ml4880 Microsemi Corporation, ml4880 Datasheet - Page 10

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

ML4880
The relationship of the inductor current to the output
current is given by the following equation:
The inductance value can be determined by setting the
ripple current portion of Equation 18 to some percentage
of the output current. Inductor ripple currents in the range
of 50% to 80% of the maximum output current are
typical. As a good starting point set the inductor ripple
current to 65% of the maximum output current:
where F
maximum output current, or:
Calculate the inductance using the volt-seconds value
given in Figure 5 at the minimum input voltage. Choose
the nearest standard value, and determine the actual
inductor ripple current at the maximum and minimum
input voltage using Equation 19 and Figure 5.
Next, determine the sense resistor value using the inductor
ripple current value calculated above using:
Having determined the values for the inductor and sense
resistor, the inductor peak current rating can now be
specified. This value is calculated at current limit and at
the maximum input voltage, and is given by:
For reliable operation, the inductor current rating should
exceed the value calculated by 10%-20%.
Now the sense resistor’s power rating can be determined.
The sense resistor must be able to carry the peak current
in the inductor, but only during the OFF-time:
where,
10
I
I
2
RMS OFF
OUT
I
L
(
)
I
SENSE
T
ON
IRC
R
V
IN MAX
SENSE
L
I
L
OUT MAX
(
V
2
1
= ratio of inductor ripple current to the
I
L PK MAX
IN MAX
V
(
(
I
. 0 65
V
IN
)
L
(
SENSE MAX
T
@
R
V
ON
I
)
OUT
V
SENSE
OUT MAX
IN
)
P
F
(
R
IRC
V
)
V
SENSE
I
IN
(
V
OUT MAX
IN MIN
V
OUT
(
I
I
SENSE MAX
IN MIN
(
SENSE MAX
)
V
IN MIN
I
(
)
(
OUT MAX
0 14
(
(
V
)
I
I
.
L MAX
2
RMS OFF
(
IN MIN
(
V
R
)
V
(
SENSE
SENSE
V
)
(
)
V
)
)
2
OUT
(
SENSE MIN
IN MIN
)
)
(
(
V
)
I
SENSE MAX
)
IN MIN
R
)
I
. 0 25
(
L MAX
T
V
SENSE
(
ON
(
R
)
OUT
(
2
V
2
1
SENSE
IN MIN
V
V
)
L
V
)
)
IN
I
(
)
L MIN
) (
3
IN
(
I
L PK MAX
(
V
I
V
L MAX
1
2
IN
V
(
@
V
)
OUT
)
IN
OUT
I
L MIN
V
(
IN
V
)
)
) (
OUT
I
)
L PK MAX
(
@
(18)
(19)
(20)
(21)
(22)
(23)
)
)
2
inductor need to be specified. In general the winding
resistance should be as low as possible, preferably in the
low milliohm range. A good rule of thumb is to allow 2
mW of winding resistance per mH of inductance. The
turns ratio is specified at 1:1 for simplicity and availability.
FLYBACK REGULATOR - MOSFET SELECTION
The internal gate drive of the ML4880 limits the selection
of the switching FET to logic level types with the ON
resistance specified at V
with a maximum total gate charge of around 70nC.
The drain to source breakdown voltage rating is
determined as follows:
A safety derating factor, F
recommended due to the voltage spikes caused by the
leakage inductance of a transformer. For a more reliable
design, look for MOSFETs that are avalanche rated.
The maximum ON resistance of the MOSFET can be
determined using the worst case power dissipation for the
package type chosen. This assumes that the major portion
of heat generated in the package is due to I
where,
FLYBACK REGULATOR — RECTIFIER SELECTION
The output diode must be able to carry the worst case
output current as well as withstand the worst case reverse
voltage. The worst case output current occurs at current
limit and at the maximum input voltage. Therefore, the
average forward current rating of the output diode can be
determined by the following:
where,
The derating factor is typically in the range of 0.5 to 0.9
depending on reliability goals. Typically a value of 0.8 is
used for most applications.
The reverse voltage rating is determined by the sum of the
output voltage and the maximum input voltage:
I
I
2
RMS ON
The winding resistance and the turns ratio of the coupled
D AVG
(
(
)
)
I
OUT MAX
V
IN MIN
(
F
ID
(
V
OUT
)
)
V
V
OUT
DS
R
. 0 25
F
SENSE
R
ID
(
(
DS ON
I
SENSE MAX
V
IN MAX
(
GS
derating factor
(
(
DS
T
ON
= 4.5V. Also, look for a FET
)
, in the range of 1.5 to 2.0 is
)
)
2
)
2
I
RMS ON
2
V
(
I
IN MAX
P
SENSE MAX
V
L
PKG
(
OUT
(
(
)
)
)
)
) (
3
V
I
L PK MAX
F
(
IN MAX
DS
(
@
2
V
R losses.
IN MAX
(
)
)
) (
V
)
I
OUT
L PK MAX
(
@
(24)
(25)
(26)
F
ID
1
)
)
2