INDUCTOR CODE L47 L68 L100 L150 L220 L330 L470 L680 H150 H220 H330 H470 H680 H1000 H1500 H2200 Table 4. Inductor Selection by Manufacturer's Part Number SCHOTT(1) PULSE ENG.(2) INDUCTOR VALUE 47 μH 671 26980 PE-53112 68 μH 671 26990 PE-92114 100 μH 671 27000 PE-92108 150 μH 671 27010 PE-53113 220 μH 671 27020 PE-52626 330 μH 671 27030 PE-52627 470 μH 671 27040 PE-53114 680 μH 671 27050 PE-52629 150 μH 671 27060 PE-53115 220 μH 671 27070 PE-53116 330 μH 671 27080 PE-53117 470 μH 671 27090 PE-53118 680 μH 671 27100 PE-53119 1000 μH 671 27110 PE-53120 1500 μH 671 27120 PE-53121 2200 μH 671 27130 PE-53122 RENCO(3) RL2442 RL2443 RL2444 RL1954 RL1953 RL1952 RL1951 RL1950 RL2445 RL2446 RL2447 RL1961 RL1960 RL1959 RL1958 RL2448 (1) Schott Corporation, (612) 475-1173, 1000 Parkers Lake Road, Wayzata, MN 55391.
(2) Pulse Engineering, (619) 674-8100, P.O. Box 12235, San Diego, CA 92112.
(3) Renco Electronics Incorporated, (516) 586-5566, 60 Jeffryn Blvd. East, Deer Park, NY 11729.
9 Power Supply Recommendations
像在任何开关稳压器中一样，布局非常重要。 与布线电感相关的快速开关电流会产生瞬态电压，这可能会引起问题。 为了使电感和接地环路最小，用粗线表示的引线长度必须尽可能短。 为了获得最佳效果，必须使用单点接地（如图所示）或接地平面结构。 当使用可调版本时，应将编程电阻器实际放置在调节器附近，以使敏感反馈布线短。
10 Layout
10.1 Layout Guidelines
电路板布局对于开关电源的正常运行至关重要。首先，接地面积必须足够用于散热。其次，必须遵循适当的准则以减少开关噪声的影响。开关模式转换器是非常快速的开关设备。在这种情况下，输入电流的快速增加与寄生走线电感的组合会产生有害的L di / dt噪声尖峰。随着输出电流的增加，这种噪声的幅度趋于增加。这种噪声可能会变成电磁干扰（EMI），还会导致设备性能出现问题。因此，请注意布局以最小化此开关噪声的影响。最重要的布局规则是保持交流电流环路尽可能小。图33显示了降压转换器中的电流。顶部示意图显示了一条虚线，该虚线代表顶部开关导通状态期间的电流。中间的示意图显示了在顶部开关断开状态期间的电流。底部示意图显示了称为交流电流的电流。这些交流电流是最关键的，因为它们会在很短的时间内变化。底部示意图的虚线是保持尽可能短和宽的迹线。这也会产生较小的环路面积，从而减小环路电感。为避免布局引起的功能问题，请查看PCB布局示例。如图 34所示
Figure 33. Current Flow in Buck Application
10.2 Layout Example
Figure 34. LM2576xx Layout Example
10.3 Grounding
为了保持输出电压的稳定性，电源接地连接必须为低阻抗（请参见图26和图32）。 对于5引线TO-220和DDPAK / TO-263型封装，接线片和引脚3都接地，并且可以使用任何一种连接方式，因为它们都是同一铜引线框架的一部分。
10.4 Heat Sink and Thermal Considerations
在许多情况下，仅需要一个小的散热器即可将LM2576的结温保持在允许的工作范围内。 对于每种应用，要确定是否需要散热器，必须确定以下几点： 1. 最高环境温度（在应用中） 2. 最大稳压器功耗（在应用中）
3. 最高允许结温（LM2576为125°C）。 为了安全，保守地设计，必须选择比最高温度低15°C的温度。 4. LM2576封装的热阻θJA和θJC。
Total power dissipated by the LM2576 can be estimated in Equation 12:
PD = (VIN)(IQ) + (VO/VIN)(ILOAD)(VSAT)
where
?
? ?
IQ (quiescent current) and VSAT can be found in the Typical Characteristics VIN is the applied minimum input voltage VO is the regulated output voltage
Heat Sink and Thermal Considerations (continued)
? ILOAD is the load current (12)
The dynamic losses during turnon and turnoff are negligible if a Schottky type catch diode is used.
When no heat sink is used, the junction temperature rise can be determined by Equation 13:
TJ = (PD)(θJA) (13)
To arrive at the actual operating junction temperature, add the junction temperature rise to the maximum ambient temperature.
TJ = TJ + TA (14)
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 Equation 15:
(15) The operating junction temperature is:
TJ = TA + TJ (16)
As in Equation 16, if the actual operating junction temperature is greater than the selected safe operating junction temperature, then a larger heat sink is required (one that has a lower thermal resistance).
J
D
JC
interface
Heat sink
T= (P)(θ+θ+θ)
11 Device and Documentation Support
11.1 Device Support
11.1.1 Device Nomenclature
11.1.1.1 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's on-time to the oscillator period.
CATCH DIODE OR CURRENT STEERING DIODE The diode which provides a return path for the load current
when the LM2576 switch is OFF.
EFFICIENCY (η) The proportion of input power actually delivered to the load.
(17)
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR) The purely resistive component of a real capacitor's
impedance (see Figure 35). It causes power loss resulting in capacitor heating, which directly affects the capacitor's operating lifetime. When used as a switching regulator output filter, higher ESR values result in higher output ripple voltages.
Figure 35. 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 to 1000 μF range generally have ESR of less than 0.15Ω.
EQUIVALENT SERIES INDUCTANCE (ESL) The pure inductance component of a capacitor (see Figure 35).
The amount of inductance is determined to a large extent on the capacitor's construction. 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's output voltage. It is usually
dominated by the output capacitor's ESR multiplied by the inductor's ripple current ( IIND). The peak-to-peak value of this sawtooth ripple current can be determined by reading Inductor Ripple Current.
CAPACITOR RIPPLE CURRENT RMS value of the maximum allowable alternating current at which a capacitor
can be operated continuously at a specified temperature.
(18)
STANDBY QUIESCENT CURRENT (ISTBY) Supply current required by the LM2576 when in the standby mode (ON /OFF pin is driven to TTL-high voltage, thus turning the output switch OFF).
INDUCTOR RIPPLE CURRENT ( IIND) The peak-to-peak value of the inductor current waveform, typically a
sawtooth waveform when the regulator is operating in the continuous mode (vs. discontinuous mode).
CONTINUOUS/DISCONTINUOUS MODE OPERATION Relates to the inductor current. In the continuous mode,
the inductor current is always flowing and never drops to zero, vs. the discontinuous mode, where the inductor current drops to zero for a period of time in the normal switching cycle.
INDUCTOR SATURATION The condition which exists when an inductor cannot hold any more magnetic flux.
LM2576HVSX-12 TI高压三端稳压器 - 图文
