LTC1872
6
1872fa
For more information
www.linear.com/LTC1872
The basic LTC1872 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 power MOSFET and the output
diode D1 is selected followed by C
IN
(= C1) and C
OUT
(= C2).
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage de-
veloped across R
SENSE
, the threshold of the comparator
determines the inductor’s peak current. The output current
the LTC1872 can provide is given by:
I
OUT
=
0.12
R
SENSE
−
I
RIPPLE
2
⎛
⎝
⎜
⎞
⎠
⎟
V
IN
V
OUT
+
V
D
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section) and V
D
is the
forward drop of the output diode at the full rated output
current.
A reasonable starting point for setting ripple current is:
I
RIPPLE
=
O.4
( )
I
OUT
(
)
V
OUT
+
V
D
V
IN
Rearranging the above equation, it becomes:
R
SENSE
=
1
10
( )
I
OUT
(
)
V
IN
V
OUT
+
V
D
⎛
⎝
⎜
⎞
⎠
⎟
for Duty Cycle <40%
However, for operation that is above 40% duty cycle, slope
compensation’s effect has to be taken into consideration
to select the appropriate value to provide the required
amount of current. Using the scaling factor (SF, in %) in
Figure 2, the value of R
SENSE
is:
R
SENSE
=
SF
10
( )
I
OUT
(
)
100
(
)
V
IN
V
OUT
+
V
D
⎛
⎝
⎜
⎞
⎠
⎟
applicaTions inForMaTion
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor ripple
current. However, this is at the expense of efficiency due
to an increase in MOSFET gate charge losses.
The inductance value also has a direct effect on ripple
current. The ripple current, I
RIPPLE
, decreases with higher
inductance or frequency and increases with higher V
OUT
.
The inductor’s peak-to-peak ripple current is given by:
I
RIPPLE
=
V
IN
f L
( )
V
OUT
+
V
D
−
V
IN
V
OUT
+
V
D
⎛
⎝
⎜
⎞
⎠
⎟
where f is the operating frequency. Accepting larger values
of I
RIPPLE
allows the use of low inductances, but results
in higher output voltage ripple and greater core losses.
A reasonable starting point for setting ripple current is:
I
RIPPLE
=
0.4 I
OUT MAX
(
)
(
)
V
OUT
+
V
D
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
In Burst Mode operation, 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
RIPPLE
≤
0.03
R
SENSE
This implies a minimum inductance of:
L
MIN
=
V
IN
f 0.03
R
SENSE
⎛
⎝
⎜
⎞
⎠
⎟
V
OUT
+
V
D
−
V
IN
V
OUT
+
V
D
⎛
⎝
⎜
⎞
⎠
⎟
A smaller value than L
MIN
could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
