ML4880CS MICRO-LINEAR [Micro Linear Corporation], ML4880CS Datasheet - Page 11

• Manufacturer Part Number: ML4880CS
• Description: Portable PC/PCMCIA Power Controller
• Manufacturer: MICRO-LINEAR [Micro Linear Corporation]

where,
Again a derating factor of 0.8 is typical for most
applications.
FLYBACK REGULATOR - INPUT CAPACITOR
SELECTION
The input capacitor should be a low ESR type and located
as close to the primary winding of the transformer as
possible. The capacitor’s ability to handle ripple current is
the selection criteria used. The input capacitor’s ripple
current is determined by the load current and the input
voltage, with the worst case condition occurring at
V
The capacitor must also be able to handle the maximum
input voltage. For reliable operation derate the capacitor
voltage rating by 20% to 50% for aluminum electrolytic
types and 50% to 70% for tantalum types.
FLYBACK REGULATOR - OUTPUT CAPACITOR
SELECTION
The output capacitors determine the flyback regulator’s
loop stability and the output ripple voltage. Use only low
ESR capacitors intended for switching power supply
applications. To ensure stability, the minimum capacitance
value is given by:
The maximum ESR value can be estimated using:
The selected capacitor must meet both the capacitance
and ESR requirements. As a final check, make sure the
output capacitor can handle the ripple current,
I
FLYBACK REGULATOR - OUTPUT VOLTAGE
The output of the flyback regulator is adjustable and can
be set to any voltage between 6.0V and 15.0V by
connecting a resistor divider to the feedback pin as shown
in Figure 3. The resistor values R1 and R2 can be
calculated using the following equation:
I
RMS C
RMS(C OUT )
IN
= V
(
IN
OUT
)
C
OUT
.
.
I
SENSE MAX
(
V
F
V
I
VR
8 6 .
RMS C
OUT
R
)
ESR
(
V
derating factor
2
OUT
OUT
1
T
ON MAX
R
I
L MAX
)
(
SENSE
I
F
L MAX
V
(
VR
(
I
V
OUT
RMS C
IN MAX
)
(
)
(
)
IN
V
I
)
V
)
OUT MAX
OUT
OUT
I
L MIN
(
(
V
V
)
IN
IN
)
(27)
(28)
(29)
(30)
(31)
The value of R2 should be 125k or less to minimize bias
current errors.
It is important to note that the accuracy of these resistors
directly affects the accuracy of the output. Use precision
resistors and set the nominal voltage approximately 1% to
2% high in order to make up for the ML4880’s load
regulation. This offset results in the best overall output
accuracy over line and load.
LAYOUT
Proximity of passive devices and adequate power and
ground planes are critical for reliable operation of the
circuit. In general, use the top layer for the high current
connections and the bottom layer for the quiet
connections such as GND, feedback and current sense.
Some more specific guidelines follow.
1. The connection from the current sense resistor to the
2. The trace lengths from the buck regulator’s input
3. The high current ground paths need to be kept
4. Note: Concentrating on keeping the current sense
5. The V
6. Remote sensing the output for improved load
I
located as close to the lead of the resistor as
possible. The trace length from the sense resistor to
the ML4880 should be kept as short as possible and
away from switching components and their traces.
capacitor to the switching MOSFET, from the
MOSFETs to the inductor, from the synchronous
rectifier MOSFET to the SENSE resistor, and from the
inductor to the output capacitor should all be as
short as possible. The flyback regulator’s trace length
from the input capacitor to the inductor, from the
inductor to the MOSFET, from the sense resistor to
the inductor, and from the diode to the output
capacitor should also be kept short to minimize
noise and ground bounce.
separate from the signal ground paths. The GND
connection should be made at a single-point star
ground. It is very important that the ground for the
ML4880 ground pins (pins 6 and 7) be made using a
separate trace.
and high current connections short as well as
keeping the switching components and traces away
from the sensitive analog components and traces
during layout will eliminate the majority of problems
created by a poor layout.
located close to the ML4880 for adequate filtering of
the IC’s internal bias voltage.
regulation can be implemented with the ML4880.
Any output can be remote sensed by using the top of
the external resistor divider as the remote sense
point.
SENSE
IN
pin should be made by a separate trace and
and V
V
OUT
REG
bypass capacitors need to be
1 25
.
V
R
1
R
2
R
2
ML4880
(32)
11