LTC1772 Linear Technology, LTC1772 Datasheet - Page 6

• Manufacturer Part Number: LTC1772
• Description: Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23
• Manufacturer: Linear Technology

APPLICATIONS
LTC1772
The basic LTC1772 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 LTC1772 can provide is given by:
where I
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
becomes:
However, for operation that is above 40% duty cycle, slope
compensation effect has to be taken into consideration to
select the appropriate value to provide the required amount
of current. Using Figure 2, the value of R
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.
6
RIPPLE
SENSE
SENSE
SENSE
I
R
R
OUT
SENSE
SENSE
RIPPLE
= (0.4)(I
(= R1). Next, the power MOSFET and the output
Selection for Output Current
is chosen based on the required output current.
R
SENSE
0 1
.
is the inductor peak-to-peak ripple current
12
12
SENSE
OUT
1
I
I
OUT
U
OUT
). Rearranging the above equation, it
SF
I
, the threshold of the comparator
RIPPLE
2
100
INFORMATION
for Duty Cycle < 40%
U
IN
(= C1)and C
W
SENSE
OUT
U
is:
(= C2).
The inductance value also has a direct effect on ripple
current. The ripple current, I
inductance or frequency and increases with higher V
V
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
I
occurs at the maximum input voltage.
In Burst Mode operation on the LTC1772, 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:
This implies a minimum inductance of:
A smaller value than L
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
RIPPLE
OUT
(Use V
I
I
L
RIPPLE
RIPPLE
RIPPLE
MIN
. The inductor’s peak-to-peak ripple current is given
= 0.4(I
IN(MAX)
allows the use of low inductances, but results in
f
V
IN
R
V
0 0288
OUT(MAX)
0 0288
R
IN
.
SENSE
.
SENSE
f L
V
= V
OUT
V
OUT
IN
). Remember, the maximum I
)
MIN
V
V
OUT
V
IN
V
OUT
could be used in the circuit;
RIPPLE
IN
V
V
D
V
D
, decreases with higher
V
D
D
RIPPLE
IN
or