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參數(shù)資料
| 型號： | LTC1772B |
| 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)： | 6/12頁 |
| 文件大?。?/td> | 196K |
| 代理商： | LTC1772B |
6
LTC1772B
APPLICATIO
S I
FOR
ATIO
U
The basic LTC1772B application circuit is shown
inFigure1. External component selection is driven by the
load requirement and begins with the selection of L1 and
R
SENSE
(= R1). Next, the power MOSFET, M1 and the
output diode D1 is selected followed by C
IN
(= C1)and
C
OUT
(= C2).
W
U
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 LTC1772B can provide is given by:
I
R
I
OUT
SENSE
RIPPLE
2
=
0 105
.
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
=
0 0875
.
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 3, the value of R
SENSE
is:
R
SF
)
I
SENSE
OUT
=
(
0 0875
)
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
D
IN
D
=
+
+
Figure 3. Maximum Output Current vs Duty Cycle
DUTY CYCLE (%)
110
100
90
80
70
60
50
40
30
20
10
S
O
/
O
1772 F03
0
70 80
90 100
60
10
20 30
40 50
I
= 0.4I
AT 5% DUTY CYCLE
I
= 0.2I
AT 5% DUTY CYCLE
V
IN
= 4.2V
Overvoltage Protection
As a further protection, the overvoltage comparator in the
LTC1772B will turn the external MOSFET off when the
feedback voltage has risen 7.5% above the reference
voltage of 0.8V. This comparator has a typical hysteresis
of 20mV.
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
I
V
10
R
PK
ITH
SENSE
=
)
– .85
when the LTC1772B is operating below 40% duty cycle.
However, once the duty cycle exceeds 40%, slope
OPERATIOU
(Refer to Functional Diagram)
compensation begins and effectively reduces the peak
inductor current. The amount of reduction is given by the
curves in Figure 3.
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