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參數(shù)資料
| 型號： | LM2647LQX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual Synchronous Buck Regulator Controller |
| 中文描述： | DUAL SWITCHING CONTROLLER, 345 kHz SWITCHING FREQ-MAX, QCC28 |
| 封裝： | LLP-28 |
| 文件頁數(shù)： | 16/25頁 |
| 文件大?。?/td> | 763K |
| 代理商： | LM2647LQX |
Application Information
CURRENT LIMIT RESISTOR
The timing scheme implemented in the LM2647 makes it
possible for the IC to continue monitoring an overcurrent
condition and to respond appropriately every cycle. This is
explained as follows.
Consider the LM2647 working under normal conditions, just
before an overload occurs.After the end of a given ON-pulse
(say ‘ton1’), the LM2647 starts sampling the current in the
low-side FET. This is the OFF-duration called ‘toff1’ in this
analysis. Therefore, if an overcurrent 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 ‘overcurrent latch’ is reset at
the very start of the next OFF-duration ‘toff2’. Therefore if the
overcurrent 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 lower FET falls below the
current limit threshold.
Note that about 150ns after LDRV first goes high (start of
low-side conduction), the current monitoring starts. There-
fore the peak current seen by the current limit detector is
almost the same as the peak inductor current.
To set the value of the current limiting resistor (‘RLIM’, be-
tween ILIM pin and SW pin), the function of the ILIM pin must
be understood. Refer to
Figure 11
to see how the voltage on
the ILIM pin changes as current ramps up. For this analysis
note that the worst case has been taken here by using the
minimum possible value of the current sourced (I
ILIM
, see
Electrical Characteristics table). Also, the maximum value of
the ‘hot’ Rds of the lower FET should be used. For example
if the chosen low-side FET is the Si4420DY from Vishay, the
typical Rds at room temperature is 10m
(but this is not the
value to be used here). The MAX is the relevant number
which is 13m
. Now applying the thumbrule that at 100C
the Rds goes up typically 1.4 times (for 30V FETs), the Rds
to be used in the actual current limit calculation is
1.4*13m
=18.2m
. Therefore using 46μA for I
ILIM
(see
Electrical Characteristics table) and Rds = 18.2m
provide the lowest value of current limit (considering toler-
ances and temperature for a chosen RLIM resistor). This
current limit must obviously be higher than the actual peak
current in the converter under normal operation to ensure
that full rated power can be delivered under all conditions by
the converter without ‘inadvertently’ hitting the worst case
(lowest value) set current limit.
The detector sets the overcurrent latch as soon the voltage
on the ILIM pin crosses below zero. Therefore the basic
design equation for calculating RLIM is:
V = v (see
Figure 11
)
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.
But what (peak) current limit value should actually be set
This depends on two factors:
a) There is a natural steady state peak current in the inductor
with the converter delivering maximum rated load. This
should be calculated at VIN
MAX
(the maximum of the input
voltage range):
b) Over and above this steady state value we need to
provide an ‘overload margin’. This margin will depend on the
step loads likely to be seen in the application and the re-
sponse expected.
The equation for calculating the steady state peak current is:
where ‘r’ is the current ripple ratio (refer to Application Note
AN-1197 at http://power.national.com for a detailed under-
standing of how ‘r’ affects all the power components). ‘r’ is
given by:
where L is in μH, f is in Hz.
Example:
Let VIN range from 5.5V to 28V, Vo=5V, Io=3A,
L=10μH, f=300kHz. What is the peak current under normal
operation
Only the highest input voltage must be used for any peak
current calculation. At VIN
MAX
the duty cycle is D=Vo/Vin=5/
28=18%. So
Note:
In general, as discussed in AN-1197, the optimum value of ‘r’ is
between 0.3 to 0.5. Large inductances (higher than ‘optimum’) may be
20056319
FIGURE 11. Understanding Current Sensing
L
www.national.com
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM2647LQX/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)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(fēng)格: 封裝 / 箱體:CPAK |
| LM2647MTC | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)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(fēng)格: 封裝 / 箱體:CPAK |
| LM2647MTC/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)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(fēng)格: 封裝 / 箱體:CPAK |
| LM2647MTCX | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)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(fēng)格: 封裝 / 箱體:CPAK |
| LM2647MTCX/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)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(fēng)格: 封裝 / 箱體:CPAK |
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