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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： | 6/24页 |
| 文件大小： | 512K |
| 代理商： | LM21215MHX |
Design Guide
OUTPUT VOLTAGE
The first step in designing the LM21215 application is setting
the output voltage. This is done by using a voltage divider
between V
OUT and AGND, with the middle node connected to
V
FB . When operating under steady-state conditions, the
LM21215 will force V
OUT such that VFB is driven to 0.6 V.
30103704
FIGURE 4. Setting V
OUT
A good starting point for the lower feedback resistor, R
FB2, is
10 k
. R
FB1 can then be calculated with the following equa-
tion:
PRECISION ENABLE
The enable (EN) pin of the LM21215 allows the output to be
toggled on and off. This pin is a precision analog input. When
the voltage exceeds 1.35V, the controller will try to regulate
the output voltage as long as the input voltage has exceeded
the UVLO voltage of 2.70V. There is an internal current
source connected to EN so if enable is not used, the device
will turn on automatically. If EN is not toggled directly the de-
vice can be preprogrammed to turn on at a certain input
voltage higher than the UVLO voltage. This can be done with
an external resistor divider from AVIN to EN and EN to AGND
as shown below in Figure 5.
30103728
FIGURE 5. Enable Startup Through Vin
The resistor values of R
A and RB can be relatively sized to
allow EN to reach the enable threshold voltage depending on
the input supply voltage. With the enable current source ac-
counted for, the equation solving for R
A is shown below:
In the above equation, R
A is the resistor from VIN to enable,
R
B is the resistor from enable to ground, IEN is the internal
enable pull-up current (2 A) and 1.35V is the fixed precision
enable threshold voltage. Typical values for R
B range from 10
k
to 100 k.
SOFT START
When EN has exceeded 1.35V, and both PVIN and AVIN
have exceeded the UVLO threshold, the LM21215 will begin
charging the output linearly to the voltage level dictated by the
feedback resistor network. The LM21215 employs a user ad-
justable soft start circuit to lengthen the charging time of the
output set by a capacitor from the soft start pin to ground. After
enable exceeds 1.35V, an internal 2 A current source begins
to charge the soft start capacitor. This allows the user to limit
inrush currents due to a high output capacitance and not
cause an over current condition. Adding a soft-start capacitor
can also reduce the stress on the input rail. Larger capacitor
values will result in longer startup times. Use the equation
below to approximate the size of the soft-start capacitor:
where I
SSis nominally 2 A and tSS is the desired startup time.
If V
IN is higher than the UVLO level and enable is toggled high
the soft start sequence will begin. There is a small delay be-
tween enable transitioning high and the beginning of the soft
start sequence. This delay allows the LM21215 to initialize its
internal circuitry. Once the output has charged to 90% of the
nominal output voltage the power good flag will transition
high. This behavior is illustrated in Figure 6.
30103731
FIGURE 6. Soft Start Timing
As shown above, the size of the capacitor is influenced by the
nominal feedback voltage level 0.6V, the soft-start charging
current I
SS (2 A), and the desired soft start time. If no soft-
start capacitor is used then the LM21215 defaults to a mini-
mum startup time of 500 s. The LM21215 will not startup
faster than 500s. When enable is cycled or the device enters
UVLO, the charge developed on the soft-start capacitor is
discharged to reset the startup process. This also happens
when the device enters short circuit mode from an over-cur-
rent event.
www.national.com
14
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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