米6体育平台手机版

ZHCSM29A September 2023 – June 2024 LM70840-Q1 , LM70860-Q1 , LM70880-Q1

PRODUCTION DATA

请参考 PDF 数据表获取器件具体的封装图。

机械数据 (封装 | 引脚)

RRX|29

散热焊盘机械数据 (封装 | 引脚)

8.1.1.3 Input Capacitors

Input capacitors are necessary to limit the input ripple voltage to the buck power stage due to switching-frequency AC currents. TI recommends using X7S or X7R dielectric ceramic capacitors to provide low impedance and high RMS current rating over a wide temperature range. To minimize the parasitic inductance in the switching loop, position the input capacitors as close as possible to the drain of the high-side MOSFET and the source of the low-side MOSFET. The input capacitor RMS current for a single-channel buck regulator is given by Equation 15.

Equation 15.

I_{C I N (r m s)} = \sqrt{D \times (I_{O U T}^{2} \times (1 - D) + \frac{{Δ I_{L}}^{2}}{12})}

The highest input capacitor RMS current occurs at D = 0.5, at which point the RMS current rating of the input capacitors must be greater than half the output current.

Ideally, the DC component of input current is provided by the input voltage source and the AC component by the input filter capacitors. Neglecting inductor ripple current, the input capacitors source current of amplitude (I_OUT − I_IN) during the D interval and sinks I_IN during the 1−D interval. Thus, the input capacitors conduct a square-wave current of peak-to-peak amplitude equal to the output current. It follows that the resultant capacitive component of AC ripple voltage is a triangular waveform. Together with the ESR-related ripple component, the peak-to-peak ripple voltage amplitude is given by Equation 16.

Equation 16.

{Δ V}_{I N} = \frac{I_{O U T} \times D \times (1 - D)}{F_{S W} \times C_{I N}} + I_{O U T} \times R_{E S R}

The input capacitance required for a particular load current, based on an input voltage ripple specification of ΔV_IN, is given by Equation 17.

Equation 17.

C_{I N} \geq \frac{D \times (1 - D) \times I_{O U T}}{F_{S W} \times {(Δ V}_{I N} - R_{E S R} \times I_{O U T})}

Low-ESR ceramic capacitors can be placed in parallel with higher valued bulk capacitance to provide optimized input filtering for the regulator and damping to mitigate the effects of input parasitic inductance resonating with high-Q ceramics. One bulk capacitor of sufficiently high current rating and two 4.7μF X7R ceramic decoupling capacitors are usually sufficient for most applications. Select the input bulk capacitor based on the ripple current rating and operating temperature range.

Of course, a two-channel buck regulator with 180° out-of-phase interleaved switching provides input ripple current cancellation and reduced input capacitor current stress. The above equations represent valid calculations when one output is disabled and the other output is fully loaded.