LTC1872BES6 LINER [Linear Technology], LTC1872BES6 Datasheet - Page 6

• Manufacturer Part Number: LTC1872BES6
• Description: Constant Frequency Current Mode Step-Up DC/DC Controller in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1872B
OPERATIO
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
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
R
diode D1 is selected followed by C
R
R
With the current comparator monitoring the voltage devel-
oped across R
determines the inductor’s peak current. The output cur-
rent the LTC1872B can provide is given by:
where I
(see Inductor Value Calculation section) and V
forward drop of the output diode at the full rated output
current.
A reasonable starting point for setting ripple current is:
Rearranging the above equation, it becomes:
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 R
6
SENSE
SENSE
SENSE
I
I
R
for Duty Cycle < 40%
RIPPLE
OUT
SENSE
RIPPLE
(= R1). Next, the power MOSFET and the output
Selection for Output Current
is chosen based on the required output current.
R
0 12
SENSE
O
10
is the inductor peak-to-peak ripple current
.
SENSE
.4
U
1
I
I
U
OUT
OUT
, the threshold of the comparator
(Refer to Functional Diagram)
I
RIPPLE
SENSE
INFORMATION
V
2
V
U
OUT
OUT
V
V
is:
IN
IN
V
OUT
IN
V
V
D
(= C1) and C
V
D
W
IN
V
D
OUT
U
D
(= C2).
is the
output. External means such as a fuse in series with the
boost inductor must be employed to handle this fault
condition.
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
inductance or frequency and increases with higher V
The inductor’s peak-to-peak ripple current is given by:
where f is the operating frequency. Accepting larger values
of I
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is:
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:
R
I
I
I
RIPPLE
RIPPLE
RIPPLE
RIPPLE
SENSE
allows the use of low inductances, but results in
0 4 .
R
f L
V
10
0 03
SENSE
IN
.
I
OUT MAX
I
V
OUT
SF
OUT
V
OUT
100
V
RIPPLE
D
V
V
OUT
D
V
V
OUT
IN
V
, decreases with higher
IN
V
IN
V
D
V
D
OUT
.