Datasheet

6
LTC1872
APPLICATIONS INFORMATION
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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 devel-
oped across R
SENSE
, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent the LTC1872 can provide is given by:
I
R
IV
VV
OUT
SENSE
RIPPLE IN
OUT D
=−






+
012
2
.
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:
IOI
VV
V
RIPPLE OUT
OUT D
IN
=
()( )
+
.4
Rearranging the above equation, it becomes:
R
I
VV
SENSE
OUT
OUT D
=
()( )
+






1
10
V
for Duty Cycle < 40%
IN
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
SF
I
V
VV
SENSE
OUT
IN
OUT D
=
()( )( )
+






10 100
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
V
fL
VVV
VV
RIPPLE
IN OUT D IN
OUT 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:
II
VV
V
RIPPLE
OUT MAX
OUT D
IN
=




+






()
04.
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
R
RIPPLE
SENSE
≤
003.
This implies a minimum inductance of:
L
V
f
R
VVV
VV
MIN
IN
SENSE
OUT D IN
OUT D
=






+−
+






003.
A smaller value than L
MIN
could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.