ML4880CS MICRO-LINEAR [Micro Linear Corporation], ML4880CS Datasheet - Page 7

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

DESIGN CONSIDERATIONS
A typical design can be implemented by using the
following design procedure. Note that this procedure is
not intended to give final values, but to give a good
starting point, and provide the relationships necessary to
make trade-off decisions. Some experimentation will be
necessary to optimize values and to verify that the design
operates over worst case conditions.
DESIGN SPECIFICATIONS
It is important to start with a clear definition of the design
specifications. Make sure the specifications reflect worst
case conditions. Key specifications include the minimum
and maximum input voltage and the output voltage and
load current for each output.
BUCK REGULATORS - INDUCTOR AND SENSE
RESISTOR SELECTION
Figure 6 shows the inductor current of the step down
regulators. The inductor current is made up of two
components: the DC current level set by the
transconductance amplifier, I
current, I
summation of I
Therefore, the selection of the inductance value
determines how much of the output current is made up of
the ripple current. Higher inductor ripple current allows
smaller inductor values, but results in higher peak
currents, lower efficiency, and higher output voltage
ripple.
I
OUT
18
16
14
12
10
8
6
4
2
0
I
SENSE
12V FLYBACK
L
3.3V BUCK
. The figure also shows that I
5V BUCK
SENSE
2
1
Figure 4. T
I
L
5
and
R
V
SENSE
SENSE
1
/
2
SENSE
I
V
ON
L
IN
:
10
T
(V)
vs. V
ON
, and the inductor ripple
IN
(
V
2
OUT
IN
L
15
is the
V
OUT
)
(3)
20
Inductor ripple currents in the range of 30% to 70% of the
maximum output current are typical. As a good starting
point set the inductor ripple current to 50% 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 maximum input voltage. Choose
the nearest standard value, realizing the trade-offs
mentioned before. Then, using the inductance value
chosen, determine the actual inductor ripple current at the
maximum and minimum input voltage using Equation 4
and Figure 5.
The sense resistor value can be determined using the
inductor ripple current value calculated above and
Equation 3 rearranged as follows:
R
SENSE
45
40
35
30
25
20
15
10
5
0
IRC
12V FLYBACK
I
OUT MAX
I
3.3V BUCK
L
5V BUCK
= ratio of inductor ripple current to the
(
V
T
Figure 5. Volt-seconds vs V
SENSE MIN
ON
)
L
5
(
2
(
1
V
T
IN
L
ON
I
)
0 5
L MIN
(
.
V
OUT
(
)
V
I
V
OUT MAX
IN
IN
10
I
OUT MAX
)
(V)
(
V
F
(
OUT
IRC
)
0 14
)
)
.
I
OUT MAX
IN
2
1
15
ML4880
I
(
L MIN
(
)
)
(4)
(5)
(6)
20
7