Datasheet

6
LTC1779
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
SENSE
(= R1). Next, the output diode D1 is selected
followed by C
IN
(= C1)and C
OUT
(= C2).
Inductor Value Calculation
The inductance value has a direct effect on ripple current.
The ripple current, I
RIPPLE
, decreases with higher induc-
tance or frequency and increases with higher V
IN
. The
inductor’s peak-to-peak ripple current is given by:
I
VV V
VL
RIPPLE
IN OUT OUT
IN
=
()
ƒ
–
••
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
current at maximum input voltage. Use the following
equations to calculate L:
II
L
VVV
VI
II
I
RIPPLE OUT MAX
IN MAX OUT OUT
IN MAX RIPPLE
L MAX OUT MAX
RIPPLE
=
=
()
ƒ
=+
04
2
.•
–•
••
()
()
()
() ()
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:
I
M
R
RIPPLE
SENSE
≤
+Ω
(. )
()
0 030
2
This implies a minimum inductance of:
L
VV
f
M
R
VV
VV
MIN
IN OUT
SENSE
OUT D
IN D
=
−
+Ω






+
+






(. )
()
0 030
2
(Use V
IN(MAX)
= V
IN
)
(Refer to Functional Diagram)
OPERATIO
U
The variable M is the ratio of the total switch current to that
portion of the switch current that flows through R
SENSE
. M
is a function of both R
SENSE
and R
OUT
of the internal power
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
PK
over the full range of supply
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
SENSE
, and hence
the resulting M, is an iterative process. For most applica-
tions, a value of R
SENSE
between 0Ω and 20Ω will be
chosen.
Figure 3. M vs Supply Voltage
APPLICATIO S I FOR ATIO
WUUU
SUPPLY VOLTAGE (V)
0
M (mA/mA)
60
55
50
45
40
35
30
25
20
8
1779 F03
246 107135 9
R
SENSE
= 0Ω
R
SENSE
= 2Ω
R
SENSE
= 18.2Ω
R
SENSE
= 14Ω
R
SENSE
= 10Ω
R
SENSE
= 6.2Ω