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参数资料
| 型号： | LM2677SDX-5.0/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 8.75 A SWITCHING REGULATOR, 280 kHz SWITCHING FREQ-MAX, DSO14 |
| 封装： | LLP-14 |
| 文件页数： | 5/26页 |
| 文件大小： | 677K |
| 代理商： | LM2677SDX-5.0/NOPB |
approximately 50%, the designer should exercise caution in
selection of the output filter components. When an application
designed to these specific operating conditions is subjected
to a current limit fault condition, it may be possible to observe
a large hysteresis in the current limit. This can affect the out-
put voltage of the device until the load current is reduced
sufficiently to allow the current limit protection circuit to reset
itself.
Under current limiting conditions, the LM267x is designed to
respond in the following manner:
1.
At the moment when the inductor current reaches the
current limit threshold, the ON-pulse is immediately
terminated. This happens for any application condition.
2.
However, the current limit block is also designed to
momentarily reduce the duty cycle to below 50% to avoid
subharmonic oscillations, which could cause the inductor
to saturate.
3.
Thereafter, once the inductor current falls below the
current limit threshold, there is a small relaxation time
during which the duty cycle progressively rises back
above 50% to the value required to achieve regulation.
If the output capacitance is sufficiently ‘large’, it may be pos-
sible that as the output tries to recover, the output capacitor
charging current is large enough to repeatedly re-trigger the
current limit circuit before the output has fully settled. This
condition is exacerbated with higher output voltage settings
because the energy requirement of the output capacitor
varies as the square of the output voltage (CV2), thus re-
quiring an increased charging current.
A simple test to determine if this condition might exist for a
suspect application is to apply a short circuit across the output
of the converter, and then remove the shorted output condi-
tion. In an application with properly selected external compo-
nents, the output will recover smoothly.
Practical values of external components that have been ex-
perimentally found to work well under these specific operating
conditions are C
OUT = 47F, L = 22H. It should be noted that
even with these components, for a device’s current limit of
I
CLIM, the maximum load current under which the possibility of
the large current limit hysteresis can be minimized is I
CLIM/2.
For example, if the input is 24V and the set output voltage is
18V, then for a desired maximum current of 1.5A, the current
limit of the chosen switcher must be confirmed to be at least
3A.
SIMPLE DESIGN PROCEDURE
Using the nomographs and tables in this data sheet (or use
the available design software at http://www.national.com) a
complete step-down regulator can be designed in a few sim-
ple steps.
Step 1:
Define the power supply operating conditions:
Required output voltage
Maximum DC input voltage
Maximum output load current
Step 2:
Set the output voltage by selecting a fixed output
LM2677 (3.3V, 5V or 12V applications) or determine the re-
quired feedback resistors for use with the adjustable LM2677
ADJ
Step 3:
Determine the inductor required by using one of the
a specific manufacturer and part number for the inductor.
Step 4:
Using Table 3 (fixed output voltage) or Table 6 (ad-
justable output voltage), determine the output capacitance
required for stable operation. Table 2 provides the specific
capacitor type from the manufacturer of choice.
Step 5:
Determine an input capacitor from Table 4 for fixed
output voltage applications. Use Table 2 to find the specific
capacitor type. For adjustable output circuits select a capac-
itor from Table 2 with a sufficient working voltage (WV) rating
greater than Vin max, and an rms current rating greater than
one-half the maximum load current (2 or more capacitors in
parallel may be required).
Step 6:
Select a diode from Table 5. The current rating of the
diode must be greater than I load max and the Reverse Volt-
age rating must be greater than Vin max.
Step 7:
Include a 0.01
μF/50V capacitor for Cboost in the de-
sign.
FIXED OUTPUT VOLTAGE DESIGN EXAMPLE
A system logic power supply bus of 3.3V is to be generated
from a wall adapter which provides an unregulated DC volt-
age of 13V to 16V. The maximum load current is 2.5A.
Through-hole components are preferred.
Step 1:
Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 2.5A
Step 2:
Select an LM2677T-3.3. The output voltage will have
a tolerance of
±2% at room temperature and ±3% over the full operating
temperature range.
Step 3:
Use the nomograph for the 3.3V device ,Figure 3. The
intersection of the 16V horizontal line (V
in max) and the 2.5A
vertical line (I
load max) indicates that L33, a 22μH inductor, is
required.
From Table 1, L33 in a through-hole component is available
from Renco with part number RL-1283-22-43 or part number
PE-53933 from Pulse Engineering.
Step 4:
Use Table 3 to determine an output capacitor. With a
3.3V output and a 22
μH inductor there are four through-hole
output capacitor solutions with the number of same type ca-
pacitors to be paralleled and an identifying capacitor code
given. Table 2 provides the actual capacitor characteristics.
Any of the following choices will work in the circuit:
1 x 220
μF/10V Sanyo OS-CON (code C5)
1 x 1000
μF/35V Sanyo MV-GX (code C10)
1 x 2200
μF/10V Nichicon PL (code C5)
1 x 1000
μF/35V Panasonic HFQ (code C7)
Step 5:
Use Table 4 to select an input capacitor. With 3.3V
output and 22
μH there are three through-hole solutions.
These capacitors provide a sufficient voltage rating and an
rms current rating greater than 1.25A (1/2 I
load max). Again
using Table 2 for specific component characteristics the fol-
lowing choices are suitable:
1 x 1000
μF/63V Sanyo MV-GX (code C14)
1 x 820
μF/63V Nichicon PL (code C24)
1 x 560
μF/50V Panasonic HFQ (code C13)
Step 6:
From Table 5 a 3A Schottky diode must be selected.
For through-hole components 20V rated diodes are sufficient
and 2 part types are suitable:
1N5820
SR302
Step 7:
A 0.01
μF capacitor will be used for Cboost.
ADJUSTABLE OUTPUT DESIGN EXAMPLE
In this example it is desired to convert the voltage from a two
battery automotive power supply (voltage range of 20V to
28V, typical in large truck applications) to the 14.8VDC alter-
13
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LM2677
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM2677SDX-ADJ | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LM2677SDX-ADJ/NOPB | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LM2677SX-12 | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LM2677SX-12/NOPB | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LM2677SX-3.3 | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
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