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参数资料
| 型号： | LM21215MHX |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 23.5 A SWITCHING REGULATOR, 525 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封装： | TSSOP-20 |
| 文件页数： | 10/24页 |
| 文件大小： | 512K |
| 代理商： | LM21215MHX |
the open-loop gain is a magnitude of 1. It is recommended
that the f
CROSSOVER not exceed one-fifth of the switching fre-
quency. The output capacitance, C
O, depends on capacitor
chemistry and bias voltage. For Multi-Layer Ceramic Capac-
itors (MLCC), the total capacitance will degrade as the DC
bias voltage is increased. Measuring the actual capacitance
value for the output capacitors at the output voltage is rec-
ommended to accurately calculate the compensation net-
work. The example given here is the total output capacitance
using the three MLCC output capacitors biased at 1.2V, as
seen in the typical application schematic, Figure 15. Note that
it is more conservative, from a stability standpoint, to err on
the side of a smaller output capacitance value in the com-
pensation calculations rather than a larger, as this will result
in a lower bandwidth but increased phase margin.
First, a the value of R
FB1 should be chosen. A typical value is
10 k
. From this, the value of R
C1 can be calculated to set
the mid-band gain so that the desired crossover frequency is
achieved:
Next, the value of C
C1 can be calculated by placing a zero at
half of the LC double pole frequency (f
LC):
Now the value of C
C2 can be calculated to place a pole at half
of the switching frequency (f
SW):
R
C2 can then be calculated to set the second zero at the LC
double pole frequency:
Last, C
C3 can be calculated to place a pole at the same fre-
quency as the zero created by the output capacitor ESR:
An illustration of the total loop response can be seen in Figure
10
100
1k
10k
100k
1M
-50
0
50
100
150
200
-40
-20
0
20
40
60
80
100
120
140
160
GAIN
(dB)
FREQUENCY (Hz)
PHASE
MARGIN
(°)
GAIN
PHASE
30103739
FIGURE 11. Loop Response
It is important to verify the stability by either observing the load
transient response or by using a network analyzer. A phase
margin between 45° and 70° is usually desired for voltage
mode systems. Excessive phase margin can cause slow sys-
tem response to load transients and low phase margin may
cause an oscillatory load transient response. If the load step
response peak deviation is larger than desired, increasing
f
CROSSOVER and recalculating the compensation components
may help but usually at the expense of phase margin.
THERMAL CONSIDERATIONS
The thermal characteristics of the LM21215 are specified us-
ing the parameter
θ
JA, which relates the junction temperature
to the ambient temperature. Although the value of
θ
JA is de-
pendant on many variables, it still can be used to approximate
the operating junction temperature of the device.
To obtain an estimate of the device junction temperature, one
may use the following relationship:
and
Where:
T
J is the junction temperature in °C, PIN is the input power in
Watts (P
IN = VIN x IIN), θJA is the junction to ambient thermal
resistance for the LM21215, T
A is the ambient temperature in
°C, and I
OUT is the output load current in A.
It is important to always keep the operating junction temper-
ature (T
J) below 125°C for reliable operation. If the junction
temperature exceeds 165°C the device will cycle in and out
of thermal shutdown. If thermal shutdown occurs it is a sign
of inadequate heatsinking or excessive power dissipation in
the device.
Figure 12, shown below, provides a better approximation of
the
θ
JA for a given PCB copper area. The PCB used in this
test consisted of 4 layers: 1oz. copper was used for the inter-
nal layers while the external layers were plated to 2oz. copper
weight. To provide an optimal thermal connection, a 3 x 5 ar-
ray of 8 mil. vias under the thermal pad were used, and an
www.national.com
18
LM21215
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM21215MHX/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 |
| LM212B | 制造商:未知厂家 制造商全称:未知厂家 功能描述:Optoelectronic |
| LM212D | 制造商:TI 制造商全称:Texas Instruments 功能描述:OPERATIONAL AMPLIFIERS/BUFFERS |
| LM212H | 制造商:Motorola Inc 功能描述: |
| LM212JG | 制造商:TI 制造商全称:Texas Instruments 功能描述:OPERATIONAL AMPLIFIERS/BUFFERS |
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