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參數(shù)資料
| 型號： | LTC1704EGN#TRPBF |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 22/28頁 |
| 文件大小： | 347K |
| 描述： | IC REG DL BUCK/LINEAR 16SSOP |
| 標(biāo)準(zhǔn)包裝： | 2,500 |
| 拓?fù)洌?/td> | 降壓（降壓）（1），線性（LDO）（1） |
| 功能： | 任何功能 |
| 輸出數(shù)： | 2 |
| 頻率 - 開關(guān)： | 550kHz |
| 電壓/電流 - 輸出 1： | 控制器 |
| 電壓/電流 - 輸出 2： | 控制器 |
| 帶 LED 驅(qū)動器： | 無 |
| 帶監(jiān)控器： | 無 |
| 帶序列發(fā)生器： | 無 |
| 電源電壓： | 3.15 V ~ 5.5 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 16-SSOP（0.154"，3.90mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 16-SSOP |
| 包裝： | 帶卷 (TR) |
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LTC1704/LTC1704B
22
1704bfa
APPLICATIO S I FOR ATIO
U
U
U
is the same as the output current. The LTC1704 current
limit circuit inverts the voltage at I
MAX
before comparing
it with the negative voltage across QB, allowing the current
limit to be set with a positive voltage.
To set the current limit, calculate the expected voltage
drop across QB at the maximum desired current:
V
PROG
= (I
LIMIT
)(R
DS(ON)
)
I
LIMIT
should be chosen to be quite a bit higher than the
expected operating current, to allow for MOSFET R
DS(ON)
changes with temperature. Setting I
LIMIT
to 150% of the
maximum normal operating current is usually safe and will
adequately protect the power components if they are
chosen properly. Note that the ringing on the switch node
can cause error for the current limit threshold (illustrated
in Figure 6). This factor will change depending on the
layout and the components used. V
PROG
is then pro-
grammed at the I
MAX
pin using the internal 10礎(chǔ) pull-up
and an external resistor:
R
IMAX
= V
PROG
/10礎(chǔ)
The resulting value of R
IMAX
should be checked in an ac-
tual circuit to ensure that the current circuit kicks in as
expected. MOSFET R
DS(ON)
specs are like horsepower
ratings in automobiles, and should be taken with a grain of
salt. Circuits that use very low values for R
IMAX
(<10k)
should be checked carefully, since small changes in R
IMAX
can cause large I
LIMIT
changes when the switch node ring-
ing makes up a large percentage of the total V
PROG
value.
If V
PROG
is set too low, the LTC1704 may fail to start up.
Accuracy Trade-Offs
The V
DS
sensing scheme used in the LTC1704 is not
particularly accurate, primarily due to uncertainty in the
R
DS(ON)
from MOSFET to MOSFET. A second error term
arises from the ringing present at the SW pin, which
causes the V
DS
to look larger than (I
LOAD
)(R
DS(ON)
) at the
beginning of QBs on-time. Another important error is due
to poor PCB layout. Care should be taken to ensure that
proper kelvin sensing of the SW pin is provided. These
inaccuracies do not prevent the LTC1704 current limit
circuit from protecting itself and the load from damaging
overcurrent conditions, but they do prevent the user from
setting the current limit to a tight tolerance if more than
one copy of the circuit is being built. The 50% factor in the
current setting equation above reflects the margin neces-
sary to ensure that the circuit will stay out of current limit
at the maximum normal load, even with a hot MOSFET that
is running quite a bit higher than its R
DS(ON)
spec.
REGULATION OVER COMPONENT
TOLERANCE/TEMPERATURE
DC Regulation Accuracy
The LTC1704s switcher controller initial DC output accu-
racy depends mainly on internal reference accuracy and
internal op amp offset. Two LTC1704 specs come into
play: feedback voltage and feedback voltage line regula-
tion. The feedback voltage spec is 800mV ?2mV over the
full temperature range and is specified at the FB pin, which
encompasses both reference accuracy and any op amp
offset. This accounts for 1.5% error at the output with a 5V
input supply. The feedback voltage line regulation spec
adds an additional 0.1%/V term that accounts for change
in reference output with change in input supply voltage.
With a 5V supply, the errors contributed by the LTC1704
itself add up to no more than 1.5% DC error at the output.
The output voltage setting resistors (see R1 and R2 in the
Typical Applications) are the other major contributor to DC
error. At a typical 1.xV output voltage, the resistors are of
roughly the same value, which tends to halve their error
terms, improving accuracy. Still, using 1% resistors for
R1 and R2 will add 1% to the total output error budget.
Using 0.1% resistors in just those two positions can nearly
halve the DC output error for very little additional cost.
Load Regulation
Load regulation is affected by feedback voltage, feedback
amplifier gain and external ground drops in the feedback
path. Feedback voltage is covered above and is within
1.5% over temperature. A full range load step might
require a 10% duty cycle change to keep the output
constant, requiring the COMP pin to move about 100mV.
With amplifier gain at 85dB, this adds up to only a 10礦
shift at FB, negligible compared to the reference accuracy
terms.
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