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參數(shù)資料
| 型號： | LM1770SMFX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SOT-23 Synchronous Buck Controller |
| 中文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO5 |
| 封裝： | SOT-23, 5 PIN |
| 文件頁數(shù)： | 8/14頁 |
| 文件大小： | 687K |
| 代理商： | LM1770SMFX |
Application Information
THEORY OF APPLICATION
The LM1770 synchronous buck switcher has a control
scheme that is referred to as constant on-time control. This
topology relies on a fixed switch on-time to regulate the
output voltage. This on-time is internally set by EEPROM
and is available with three different set-points to allow for
different frequency options. The LM1770 automatically ad-
justs the on-time during operation inversely with the input
voltage (V
IN
) to maintain a constant frequency. Therefore the
switching frequency during continuous conduction mode is
independent of the inductor and capacitor size unlike hyster-
etic switchers.
At the beginning of the cycle the LM1770 turns on the high
side PFET for a fixed duration. This on-time is predetermined
(internally set by EEPROM and adjusted by V
) and the
switch will not turn off until the timer has completed its
period. The PFET will then turn off for a minimum pre-
determined time period. This minimum T
of 150ns is
internally set and cannot be adjusted. This is to prevent false
triggering from occurring on the comparator due to noise
from the SW node transition. After the minimum T
period
has expired, the PFET will remain off until the comparator
trip-point has been reached. Upon passing this trip-point (set
at 0.8V at the feedback pin), the PFET will turn back on and
the process will repeat, thus regulating the output.
The NFET control is complementary to the PFET control with
the exception of a short dead-time to prevent shoot through
from occurring.
DEVICE OPERATION
Timing Opinion
Three versions of the LM1770 are available each with a
predetermined T
ON
set internally by EEPROM. This T
ON
setting will determine the switching frequency for the appli-
cation. Derivation and calculation of the switching frequen-
cy’s dependence on V
IN
and T
ON
can be seen in the follow-
ing section.
In a PWM buck switcher the following equations can be
manipulated to obtain the switching frequency. The first
equation shows the standard duty-cycle equation given by
the volts-seconds balance on the inductor with the following
equations defining standard relationships:
T
ON
= D x T
P
Using this equations and solving for duty-cycle:
D = f
SW
x T
ON
Frequency can now be expressed as:
Or simply written as:
where,
α
= V
IN
x T
ON
To maintain a set frequency in an application,
α
is always
held constant by varying T
inversely with V
. The three
versions of the LM1770 are identified by the on times at a
V
of 3.3V for consistency. For clarification see the table
below:
Product ID
LM1770S
LM1770T
LM1770U
T
ON
@
3.3V
0.5μs
1.0μs
2.0μs
α
(V μs)
1.65
3.3
6.6
The variation of T
versus V
can also be expressed
graphically. These graphs can be found in the typical curves
section of the datasheet.
With
α
being a constant regardless of the version of the
LM1770 used, equation [6] shows that the only dependent
variable remaining is V
OUT
. Since V
OUT
will be a constant in
any application, the frequency will also remain constant. The
switching frequency at which the application runs depends
upon the V
OUT
desired and the LM1770 version chosen. For
any V
OUT
, three frequency options (LM1770 versions) can
be selected. This can be seen in the table below. The rec-
ommended frequency range of operation is 100kHz to
1000kHz.
VOUT
0.8
1
1.2
1.5
1.8
2.5
3.3
Timing Options
1000ns
242
303
364
455
545
758
1000
500ns
485
606
727
909
1091
1515
2000
2000ns
121
152
182
227
273
379
500
Switching Frequency (kHz) of LM1770 based on output voltage and timing
option.
SHORT-CIRCUIT PROTECTION
The LM1770 has an internal short circuit comparator that
constantly monitors the feedback node (except during soft-
start). If the feedback voltage drops below 0.55V (equivalent
to the output voltage dropping below 68% of nominal), the
comparator will trip causing the part to latch off. The LM1770
will not resume switching until the input voltage is taken
below the UVLO threshold and then brought back into its
normal operating range. The purpose of this function is to
prevent a severe short circuit from causing damage to the
application. Due to the fast transient response of the
LM1770 a severe short on the output causing the feedback
to drop would only occur if the load applied had an effective
resistance that approaches the PMOS R
DS(ON)
.
SOFT-START
To limit in-rush current and allow for a controlled startup the
LM1770 incorporates an internal soft-start scheme. Every
time the input voltage rises through the UVLO threshold the
LM1770 goes through an adaptive soft-start that limits the
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相關代理商/技術參數(shù) |
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|---|---|
| LM1770SMFX/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770TMF | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770TMF/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770TMFX | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM1770TMFX/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
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