The dynamic losses during turn-on and turn-off are negligible if a Schottky type catch diode is used.
When no heat sink is used, the junction temperature rise can be determined by the following:
TJ = (PD) (θJA)
(11)
To arrive at the actual operating junction temperature, add the junction temperature rise to the maximum ambient temperature.
TJ = TJ + TA (12)
If the actual operating junction temperature is greater than the selected safe operating junction temperature determined in step 3, then a heat sink is required.
When using a heat sink, the junction temperature rise can be determined by the following:
The operating junction temperature will be:
TJ = TA + TJ
J
D
JC
interface
Heat sink
T= (P) (θ+θ+θ)
(13) (14)
如公式14所示，如果实际工作结温高于选定的安全工作结温，则需要更大的散热器（散热片的热阻较低）。
在塑料CDIP或表面贴装SOIC封装中使用LM2575时，应了解有关封装热性能的几项内容。大部分热量通过引线传导到封装中，而一小部分则通过封装的塑料部分传导出去。由于引线框架是实心铜，因此来自芯片的热量很容易通过引线传导至充当散热器的印刷电路板铜。
为了获得最佳的散热性能，应将接地引脚和所有未连接的引脚焊接到大量印刷电路板铜上，例如接地层。大面积的铜可以将热量最好地传递到周围的空气中。即使两侧之间没有直接的铜接触，电路板两侧的铜也有助于将热量散发到封装中。通过精心设计的印刷电路板，可以实现SOIC封装的热阻值低至40°C / W，而CDIP封装的热阻值低至30°C / W。
Switchers Made Simple设计软件中包含一个更精确的（非线性）热模型，该模型可用于确定具有不同输入输出参数或不同组件值的结温。它还可以计算将调节器的结温保持在最大工作温度以下所需的散热器热阻。
ADDITIONAL APPLICATIONS
INVERTING REGULATOR
图32显示了buck-boost配置的LM2575-12，可从正输入电压产生负12V输出。该电路将调节器的接地引脚自举到负输出电压，然后通过将反馈引脚接地，调节器检测反相的输出电压并将其调节至-12V。
对于12V或更高的输入电压，此配置中的最大可用输出电流约为0.35A。在较轻的负载下，所需的最小输入电压降至约4.7V。
该降压-升压配置中的开关电流高于标准降压模式设计中的开关电流，因此降低了可用的输出电流。同样，降压-升压转换器的启动输入电流高于标准的降压模式调节器，这可能会使输入电源的电流限制小于1.5A。使用延迟的导通或欠压锁定电路（在负升压稳压器部分中进行了介绍）将允许输入电压上升到足够高的水平，然后才允许开关打开。
由于降压和降压-升压调节器拓扑结构之间的结构差异，因此降压调节器的设计过程部分不能用于选择电感器或输出电容器。降压-升压设计的推荐电感值范围在68μH至220μH之间，并且输出电容器的值必须大于降压设计通常所需的值。低输入电压或高输出电流要求使用大容量输出电容器（以数千微法拉为单位）。
The peak inductor current, which is the same as the peak switch current, can be calculated from the following formula:
where
? fosc = 52 kHz.
(15)
Under normal continuous inductor current operating conditions, the minimum VIN represents the worst case. Select an inductor that is rated for the peak current anticipated.
Also, the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage. For a ?12V output, the maximum input voltage for the LM2575 is +28V, or +48V for the LM2575HV.
The Switchers Made Simple (version 3.3) design software can be used to determine the feasibility of regulator designs using different topologies, different input-output parameters, different components, and so on.
Figure 32. Inverting Buck-Boost Develops ?12V
NEGATIVE BOOST REGULATOR
降压-升压拓扑的另一种变化是负升压配置。 图33中的电路接受-5V至-12V的输入电压，并提供稳定的-12V输出。 输入电压大于-12V将导致输出升高至-12V以上，但不会损坏稳压器。
由于这种类型的调节器具有升压功能，因此开关电流相对较高，尤其是在低输入电压下。 输出负载电流限制是开关最大额定电流的结果。 此外，升压调节器在负载短路的情况下也不能提供限流负载保护，因此可能需要其他一些方法（例如保险丝）。
V
IN
1
LM2575-12
Feedback
4
Output
+ COUT
1000 PF Low ESR
+ CIN
100 PF
3 GND
5
ON/OFF
2
1N5817
VOUT = -12V
-VIN
150 PH
-5V to -12V
Typical Load Current 200 mA for VIN = ?5.2V
500 mA for VIN = ?7V
Pin numbers are for TO-220 package.
Figure 33. Negative Boost
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold. An undervoltage lockout circuit which accomplishes this task is shown in Figure 34, while Figure 35 shows the same circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches a predetermined level.
VTH ≈ VZ1 + 2VBE (Q1) (16)
DELAYED STARTUP
The ON /OFF pin can be used to provide a delayed startup feature as shown in Figure 36. With an input voltage of 20V and for the part values shown, the circuit provides approximately 10 ms of delay time before the circuit begins switching. Increasing the RC time constant can provide longer delay times. But excessively large RC time constants can cause problems with input voltages that are high in 60 Hz or 120 Hz ripple, by coupling the ripple into the ON /OFF pin.
ADJUSTABLE OUTPUT, LOW-RIPPLE POWER SUPPLY
A 1A power supply that features an adjustable output voltage is shown in Figure 37. An additional L-C filter that reduces the output ripple by a factor of 10 or more is included in this circuit.
Complete circuit not shown.
Pin numbers are for the TO-220 package.
Figure 34. Undervoltage Lockout for Buck Circuit
Complete circuit not shown (see Figure 32). Pin numbers are for the TO-220 package.
Figure 35. Undervoltage Lockout
for Buck-Boost Circuit
Complete circuit not shown.
Pin numbers are for the TO-220 package.
Figure 36. Delayed Startup
Pin numbers are for the TO-220 package.
Figure 37. 1.2V to 55V Adjustable 1A Power Supply with Low Output Ripple
Definition of Terms
BUCK REGULATOR A switching regulator topology in which a higher voltage is converted to a lower voltage.
Also known as a step-down switching regulator. BUCK-BOOST REGULATOR A switching regulator topology in which a positive voltage is converted to a
negative voltage without a transformer.
DUTY CYCLE (D) Ratio of the output switch'son-time to the oscillator period.
(17)
CATCH DIODE OR CURRENT STEERING DIODE The diode which provides a return path for the load current
when the LM2575 switch is OFF.
EFFICIENCY (η) The proportion of input power actually delivered to the load.
(18)
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR) The purely resistive component of a real capacitor's
impedance (see Figure 38). It causes power loss resulting in capacitor heating, which directly affects the capacitor'soperating lifetime. When used as a switching regulator output filter, higher ESR values result in higher output ripple voltages.
Figure 38. Simple Model of a Real Capacitor
Most standard aluminum electrolytic capacitors in the 100 μF–1000 μF range have 0.5Ω to 0.1Ω ESR. Higher-grade capacitors (“low-ESR”, “high-frequency”, or “low-inductance”')in the 100 μF–1000 μF range generally have ESR of less than 0.15Ω.
EQUIVALENT SERIES INDUCTANCE (ESL) The pure inductance component of a capacitor (see Figure 38).
The amount of inductance is determined to a large extent on the capacitor'sconstruction. In a buck regulator, this unwanted inductance causes voltage spikes to appear on the output. OUTPUT RIPPLE VOLTAGE The AC component of the switching regulator'soutput voltage. It is usually
dominated by the output capacitor'sESR multiplied by the inductor'sripple current ( IIND). The peak-to-peak value of this sawtooth ripple current can be determined by reading INDUCTOR RIPPLE CURRENT.
LM2575HVS-ADJTI高压三端稳压器 - 图文
