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參數(shù)資料
| 型號： | LTC3408EDD |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.5MHz, 600mA Synchronous Step-Down Regulator with Bypass Transistor |
| 中文描述： | 1.3 A SWITCHING REGULATOR, 1800 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 3 X 3 MM, 0.75 MM HEIGHT, PLASTIC, M0-229-WEED-1, DFN-8 |
| 文件頁數(shù)： | 7/12頁 |
| 文件大?。?/td> | 156K |
| 代理商： | LTC3408EDD |
7
LTC3408
3408f
OPERATIOU
off and on the bypass P-channel MOSFET with a frequency
of approximately 50kHz to 100kHz at 1.6A peak current.
This will continue until the short is removed. While the
bypass P-channel MOSFET is pulsing intermittently, the
inherent current limit of the step-down regulator limits its
peak current to about 1A.
(Refer to Functional Diagram)
Dropout Operation
If the reference voltage would cause V
OUT
to exceed V
IN
,
the LTC3408 enters dropout operation. During dropout,
the main switch remains on continuously and operates at
100% duty cycle. If the voltage at REF is less than 1.2V, the
bypass P-channel MOSFET will stay off even in dropout
operation. The output voltage is then determined by the
input voltage minus the voltage drop across the main switch
and the inductor. If the voltage at REF is greater than 1.2V,
Figure 2. Maximum Output Current vs Input Voltage
SUPPLY VOLTAGE (V)
2.5
M
1200
1000
800
600
400
200
0
3.0
3.5
4.0
4.5
3408 F02
5.0
5.5
V
OUT
= 1.8V
V
OUT
= 1.5V
V
OUT
= 2.5V
but less than V
IN
/3, the bypass P-channel MOSFET will be
on, but the main switch will be off. For best performance
and lowest voltage drop from V
IN
to V
OUT
, always ensure
that the REF voltage is greater than both 1.2V and V
IN
/3.
An important detail to remember is that at low input
supply voltages, the R
DS(ON)
of the P-channel switch
increases (see Typical Performance Characteristics).
Therefore, the user should calculate the power dissipa-
tion when the LTC3408 is used at 100% duty cycle with
low input voltage (See Thermal Considerations in the
Applications Information section).
Low Supply Operation
The LTC3408 will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles >40%. However, the LTC3408 uses a
patent-pending scheme that counteracts this compensat-
ing ramp, which allows the maximum inductor peak
current to remain unaffected throughout all duty cycles.
APPLICATIOU
The basic LTC3408 application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection of L followed by
C
IN
and C
OUT
.
W
U
U
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 4
μ
H to 6
μ
H. Its value is chosen based on the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. As Equation 1 shows, a greater difference be-
tween V
IN
and V
OUT
produces a larger ripple current.
Where these voltages are subject to change, the highest
V
IN
and lowest V
OUT
will determine the maximum ripple
current. A reasonable starting point for setting ripple
current is I
L
= 120mA (20% of the maximum load, 600mA).
=
I
f L
( )( )
V
V
V
L
OUT
OUT
IN
1
1
–
(1)
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