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參數(shù)資料
| 型號： | LTC1772 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23 |
| 中文描述： | 恒定頻率電流模式降壓DC / DC控制器，采用SOT - 23 |
| 文件頁數(shù)： | 6/16頁 |
| 文件大小： | 188K |
| 代理商： | LTC1772 |
6
LTC1772
APPLICATIO
S I
N
FOR
ATIO
U
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
SENSE
(= R1). Next, the power MOSFET and the output
diode D1 is selected followed by C
IN
(= C1)and C
OUT
(= C2).
W
U
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 LTC1772 can provide is given by:
I
R
I
OUT
SENSE
RIPPLE
2
=
0 1
.
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
RIPPLE
= (0.4)(I
OUT
). Rearranging the above equation, it
becomes:
R
I
SENSE
OUT
=
( )(
12
)
1
for Duty Cycle < 40%
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
SENSE
is:
R
SF
I
SENSE
OUT
=
( )(
12
)(
)
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
IN
or
V
OUT
. The inductor’s peak-to-peak ripple current is given
by:
I
V
V
f L
V
V
V
V
RIPPLE
IN
OUT
OUT
IN
D
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)
). Remember, the maximum I
RIPPLE
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:
I
R
RIPPLE
SENSE
≤
0 0288
.
This implies a minimum inductance of:
L
V
V
f
R
V
V
V
V
MIN
IN
OUT
SENSE
OUT
IN
D
D
=
.
+
+
0 0288
(Use V
IN(MAX)
= V
IN
)
A smaller value than L
MIN
could be used in the circuit;
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
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