ml4880 Microsemi Corporation, ml4880 Datasheet - Page 6

• Manufacturer Part Number: ml4880
• Description: Portable Pc/pcmcia Power Controller
• Manufacturer: Microsemi Corporation

ML4880
or the buck regulator is operating in a discontinuous
current mode (DCM). Therefore, the synchronous rectifier
comparator is used only for DCM operation. A timing
diagram is shown in Figure 2.
FLYBACK CONTROLLER
A block diagram of the flyback regulator is shown in
Figure 3. The circuit also utilizes a constant ON-time PFM
control architecture. As with the buck controllers, the
flyback controller determines the OFF-time by waiting for
the inductor current to drop to a level set by the feedback
voltage V
of the inductor current ripple is programmed instead of
the peak, and features an automatic transition into and out
of burst mode which does not require additional control
circuitry.
The flyback controller’s oscillator/one shot block has a
constant ON-time and a minimum OFF-time similar to
that of the buck controllers. The OFF-time is extended for
as long as the current comparator’s output stays low.
Again, since the inductor current flows in the current
sense resistor during the OFF-time, a minimum OFF-time
is required to allow for the finite circuit delays in sensing
the inductor current. The ON-time is triggered when the
current comparator’s output goes high. However, unlike
conventional fixed ON-time controllers, the ML4880’s
one shot has an inverse relationship with the input voltage
as shown in Figure 4. Figure 5 plots the inductor voltage-
ON-time product. Note that the volt-second product is
nearly constant over the input voltage range. This results
in an inductor current ripple of:
6
FB
. As a result of the control scheme, the valley
OSCILLATOR/
ONE SHOT
I
L
COMPARATOR
CURRENT
V
T
C
ON
L
T
ON
V
IN
+
–
Figure 3. Flyback Controller Functional Diagram
TRANSCONDUCTANCE
AMPLIFIER
Rgm
(2)
+
–
It is important to note that the ripple current does not vary
in proportion with V
wide input voltage range.
The transconductance amplifier and current comparator
operate in the same manner as the buck regulator. The
current programming comparator controls the length of
the OFF-time by waiting until the inductor current
decreases to a value determined by the transconductance
amplifier before starting a new ON-time.
Like the buck controllers, when the flyback’s output
voltage drops due to a load increase, the
transconductance amplifier will increase its output current
and generate a larger voltage across R
causes the current comparator’s output to go positive
earlier or shortens the OFF-time. Conversely, when the
output voltage is too high, the transconductance
amplifier’s output current will eventually become
negative. This holds the current comparator’s output low
and extends the OFF-time causing the converter to stop
operation until the output voltage drops enough to
increase the output current of the transconductance amp
above zero.
In summary, the flyback has three operation modes that
can be defined by the voltage at the SENSE pin at the end
of the OFF-time:
V
0V > V
–140mV > VSENSE > –250mV Current limit
SENSE
1.25V
SENSE
0V
> –140mV
N DRV
I
V
SENSE
FB
IN
, but remains nearly constant over a
V
IN
Discontinuous
current mode
Continuous
current mode
gm
, which in turn