
110
=
limit of 16.5V (max) and
limit of 16.5V (max) and
180
nF
nC
Ω
=
11
SS
SS
mA
:
:
ML4801
(6)
(6)
9