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參數(shù)資料
| 型號(hào)： | LTC1779 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 250mA Low Dropout Current Mode Step-Down DC/DC Converter in SOT-23(2.5V輸出,250mA電流模式，低壓差,步降DC/DC變換器（SOT-23封裝）) |
| 中文描述： | 250mA的低壓差電流模式降壓DC / DC轉(zhuǎn)換器采用SOT - 23（2.5V的輸出，250mA的電流模式，低壓差，步降直流/直流變換器（采用SOT - 23封裝）） |
| 文件頁數(shù)： | 8/12頁 |
| 文件大小： | 174K |
| 代理商： | LTC1779 |
8
LTC1779
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS, AVX TPSV and KEMET T510 series of surface mount
tantalum, available in case heights ranging from 2mm to
4mm. Other capacitor types include Sanyo OS-CON,
Nichicon PL series and Panasonic SP.
Low Supply Operation
Although the LTC1779 can function down to approxi-
mately 2.0V, the maximum allowable output current is
reduced when V
IN
decreases below 3V. Figure 3 shows the
amount of change as the supply is reduced down to 2V.
Also shown in Figure 4 is the effect of V
IN
on V
REF
as V
IN
goes below 2.3V.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the internal
P-channel MOSFET is a square wave of duty cycle
(V
OUT
+ V
D
)/(V
IN
+ V
D
). To prevent large voltage tran-
sients, a low ESR input capacitor sized for the maximum
RMS current must be used. The maximum RMS capaci-
tor current is given by:
C
I
V
[
V
V
V
IN
MAX
OUT
IN
OUT
IN
Required I
RMS
≈
(
)
]
1 2
/
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC1779, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple (
V
OUT
) is approximated by:
V
I
ESR
fC
OUT
RIPPLE
OUT
≈
+
1
4
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since
I
L
increases with input voltage.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR (size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for C
OUT
has been
met, the RMS current rating generally far exceeds the
I
RIPPLE(P-P)
requirement.
Figure 4. Line Regulation of V
REF
and V
ITH
INPUT VOLTAGE (V)
2.0
N
105
100
95
90
85
80
75
2.2
2.4
2.6
2.8
1779 F04
3.0
V
REF
V
ITH
Setting Output Voltage
The LTC1779 develops a 0.8V reference voltage between
the feedback (Pin 3) terminal and ground (see Figure 5). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
V
R
R
OUT
=
+
0.
2
1
APPLICATIOU
W
U
U
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