DIODES AP3428 Step-Down DC-DC Converter User Manual

AP3428 Application Note
AE Department
User Manual

Revision Information

General Description

The AP3428 is a 1A step-down DC-DC converter. At heavy load, the constant- frequency PWM control performs excellent stability and transient response. No external compensation components are required.
The AP3428 supports a range of input voltages from 2.5V to 5.5V, allowing the use of a single Li+/Li-polymer cell, multiple Alkaline/NiMH cell, and other standard power sources.
The output voltage is adjustable from 0.6V to the input voltage. The AP3428 employs internal power switch and synchronous rectifier to minimize external part count and realize high efficiency. During shutdown, the input is disconnected from the output and the shutdown current is less than 1μA. Other key features include over-temperature and short circuit protection, and under- voltage lockout to prevent deep battery discharge.
The AP3428 delivers 1A maximum output current while consuming only 40μA of no- load quiescent current. Ultra-low RDS(ON) integrated MOSFETs and 100% duty cycle operation make the AP3428 an ideal choice for high output voltage, high current applications which require a low dropout threshold. The AP3428 is available in SOT23-5, DFN2020-6 packages.

Key Features

• Output Current: Up to 1A
• Output Voltage: 0.6V to VIN
• Input Voltage: 2.5V to 5.5V
• 0.6V Reference Voltage With ±2% Precision
• 40μA (Typ) No Load Quiescent Current
• Shutdown Current: <1μA
• 100% Duty Cycle Operation
• 1.5 MHz Switching Frequency
• Internal Soft Start
• No External Compensation Required
• Current Limit Protection
• Thermal Shutdown

Applications

• Post DC-DC voltage regulation
• Set Top Boxes
• Notebook computer
• PAD

Typical Application Schematic

Application Information

1. Setting the output voltage
   The output voltage is set using a resister voltage divider from the output to FB. The output voltage is calculated as below: First, select a value for R2, then R1 is determined. The output voltage is given by Table 1.

Table 1: Resistor selection for output voltage setting

2. Inductor selection
   The inductor is used to supply smooth current to output when it is driven by a switching voltage. Its value is determined based on the operating frequency, load current, ripple current, and duty cycle. For most application, the value of the inductor will fall in the range of 2.2uH to 4.7uH. Choose an inductor that has small DC resistance, has enough current rating and is hard to cause magnetic saturation.

3. Input capacitor
   Good quality input capacitor is necessary to filter noise at input voltage source and limit the ripple voltage of input while supplying most of the switch current during the on-time. For input capacitor selection, ceramic capacitors are recommended because they provide both low impedance and small footprint. But tantalum or low electrolytic capacitor is also sufficed.
   The important parameters for the input capacitor are the voltage rating and the RMS current rating. The voltage rating should be at least 1.25 times greater than the maximum input voltage, while a voltage rating of 1.5 times is conservative. The RMS of input capacitor current is calculated as: : The RMS of input capacitor current
   As indicated by the RMS ripple current equation, highest requirement for RMS current rating occurs at 50% duty cycle. So the RMS ripple current rating of input capacitor should be greater than half the output current under this worse case. For reliable operation and best performance, ceramic capacitors are preferred for input capacitor because their low ESR and high ripple current rating. And X5R or X7R type dielectric ceramic capacitors are preferred for their better temperature and voltage characteristic. Additional, when selecting ceramic capacitor, make sure it has enough capacitance to provide sufficient charge to prevent excessive input voltage ripple. The capacitance of CIN should be more than or equal to10uF.

4. Output capacitor
   The output capacitor is selected based upon the desired output ripple and transient response.
   The output voltage ripple depends directly on the ripple current and it is affected by two parameters from the output capacitor: total capacitance and the equivalent series resistance (ESR). The output ripple voltage can be found from: : The output ripple voltage.
   : The equivalent series resistance of output capacitor.
   For lower output ripple voltage across the entire operating temperature range, X5R or X7R dielectric type of ceramic, or other low ESR tantalum capacitor or aluminum electrolytic capacitor is recommended. The capacitance of COUT should be more than or equal to 10uF and the output capacitor voltage rating should be greater than 1.5 times of maximum output voltage.

5. PCB Layout Guideline
   PCB layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of the converter and surrounding circuitry by contributing to EMI.

6. power path length
   The power path of AP3428 includes an input capacitor, output inductor and output capacitor. Place them on the same side of PCB. The GND trace, the SW trance and the VIN trance should be kept short, direct and wide. CIN must be close to Pins VIN and GND. The loop area formed by CIN and GND must be minimized.

7. Feedback Net
   Special attention should be paid to the route of the feedback wring. The feedback trace should be routed far away from the inductor and noisy power trace. Try to minimize trace length to the FB pin and connect feedback network behind the output capacitors.

8. Via Hole
   Be careful to via hole. Via will result high resistance and inductance to the power path. If heavy switching currents must be routed through vias and/or internal planes, use multiple vias parallel to reduce their resistance and inductance. Figure 6 and 7 are examples of AP3428 layer.

9. Pad PCB Design
   It is recommended that the PCB lead finger pad be designed 0.4mm toe length beyond the package body as a stander to optimize solder volume and extended 0.05mm towards the center line of the package. The dimensions of the thermal pad on the PCB should be equal to the exposed pad on the DFN. The pad must be physically connected to the PCB substrate with solder.

Rev. V1.0
Mar. 2016

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