ZHCSTL8F April 2007 – November 2023 LM25116
PRODUCTION DATA
- 1
- 1 特性
- 2 应用
- 3 说明
- 4 Pin Configuration and Functions
- 5 Specifications
- 6 Detailed Description
-
7 Application and Implementation
- 7.1 Application Information
- 7.2
Typical Application
- 7.2.1 Design Requirements
- 7.2.2
Detailed Design Procedure
- 7.2.2.1 Timing Resistor
- 7.2.2.2 Output Inductor
- 7.2.2.3 Current Sense Resistor
- 7.2.2.4 Ramp Capacitor
- 7.2.2.5 Output Capacitors
- 7.2.2.6 Input Capacitors
- 7.2.2.7 VCC Capacitor
- 7.2.2.8 Bootstrap Capacitor
- 7.2.2.9 Soft Start Capacitor
- 7.2.2.10 Output Voltage Divider
- 7.2.2.11 UVLO Divider
- 7.2.2.12 MOSFETs
- 7.2.2.13 MOSFET Snubber
- 7.2.2.14 Error Amplifier Compensation
- 7.2.2.15 Comprehensive Equations
- 7.2.3 Application Curves
- 7.3 Power Supply Recommendations
- 7.4 Layout
- 8 Device and Documentation Support
- 9 Revision History
- 10Mechanical, Packaging, and Orderable Information
7.2.2.6 Input Capacitors
The regulator supply voltage has a large source impedance at the switching frequency. Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the on-time. When the buck switch turns on, the current into the switch steps to the valley of the inductor current waveform, ramps up to the peak value, and then drops to zero at turnoff. The input capacitors must be selected for RMS current rating and minimum ripple voltage. A good approximation for the required ripple current rating is IRMS > IOUT / 2.
Quality ceramic capacitors with a low ESR were selected for the input filter. To allow for capacitor tolerances and voltage rating, four 2.2-µF ceramic capacitors were used for the typical application circuit. With ceramic capacitors, the input ripple voltage is triangular and peak at 50% duty cycle. Considering the capacitance change with DC bias, the input ripple voltage is approximated with Equation 17.
When the converter is connected to an input power source, a resonant circuit is formed by the line impedance and the input capacitors. If step input voltage transients are expected near the maximum rating of the LM25116, a careful evaluation of the ringing and possible overshoot at the device VIN pin must be completed. To minimize overshoot make CIN > 10 × LIN. The characteristic source impedance and resonant frequency are in Equation 18.
The converter exhibits a negative input impedance which is lowest at the minimum input voltage in Equation 19.
The damping factor for the input filter is given by Equation 20.
where
- RIN is the input wiring resistance
- ESR is the series resistance of the input capacitors
The term ZS / ZIN is always negative due to ZIN. When δ = 1, the input filter is critically damped. This may be difficult to achieve with practical component values. With δ < 0.2, the input filter exhibits significant ringing. If δ is zero or negative, there is not enough resistance in the circuit and the input filter sustains an oscillation. When operating near the minimum input voltage, an aluminum electrolytic capacitor across CIN may be required to damp the input for a typical bench test setup. Any parallel capacitor must be evaluated for its RMS current rating. The current splits between the ceramic and aluminum capacitors based on the relative impedance at the switching frequency.