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參數(shù)資料
| 型號(hào)： | LTC3801ES6#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封裝： | MO-193, PLASTIC, TSOT-23 |
| 文件頁(yè)數(shù)： | 10/12頁(yè) |
| 文件大?。?/td> | 229K |
| 代理商： | LTC3801ES6#PBF |
7
LTC3801/LTC3801B
sn3801 3801fs
OPERATIO
U
(Refer to the Functional Diagram)
inductor current. The amount of reduction is given by the
curve in Figure 3.
Soft-Start
An internal default soft-start circuit is employed at power-
up and/or when coming out of shutdown. The soft-start
circuit works by internally clamping the voltage at the
ITH/RUN pin to the corresponding zero current level and
gradually raising the clamp voltage such that the minimum
time required for the programmed switch current to reach
its maximum is approximately 0.6ms. After the soft-start
circuit has timed out, it is disabled until the part is put in
shutdown again or the input supply is cycled.
Figure 3. Maximum Current Limit Trip Voltage vs Duty Cycle
DUTY CYCLE (%)
20
TRIP
VOLTAGE
(mV)
75
95
100
3801 F03
55
35
40
60
80
30
50
70
90
115
65
85
45
105
LTC3801
SLOPE
FACTOR
(%)
70
50
30
100
60
80
40
90
VIN = 4.2V
TA = 25°C
LTC3801
LTC3801B
APPLICATIO S I FOR ATIO
WU
UU
The basic LTC3801/LTC3801B application circuit is shown
on the front page of this data sheet. External component
selection is driven by the load requirement and begins with
the selection of the inductor and RSENSE. Next, the power
MOSFET and the output diode are selected followed by the
input bypass capacitor CIN and output bypass capacitor
COUT.
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
With the current comparator monitoring the voltage
developed across RSENSE, the threshold of the compara-
tor determines the inductor’s peak current. The output
current the LTC3801 can provide is given by:
I
R
I
OUT
SENSE
RIPPLE
=
0 117
2
.
where IRIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section). For the LTC3801B
use 104mV in the previous equation and follow through
the analysis using that number.
A reasonable starting point for setting ripple current is
IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it
becomes:
R
I
SENSE
OUT
=<
1
10
()(
)
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 RSENSE is:
R
SF
I
SENSE
OUT
=
()(
)(
)
10
100
where SF is the “Slope Factor.”
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 cur-
rent. The ripple current, IRIPPLE, decreases with higher in-
ductance or frequency and increases with higher VIN or
VOUT.Theinductor’speak-to-peakripplecurrentisgivenby:
I
VV
fL
VV
RIPPLE
IN
OUT
D
IN
D
=
+
()
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
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