ZHCS573I August 2010 – April 2018 LM25119
PRODUCTION DATA.
- 1 特性
- 2 应用
- 3 说明
- 4 修订历史记录
- 5 Pin Configuration and Functions
- 6 Specifications
-
7 Detailed Description
- 7.1 Overview
- 7.2 Functional Block Diagram
- 7.3
Feature Description
- 7.3.1 High Voltage Start-Up Regulator
- 7.3.2 UVLO
- 7.3.3 Enable 2
- 7.3.4 Oscillator and Sync Capability
- 7.3.5 Error Amplifiers and PWM Comparators
- 7.3.6 Ramp Generator
- 7.3.7 Current Limit
- 7.3.8 Hiccup Mode Current Limiting
- 7.3.9 Soft Start
- 7.3.10 HO and LO Output Drivers
- 7.3.11 Maximum Duty Cycle
- 7.3.12 Thermal Protection
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1
Dual-output Design Example
- 8.2.1.1 Design Requirements
- 8.2.1.2
Detailed Design Procedure
- 8.2.1.2.1 Timing Resistor
- 8.2.1.2.2 Output Inductor
- 8.2.1.2.3 Current Sense Resistor
- 8.2.1.2.4 Ramp Resistor and Ramp Capacitor
- 8.2.1.2.5 Output Capacitors
- 8.2.1.2.6 Input Capacitors
- 8.2.1.2.7 VCC Capacitor
- 8.2.1.2.8 Bootstrap Capacitor
- 8.2.1.2.9 Soft Start Capacitor
- 8.2.1.2.10 Restart Capacitor
- 8.2.1.2.11 Output Voltage Divider
- 8.2.1.2.12 UVLO Divider
- 8.2.1.2.13 MOSFET Selection
- 8.2.1.2.14 MOSFET Snubber
- 8.2.1.2.15 Error Amplifier Compensation
- 8.2.1.3 Application Curves
- 8.2.2 Two-Phase Design Example
- 8.2.1
Dual-output Design Example
- 9 Power Supply Recommendations
- 10Layout
- 11器件和文档支持
- 12机械、封装和可订购信息
8.2.1.2.6 Input Capacitors
The regulator input supply voltage typically has high 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 buck switch steps to the valley of the inductor current waveform, ramps up to the peak value, and then drops to the zero at turnoff. The input capacitance must be selected for RMS current rating and minimum ripple voltage. A good approximation for the required ripple current rating necessary is IRMS> IOUT / 2. Seven 2.2-μF ceramic capacitors were used for each channel. With ceramic capacitors, the input ripple voltage is triangular. The input ripple voltage with one channel operating is approximately Equation 24 and the example (Equation 25).
The ripple voltage of the input capacitors is reduced significantly with dual-channel operation because each channel operates 180 degrees out of phase from the other. Capacitors connected in parallel must be evaluated for RMS current rating. The current splits between the input capacitors based on the relative impedance of the capacitors at the switching frequency.
When the converter is connected to an input power source, a resonant circuit is formed by the line inductance and the input capacitors. To minimize overshoot make CIN> 10 x LIN. The characteristic source impedance (ZS) and resonant frequency (fS) are Equation 26 and the example (Equation 27).
where
- LIN is the inductance of the input wire
The converter exhibits negative input impedance which is lowest at the minimum input voltage in Equation 28.
The damping factor for the input filter is given by Equation 29.
where
- RIN is the input wiring resistance
- ESR is the equivalent series resistance of the input capacitors
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, a bulk aluminum electrolytic capacitor across CIN may be needed to damp the input for a typical bench test setup.