LTC1779 LINER [Linear Technology], LTC1779 Datasheet - Page 6

• Manufacturer Part Number: LTC1779
• Description: 250mA Current Mode Step-Down DC/DC Converter in ThinSOT
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC1779
The variable M is the ratio of the total switch current to that
portion of the switch current that flows through R
is a function of both R
switch, which in turn, is a strong function of supply
voltage. For values of M refer to Figure 3. In order to
guarantee the desired I
voltage, the minimum value of M, corresponding to the
minimum supply voltage seen in the application, should
be chosen. Note that the selection of R
the resulting M, is an iterative process. For most applica-
tions, a value of R
chosen.
APPLICATIO S I FOR ATIO
The basic LTC1779 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of L1 and
R
followed by C
Inductor Value Calculation
The inductance value has a direct effect on ripple current.
The ripple current, I
tance or frequency and increases with higher V
inductor’s peak-to-peak ripple current is given by:
where f is the operating frequency fixed at 550kHz in the
LTC1779.
A smaller value of L results in higher current ripple and
output voltage ripple as well as greater core losses. Larger
values of L decrease the ripple, but require finding physi-
cally larger inductors since maximum DC current rating
decreases significantly as inductance increases within
inductor product types. Generally, by choosing the de-
sired ripple current based on the maximum output cur-
rent, the inductor value can be calculated from the previous
equation. It is typical to choose the inductor so that the
ripple current is about 40% of the maximum output
6
SENSE
I
RIPPLE
(= R1). Next, the output diode D1 is selected
IN
V
IN
(= C1)and C
V
U
–
IN
SENSE
U
V
RIPPLE
OUT
• •
SENSE
(Refer to Functional Diagram)
PK
L
U
V
between 0
, decreases with higher induc-
and R
over the full range of supply
OUT
OUT
(= C2).
OUT
W
of the internal power
SENSE
and 20
, and hence
U
SENSE
IN
will be
. The
. M
current at maximum input voltage. Use the following
equations to calculate L:
and then choose an appropriate L and recalculate the
ripple current.
In Burst Mode operation on the LTC1779, the ripple
current is normally set such that the inductor current is
continuous during the burst periods. Therefore, the peak-
to-peak ripple current must not exceed:
This implies a minimum inductance of:
(Use V
I
L
I
I
L
RIPPLE
L MAX
RIPPLE
MIN
(
V
IN MAX
)
V
IN(MAX)
IN MAX
60
55
50
45
40
35
30
25
20
f
(
(
I
0 4
(
0
(
OUT MAX
R
. •
R
V
SENSE
Figure 3. M vs Supply Voltage
M
SENSE
)
M
1
IN
(
–
)
= V
( .
I
( .
0 030
OUT MAX
• •
0 030
2
V
OUT
V
IN
SUPPLY VOLTAGE (V)
)
3
(
OUT
)
I
2
RIPPLE
2
)
4
)
I
RIPPLE
•
)
)
V
)
5
2
OUT
R
6
SENSE
V
V
R
OUT
R
R
7
SENSE
IN
R
R
SENSE
SENSE
SENSE
SENSE
= 18.2
8
= 6.2
= 14
= 10
V
= 2
= 0
V
9
1779 F03
D
D
10