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参数资料
| 型号： | LM27402SQX |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 1350 kHz SWITCHING FREQ-MAX, QCC16 |
| 封装： | 4 X 4 MM, 0.80 MM HEIGHT, 0.65 MM PITCH, LLP-16 |
| 文件页数： | 8/32页 |
| 文件大小： | 653K |
| 代理商： | LM27402SQX |
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30092643
FIGURE 7. Soft-Start Timing
TRACKING
The SS/TRACK pin also functions as a tracking pin when ex-
ternal power supply tracking is needed. Tracking is achieved
by simply dividing down the external supply voltage with a
simple resistor network shown in Figure 8. With the correct
resistor divider configuration, the LM27402 can track an ex-
ternal voltage source to obtain a coincident or ratiometric
startup behavior.
30092615
FIGURE 8. Tracking an External Power Supply
Since the soft-start charging current I
SS is sourced from the
SS/TRACK pin, the size of R
2 should be less than 10 k to
minimize errors in the tracking output. Once a value for R
2 is
selected, the value for R
1 can be calculated using the appro-
priate equation in Figure 9 to give the desired startup se-
quence. Figure 9 shows two common startup sequences; the
top waveform shows a coincidental startup while the bottom
waveform illustrates a ratiometric startup. A coincidental con-
figuration provides a robust startup sequence for certain ap-
plications since it avoids turning on any parasitic conduction
paths that may exist between loads. A ratiometric configura-
tion is preferred in applications where both supplies need to
be at the final value at the same time.
30092616
FIGURE 9. Tracking Startup Sequences
Similar to the soft-start function, the fastest possible startup
time is 1.28 ms regardless of the rise time of the tracking volt-
age. When using the track feature, the final voltage seen by
the SS/TRACK pin should exceed 0.8V to provide sufficient
overdrive and transient immunity.
SETTING THE SWITCHING FREQUENCY
There are two options for setting the switching frequency of
the LM27402. The frequency can be adjusted by an external
resistor from FADJ to GND, or the user can synchronize the
LM27402 to an external clock signal through SYNC. The
LM27402 will only synchronize to frequencies above the fre-
quency set by the R
FADJ resistor. The clock signal must range
from less than 0.8V to greater than 2.0V to ensure proper op-
eration. If the clock signal ceases, the switching frequency will
reduce to the frequency set by the FADJ resistor. The fre-
quency range is 200 kHz to 1.2 MHz. The sync-in clock can
synchronize a maximum of 400 kHz above the frequency set
by the resistor. To find the value of resistance needed for a
given frequency use the following equation: (f
SW (kHz),
R
FADJ (k))
SETTING THE CURRENT LIMIT THRESHOLD
The LM27402 exploits the filter inductor DCR (DC resistance)
to detect over current events. If desired, the user can employ
inductors with low tolerance DCR to increase the accuracy of
the current limit threshold. The most common topology for
sensing the DCR is shown in Figure 10.
www.national.com
16
LM27402
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM27402SQX/NOPB | 功能描述:DC/DC 开关控制器 RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK |
| LM27403 | 制造商:TI 制造商全称:Texas Instruments 功能描述:Synchronous Buck Controller with Temperature-Compensated, Inductor-DCR-Based Overcurrent Protection and Programmable Thermal Shutdown |
| LM27403EVM | 制造商:Texas Instruments 功能描述:LM27403EVM - Boxed Product (Development Kits) |
| LM27403EVM-POL600 | 制造商:Texas Instruments 功能描述:BOARD EVAL LM27403 |
| LM27403SQ | 制造商:TI 制造商全称:Texas Instruments 功能描述:Synchronous Buck Controller with Temperature-Compensated, Inductor-DCR-Based Overcurrent Protection and Programmable Thermal Shutdown |
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