ML4802
FUNCTIONAL DESCRIPTION
(Continued)
PWM SECTION
Pulse Width Modulator
The PWM section of the ML4802 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 ML4802). The
PWM is primarily intended for current-mode operation. In
current-mode applications, the PWM ramp (RAMP 2) is
usually derived directly from a current sensing resistor in
the primary of the output stage, and is thereby
representative of the current flowing in the converter’s
output stage. DC ILIMIT, which provides cycle-by-cycle
current limiting, is internally connected to RAMP 2.
No voltage error amplifier is included in the PWM stage
of the ML4802, as this function is generally performed on
the output side of the PWM’s isolation boundary. To
facilitate the design of optocoupler feedback circuitry, an
offset has been built into the PWM’s RAMP 2 input which
allows VDC to command a zero percent duty cycle for
input voltages below 1.25V.
VIN OK Comparator
The VIN OK comparator monitors the DC output of the
PFC and inhibits the PWM if this voltage on VFB is less
than its nominal 2.5V. Once this voltage reaches 2.5V,
which corresponds to the PFC output capacitor being
charged to its rated boost voltage, the soft-start
commences.
PWM Control (RAMP 2)
RAMP 2 is the sampling point for a voltage representing
the current in the primary of the PWM’s output
transformer, derived from a current sensing resistor.
PWM Current Limit
The DC ILIMIT pin is a cycle-by-cycle current limiter for
the PWM section. It is connected internally to the PWM
control pin. Should the input voltage at this pin ever
exceed 1.5V, the output of the PWM will be disabled until
the output flip-flop is reset by the clock pulse at the start
of the next PWM power cycle.
Soft Start
Start-up of the PWM is controlled by the selection of the
external capacitor at SS. A current source of 25µA
supplies the charging current for the capacitor, and start-
up of the PWM begins at 1.25V. Start-up delay can be
programmed by the following equation:
25
m
A
CSS
=
tDELAY
1.25V
L1
+
I1
VIN
SW1
C1
RL
RAMP
SW2
I2
I3
I4
It is important that the time constant of the PWM soft-start
allow the PFC time to generate sufficient output power for
the PWM section. The PWM start-up delay should be at
least 5ms.
Solving for the minimum value of CSS:
CSS
=
5ms
25
m
A
@
200nF
1.25V
Generating VCC
The ML4802 is a voltage-fed part. It requires an external
15V±10% (or better) Zener shunt voltage regulator, or
other controlled supply, to maintain the voltage supplied
to the part at 15V nominal. This allows a low power
dissipation while at the same time delivering 13V
nominal of gate drive at the PWM OUT and PFC OUT
outputs.
LEADING/TRAILING MODULATION
Conventional Pulse Width Modulation (PWM) techniques
employ trailing edge modulation in which the switch will
turn on right after the trailing edge of the system clock.
The error amplifier output voltage is then compared with
the modulating ramp. When the modulating ramp reaches
the level of the error amplifier output voltage, the switch
will be turned OFF. When the switch is ON, the inductor
current will ramp up. The effective duty cycle of the
trailing edge modulation is determined during the ON of
the switch. Figure 5 shows a typical trailing edge control
scheme.
DC
VEAO
REF
U3
+
–EA
DFF
RAMP
OSC
U4
CLK
+
–
U1
R
Q
D U2
Q
CLK
VSW1
TIME
TIME
Figure 5. Typical Trailing Edge Control Scheme
where CSS is the required soft start capacitance, and
tDELAY is the desired start-up delay.
August 2000
Datasheet
11
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
