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參數(shù)資料
| 型號： | LM27241 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Synchronous Buck Regulator Controller for Mobile Systems |
| 中文描述： | 同步降壓穩(wěn)壓控制器，用于移動通信系統(tǒng) |
| 文件頁數(shù)： | 16/27頁 |
| 文件大小： | 1336K |
| 代理商： | LM27241 |
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Application Information
CURRENT LIMIT RESISTOR
The timing scheme implemented in the LM27241 makes it
possible for the IC to continue monitoring an over current
condition and to respond appropriately every cycle. This is
explained as follows.
Consider the LM27241 working under normal conditions, just
before an overload occurs.After the end of a given ON-pulse
(say ‘ton1’), the LM27241 starts sampling the current in the
low-side FET. This is the OFF-duration called ‘toff1’ in this
analysis. Therefore, if an over-current condition is detected
during this OFF-duration ‘toff1’, the controller will decide to
omit the next ON-pulse (which would have occurred during
the duration ‘ton2’). This is done by setting an internal ‘over-
current latch’ which will keep HDRV low. The LDRV will now
not only stay high during the present OFF-duration (‘toff1’)
but during the duration of the next (omitted) ON-pulse
(‘ton2’), and then as expected also during the succeeding
OFF-duration (‘toff2’). But the ‘over-current latch’ is reset at
the very start of the next OFF-duration ‘toff2’. Therefore if the
over-current condition persists, it can be recognized during
‘toff2’ and a decision to skip the next ON-pulse (duration
‘ton3’) can be taken. Finally, several ON-pulses may get
skipped until the current in the low-side FET falls below the
current limit threshold.
For this analysis, the nominal value of current sourced ILIM,
(see Electrical Characteristics table) and the R
DS_ON
of the
low-side FET at 100C should be used. This will ensure
adequate headroom without the need for excessively large
components. From the MOSFET data sheet typical R
DS_ON
at 25C is given to be 3.7m
. This value is not to be used in
the current limit calculation. The maximum FET R
DS_ON
at
room temperature is 4.8m
. During normal circuit operation,
the FET temperature will rise to a temperature greater than
25C. Therefore R
at 100C is obtained. From the
datasheet, at 100C the R
goes up typically 1.3 times
its 25C value. Therefore, the R
DS_ON
to be used in the
actual current limit calculation is:
R
DS_ON
= 1.3 x 4.8 m
= 6.42 m
Using I
ILIM
= 62μA and a value of 6.42m
for maximum
R
considering tolerances and temperature (for a given R
resistor). In order to allow for output load transients, it is a
good design practice to add margin to this calculation.
At the point where current limiting occurs (peak inductor
current becomes equal to current limit) the resistor for setting
the current limit can be calculated. The (peak) current limit
value depends on two factors:
1) The peak current in the inductor with the converter deliv-
ering maximum rated load. This should be calculated at
VIN
MAX
2) The ‘overload margin’ (above maximum load) that needs
to be maintained. This will depend on the step loads likely to
be seen in the application and the response expected. The
peak inductor current under normal operation (maximum
load) depends on the load and the inductance. It is given by:
where I
RIPPLE
was determined in the output filter section.
Example:
Let I
RIPPLE
be 2A. The peak current under normal
operation is
Usually it is necessary to set the current limit about 20%
higher than the peak inductor current. This overload margin
helps handle sudden load changes. A 20% margin will re-
quire a current limit of 11A x 1.2 = 13.2A.
A standard value of 1.37k
may be chosen
.
A larger overload margin greater than 20% (say 40%) would
help in obtaining good dynamic response. This is necessary
if the load steps from an extremely low value (say zero) up to
maximum load current. A larger current limit will, however,
generate stresses in the FETs during abnormal load condi-
tion (such as a shorted output).
A standard value of 1.58k
may be chosen
.
Summarizing, for a 1.2V/10A rated output, using a 1.9μH
inductor and any low side equivalent FET (same R
DS_ON
as
IRF7832).
For 20% overload margin, select current limit resistor to
be 1.37k
For 40% overload margin, select current limit resistor to
be 1.58k
INDUCTOR AND OUTPUT CAPACITOR
The designer is again referred to AN-1197 for the equations
required here. The design table in the referencedApplication
Note uses V
as the drop across the diode in an asynchro-
nous configuration. Also, V
is the drop across the switch
(high-side FET). In the case of the LM27241 a reasonable
approximation is to set V
= V
= 0 in the design table
available in AN-1197. Then the table can be used easily for
selection of the inductor and output capacitor. A step by step
example is also provided for a general buck regulator in the
Application Note AN-1207 at http://power.national.com.
20120119
FIGURE 12. Understanding Current Sensing
L
www.national.com
16
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| LM27241MTC | 制造商:Texas Instruments 功能描述:PWM CONTROLLER SMD 27241 TSSOP20 |
| LM27241MTC/NOPB | 功能描述:IC REG CTRLR BUCK PWM VM 20TSSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:4,000 系列:- PWM 型:電壓模式 輸出數(shù):1 頻率 - 最大:1.5MHz 占空比:66.7% 電源電壓:4.75 V ~ 5.25 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 85°C 封裝/外殼:40-VFQFN 裸露焊盤 包裝:帶卷 (TR) |
| LM27241MTCX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Synchronous Buck Regulator Controller for Mobile Systems |
| LM27241MTCX/NOPB | 功能描述:IC REG CTRLR BUCK PWM VM 20TSSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標準包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:100% 電源電壓:8.2 V ~ 30 V 降壓:無 升壓:無 回掃:是 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:是 工作溫度:0°C ~ 70°C 封裝/外殼:8-DIP（0.300"，7.62mm） 包裝:管件 產(chǎn)品目錄頁面:1316 (CN2011-ZH PDF) |
| LM27243A E WAF | 制造商:Texas Instruments 功能描述: |
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