Voltage stabilizing chips (such as LDOs, switching regulators, etc.) must be combined with other peripheral components in practical circuit design to work stably, reliably, and safely. The functions of these supporting components include filtering, energy storage, feedback, protection, heat dissipation, etc. The following are the most common supporting components and their functions:
1、 Core essential components
Input/output capacitance:
Input capacitance (Cin):
Function: Energy storage (smoothing input voltage), filtering out noise and ripple at the input end, providing instantaneous current during chip startup/load transients.
Type: Usually uses ceramic capacitors (MLCC) (low ESR, good high-frequency characteristics), sometimes parallel aluminum electrolytic capacitors or tantalum capacitors (large capacity, low-frequency energy storage). Place it close to the input pins of the chip!
Output capacitance (Cout):
Function: Stabilize output voltage, reduce output ripple and noise, improve transient response capability, key components for loop stability (especially LDO).
Type: Low ESR ceramic capacitors (MLCC) are preferred. Some chips (such as some LDOs) have specific requirements for ESR and may require specific series or parallel capacitors. Electrolytic capacitors can be connected in parallel in high current or high ripple situations. Place it close to the output pins of the chip!
Capacity selection: Strictly follow the recommended values in the chip data manual, usually with minimum values and type requirements. Being too small may lead to instability or excessive ripple; Excessive size may increase costs and size, and result in high surge current during startup.
Feedback resistor network (adjustable output regulator):
Function: For voltage regulators with adjustable output voltage (such as LM317, FB pin of switching power supply), a portion of the output voltage is fed back to the error amplifier inside the chip through a resistive voltage divider network, setting the precise output voltage.
Selection: Use resistors with higher accuracy (such as 1%). The selection of resistance value should refer to the data manual. Usually, the feedback current is very small (uA level), and the resistance value is in the k Ω level (such as tens of k Ω) to avoid being too large (susceptible to noise interference) or too small (increasing power consumption). Calculation: Vout=Vref (1+R1/R2).
2、 Important enhancement/protection components
Freewheeling diode (Buck topology for switching regulators):
Function: In a Buck converter, when the switching transistor (usually an internal MOSFET) is turned off, it provides a freewheeling loop for the inductor current and maintains continuous current.
Type: Fast recovery diode or Schottky diode (low forward voltage drop Vf, fast switching characteristics) must be used. Ordinary diodes are not suitable!
Selection: The rated current should be greater than the maximum output current, and the reverse withstand voltage should be greater than the maximum input voltage. Place close to the SW pins and inductors of the chip!
Power inductor (switching regulator):
Function: The core energy storage component of a switching power supply (Buck, Boost, BuckBoost). By controlling the on/off of the switch tube, voltage conversion and filtering are achieved during energy storage (increasing current) and energy release (decreasing current) processes.
Selection: Key parameters include inductance value, saturation current, DC resistance, and self resonant frequency.
Inductance value: strictly calculate according to the formula in the data manual or select the recommended value. Too small leads to large ripple current, low efficiency, and possible instability; Being too large leads to slow dynamic response, large size, and high cost.
Saturation current: It must be greater than the peak current in the circuit (usually the maximum output current+1/2 ripple current), otherwise the inductance will saturate and fail.
DCR: The lower the DC resistance, the smaller the conduction loss and the higher the efficiency.
Type: Commonly used shielded power inductor (reduces EMI). Place near the SW pin and freewheeling diode of the chip!
Bootstrap capacitor (switching regulator high side drive):
Function: For Buck chips that use N-channel MOSFETs as high side switches, a bootstrap circuit is required to provide a driving voltage higher than the input voltage to its gate. The bootstrap capacitor stores energy for the circuit.
Selection: Use low ESR ceramic capacitors with a capacity strictly in accordance with the data manual requirements (usually 0.1uF 1uF). Place near the BOOT and SW pins of the chip!
Input protection diode:
Function:
Reverse connection protection: Connect a diode in series at the input end to prevent damage to the chip caused by reverse connection of the power supply. It will bring voltage drop and power consumption.
Reverse current blocking: prevents current backflow and damage to the chip or input capacitor when the output voltage is higher than the input voltage (such as power failure or hot plugging). Schottky diodes (low voltage drop) are usually used.
Selection: The rated current and voltage meet the requirements, taking into account power consumption.
3、 Specific situations/enhanced performance components
Enable/disable (EN/SHDN) pull-up/pull-down resistors:
Function: To provide a determined high (on) or low (off) state for the enable pin of the voltage regulator. Avoid floating and causing unexpected behavior.
Selection: If the resistance is high (such as 10k Ω 100k Ω), the power consumption should be ignored.
Soft start capacitor:
Function: Control the slope of the output voltage rise during the start-up of voltage regulators (especially switching regulators and high current LDOs). Limit the surge current during startup to prevent input voltage drop or output overshoot.
Implementation: Connect to the soft start pin (SS) of the chip. The charging time of the capacitor determines the start-up time.
Compensation network:
Function: Some voltage regulators (especially switching power supplies) may require an external RC network to optimize the stability and transient response of the control loop.
Choice: It is necessary to strictly follow the design guidance of the data manual. Random changes may lead to oscillation or instability.
Heat sink/cooling measures:
Function: For medium to high power applications, the chip's own power consumption P=(Vin Vout) Iout (LDO) or switching losses (switching regulator) will generate heat. It is necessary to ensure that the junction temperature of the chip does not exceed the safe value.
Measures:
PCB copper foil heat dissipation: Fully utilize the heat dissipation pads of the chip, design a large area of copper and connect it to the ground plane, and use a via array to conduct heat to the inner or back copper layer.
External heat sink: When the PCB heat dissipation is insufficient, heat sinks need to be installed on the chip package.
Forced air cooling: In extreme cases, a fan is required.
EMI filter:
Function: Switching power supplies can generate high-frequency switching noise, which may interfere with other circuits or conduct back to the grid. Add common mode inductance, differential mode inductance, X capacitor, Y capacitor, etc. at the input end to form a filter.
Selection: Design according to noise spectrum and EMC standard requirements.
TVS diode/varistor:
Function: Provides transient overvoltage protection (such as surge and electrostatic discharge) at the input end, absorbs high-energy pulses, and protects the voltage regulator chip.
Selection: The clamping voltage should be lower than the maximum withstand voltage of the chip, and the energy absorption capacity should meet the requirements.
Fuse:
Function: Connect in series on the input power line to prevent fire or damage to the power supply in the event of a serious short circuit fault in the subsequent circuit (including the voltage regulator). Commonly used self recovering fuses (PPTC).
Selection: The rated current is slightly higher than the maximum operating current.
Summary: Core principles of supporting selection
Data sheet first: absolute priority and strict adherence to the official data sheet and application note provided by the chip manufacturer. Above are the most accurate and complete recommended circuits, component parameter calculation methods, and layout guidelines.
Close placement: Components on high-frequency, high current paths such as input/output capacitors, freewheeling diodes, power inductors, and bootstrap capacitors must be placed as close as possible to the corresponding pins of the chip to minimize loop area and parasitic parameters (inductance, resistance), which are crucial for stability, efficiency, and EMI.
Consider actual operating conditions: When selecting component parameters, consider the worst-case operating conditions (highest/lowest input voltage, maximum output current, highest operating temperature).
Stability priority: Especially the design of output capacitors and (if necessary) compensation networks, which directly affect whether the system oscillates.
Heat dissipation design: It is necessary to conduct thermal design calculations to ensure that the chip junction temperature is within a safe range.
Protection mechanism: Consider whether reverse connection protection, overvoltage protection, overcurrent protection (fuse), etc. are needed based on the application environment.
Simply put, designing a reliable voltage regulator circuit is not just about choosing a voltage regulator chip. Understanding its working principle and carefully selecting and arranging peripheral components in strict accordance with the manual is the key to success. Neglecting any link can lead to circuit instability, low efficiency, severe heating, and even damage.