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參數(shù)資料
| 型號： | LM5008SD/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.61 A SWITCHING REGULATOR, DSO8 |
| 封裝： | 4 X 4 MM, LLP-8 |
| 文件頁數(shù)： | 16/16頁 |
| 文件大?。?/td> | 359K |
| 代理商： | LM5008SD/NOPB |
On-Time Generator and Shutdown
The on-time for the LM5008 is determined by the R
ON resistor,
and is inversely proportional to the input voltage (Vin), result-
ing in a nearly constant frequency as Vin is varied over its
range. The on-time equation for the LM5008 is:
T
ON = 1.25 x 10
-10
x R
ON / VIN
(2)
See Figure 3. R
ON should be selected for a minimum on-time
(at maximum V
IN) greater than 400 ns, for proper current limit
operation. This requirement limits the maximum frequency for
each application, depending on V
The LM5008 can be remotely disabled by taking the R
ON/SD
pin to ground. See Figure 11. The voltage at the R
ON/SD pin
is between 1.5 and 3.0 volts, depending on Vin and the value
of the R
ON resistor.
20097907
FIGURE 11. Shutdown Implementation
Current Limit
The LM5008 contains an intelligent current limit OFF timer. If
the current in the Buck switch exceeds 0.5A the present cycle
is immediately terminated, and a non-resetable OFF timer is
initiated. The length of off-time is controlled by an external
resistor (R
CL) and the FB voltage (see Figure 5). When FB =
0V, a maximum off-time is required, and the time is preset to
35s. This condition occurs when the output is shorted, and
during the initial part of start-up. This amount of time ensures
safe short circuit operation up to the maximum input voltage
of 95V. In cases of overload where the FB voltage is above
zero volts (not a short circuit) the current limit off-time will be
less than 35s. Reducing the off-time during less severe
overloads reduces the amount of foldback, recovery time, and
the start-up time. The off-time is calculated from the following
equation:
T
OFF = 10
-5
/ (0.285 + (V
FB / 6.35 x 10
-6
x R
CL))
(3)
The current limit sensing circuit is blanked for the first 50-70ns
of each on-time so it is not falsely tripped by the current surge
which occurs at turn-on. The current surge is required by the
re-circulating diode (D1) for its turn-off recovery.
N - Channel Buck Switch and Driver
The LM5008 integrates an N-Channel Buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor and an internal high voltage diode. A 0.01F ceramic
capacitor (C4) connected between the BST pin and SW pin
provides the voltage to the driver during the on-time.
During each off-time, the SW pin is at approximately 0V, and
the bootstrap capacitor charges from Vcc through the internal
diode. The minimum OFF timer, set to 300ns, ensures a min-
imum time each cycle to recharge the bootstrap capacitor.
An external re-circulating diode (D1) carries the inductor cur-
rent after the internal Buck switch turns off. This diode must
be of the Ultra-fast or Schottky type to minimize turn-on losses
and current over-shoot.
Thermal Protection
The LM5008 should be operated so the junction temperature
does not exceed 125°C during normal operation. An internal
Thermal Shutdown circuit is provided to protect the LM5008
in the event of a higher than normal junction temperature.
When activated, typically at 165°C, the controller is forced into
a low power reset state, disabling the buck switch and the
V
CC regulator. This feature prevents catastrophic failures from
accidental device overheating. When the junction tempera-
ture reduces below 140°C (typical hysteresis = 25°C), the Vcc
regulator is enabled, and normal operation is resumed.
Applications Information
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values will be illus-
trated with a design example. Refer to Figure 1. The following
steps will configure the LM5008 for:
Input voltage range (Vin): 12V to 95V
Output voltage (V
OUT1): 10V
Load current (for continuous conduction mode): 100 mA
to 300 mA
Maximum ripple at V
OUT2: 100 mVp-p at maximum input
voltage
R1 and R2:
From Figure 1, V
OUT1 = VFB x (R1 + R2) / R2, and
since V
FB = 2.5V, the ratio of R1 to R2 calculates as 3:1.
Standard values of 3.01 k
(R1) and 1.00 k (R2) are cho-
sen. Other values could be used as long as the 3:1 ratio is
maintained. The selected values, however, provide a small
amount of output loading (2.5 mA) in the event the main load
is disconnected. This allows the circuit to maintain regulation
until the main load is reconnected.
F
s and RON: The recommended operating frequency range
for the LM5008 is 50kHz to 600 kHz. Unless the application
requires a specific frequency, the choice of frequency is gen-
erally a compromise since it affects the size of L1 and C2, and
9
www.national.com
LM5008
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM5008SDX | 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 |
| LM5008SDX/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 |
| LM5009 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:150 mA, 100V Step-Down Switching Regulator |
| LM5009_08 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:150 mA, 100V Step-Down Switching Regulator |
| LM5009A | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:100V, 150 mA Constant On-Time Buck Switching Regulator |
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