LTC1872BES6 データシートの表示（PDF） - Linear Technology
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LTC1872BES6 Datasheet PDF : 12 Pages
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LTC1872B
U
OPERATIO (Refer to Functional Diagram)
Short-Circuit Protection
Since the power switch in a boost converter is not in series
with the power path from input to load, turning off the
switch provides no protection from a short-circuit at the
output. External means such as a fuse in series with the
boost inductor must be employed to handle this fault
condition.
APPLICATIONS INFORMATION
The basic LTC1872B application circuit is shown in
Figure␣ 1. External component selection is driven by the
load requirement and begins with the selection of L1 and
RSENSE (= R1). Next, the power MOSFET and the output
diode D1 is selected followed by CIN(= C1) and COUT(= C2).
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across RSENSE, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent the LTC1872B can provide is given by:
IOUT
=
0.12
RSENSE
−
IRIP2PLE
VIN
VOUT +
VD
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section) and VD is the
forward drop of the output diode at the full rated output
current.
A reasonable starting point for setting ripple current is:
IRIPPLE
=
(O.4)(IOUT
)
VOUT +
VIN
VD
Rearranging the above equation, it becomes:
RSENSE
=
1
(10)(IOUT)
VIN
VOUT +
VD
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 3, the value of RSENSE is:
6
RSENSE
=
SF
(10)(IOUT)(100)
VIN
VOUT +
VD
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, IRIPPLE, decreases with higher
inductance or frequency and increases with higher VOUT.
The inductor’s peak-to-peak ripple current is given by:
( ) IRIPPLE
=
VIN
fL
VOUT + VD − VIN
VOUT + VD
where f is the operating frequency. Accepting larger values
of IRIPPLE 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:
( ) IRIPPLE =
0.4
IOUT(MAX)
VOUT +
VIN
VD
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:
IRIPPLE
≤
0.03
RSENSE
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