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參數(shù)資料
| 型號： | LM25011Q1MY |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, 2000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | MSOP-10 |
| 文件頁數(shù)： | 3/20頁 |
| 文件大?。?/td> | 458K |
| 代理商： | LM25011Q1MY |
plitude. The approximate operating frequency is calculated as
follows:
(1)
The buck switch duty cycle is approximately equal to:
(2)
When the load current is less than one half the inductor’s rip-
ple current amplitude the circuit operates in discontinuous
conduction mode. The off-time is longer than in continuous
conduction mode while the inductor current is zero, causing
the switching frequency to reduce as the load current is re-
duced. Conversion efficiency is maintained at light loads
since the switching losses are reduced with the reduction in
load and frequency. The approximate discontinuous operat-
ing frequency can be calculated as follows:
(3)
where R
L = the load resistance, and L1 is the circuit’s inductor.
The output voltage is set by the two feedback resistors
(R
FB1, RFB2 in the Block Diagram). The regulated output volt-
age is calculated as follows:
V
OUT = 2.51V x (RFB1 + RFB2) / RFB1
(4)
Ripple voltage, which is required at the input of the regulation
comparator for proper output regulation, is generated inter-
nally in the LM25011, and externally when the LM25011A is
used. In the LM25011 the ERM (Emulated Ripple Mode) con-
trol circuit generates the required internal ripple voltage from
the ripple waveform at the CS pin. The LM25011A, which is
designed for higher frequency operation, requires additional
ripple voltage, which must be generated externally and pro-
vided to the FB pin. This is described in the Applications
Information section.
On-Time Timer
The on-time for the LM25011/LM25011A is determined by the
R
T resistor and the input voltage (VIN), calculated from:
(5)
The inverse relationship with V
IN results in a nearly constant
frequency as V
IN is varied. To set a specific continuous con-
duction mode switching frequency (F
S), the RT resistor is
determined from the following:
(6)
The on-time must be chosen greater than 90 ns for proper
operation. Equations 1, 5 and 6 are valid only during normal
operation - i.e., the circuit is not in current limit. When the
LM25011 operates in current limit, the on-time is reduced by
40% (this feature is not present in LM25011A). This feature
reduces the peak inductor current which may be excessively
high if the load current and the input voltage are simultane-
ously high. This feature operates on a cycle-by-cycle basis
until the load current is reduced and the output voltage re-
sumes its normal regulated value. The maximum continuous
current into the RT pin must be less than 2 mA. For high fre-
quency applications, the maximum switching frequency is
limited at the maximum input voltage by the minimum on-time
one-shot period (90 ns). At minimum input voltage the maxi-
mum switching frequency is limited by the minimum off-time
one-shot period, which, if reached, prevents achievement of
the proper duty cycle.
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the voltage across the external current sense resistor
R
S. Referring to the Block Diagram, during the off-time the
recirculating current flows through the inductor, through the
load, through the sense resistor, and through D1 to the in-
ductor. If the voltage across the sense resistor exceeds the
threshold (V
ILIM) the current limit comparator output switches
to delay the start of the next on-time period. The next on-time
starts when the recirculating current decreases such that the
voltage across R
S reduces to the threshold and the voltage at
FB is below 2.51V. The operating frequency is typically lower
due to longer-than-normal off-times. When current limit is de-
tected, the on-time is reduced by
40% (only in LM25011) if
the voltage at the FB pin is below its threshold when the volt-
age across R
S reduces to its threshold (VOUT is low due to
current limiting).
Figure 2 illustrates the inductor current waveform during nor-
mal operation and in current limit. During the first “Normal
Operation” the load current is I
01, the average of the inductor
current waveform. As the load resistance is reduced, the in-
ductor current increases until the lower peak of the inductor
ripple current exceeds the threshold. During the “Current Lim-
ited” portion of Figure 2, each on-time is reduced by
40%,
resulting in lower ripple amplitude for the inductor’s current.
During this time the LM25011 is in a constant current mode
with an average load current equal to the current limit thresh-
old plus half the ripple amplitude (I
OCL), and the output voltage
is below the normal regulated value. Normal operation re-
sumes when the load current is reduced (to I
O2), allowing
V
OUT and the on-time to return to their normal values. Note
that in the second period of “Normal Operation”, even though
the inductor’s peak current exceeds the current limit threshold
during part of each cycle, the circuit is not in current limit since
the inductor current falls below the current limit threshold dur-
ing each off time. The peak current allowed through the buck
switch is 3.5A, and the maximum allowed average current is
2.0A.
11
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LM25011/LM25011QLM25011A/LM25011AQ
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM25011Q1MY/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM25011Q1MYX | 制造商:TI 制造商全稱:Texas Instruments 功能描述:42V 2A Constant On-Time Switching Regulator With Adjustable Current Limit |
| LM25011Q1MYX/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM25017 | 制造商:TI 制造商全稱:Texas Instruments 功能描述:48V, 650mA Constant On-Time Synchronous Buck Regulator |
| LM25017MR/NOPB | 功能描述:直流/直流開關(guān)調(diào)節(jié)器 48V 650mA Sync SD Reg w/ Intg MOSFETs RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5 |
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