feature

4.5V to 18V operating input voltage range; synchronous buck: 80mΩ internal high-side switch; 30mΩ internal low-side switch with integrated Schottky diode; high efficiency: up to 95%; internal soft-start; output voltage adjustable to 0.8V; 3A Continuous output current; fixed 600kHz PWM operation; z-pulse skipping at light loads for high efficiency over the entire load range; cycle-by-cycle current limit; pre-bias start-up; short circuit protection; thermal shutdown; SO-8 package.

application

Point Load DC/DC Converters; LCD TVs; Set-Top Boxes; DVD/Blu-ray Players/Recorders; Cable Modems; PCIe Graphics Cards; Telecom/Network/Data Communication Equipment.

General Instructions

The AOZ1031A is a high-efficiency, easy-to-use 3A synchronous buck regulator. The AOZ1031A operates from an input voltage range of 4.5V to 18V, provides up to 3A of continuous output current, and the output voltage is adjustable down to 0.8V.

The AOZ1031A is available in an SO-8 package and is rated for an ambient temperature range of -40°C to +85°C.

typical application

block diagram

Typical performance characteristics

The circuit of Figure 1. TA=25°C, VIN=VEN=12V, VOUT=3.3V unless otherwise specified.

AOZ1031A Efficiency

Thermal decay effect

Thermal derating curves for typical input and output conditions for an SO 8 package based on the evaluation board. 25°C ambient temperature and natural convection (wind speed <50LFM) unless otherwise specified.

Detailed description

The AOZ1031A is a current mode buck regulator with integrated high side PMOS switch and low side NMOS switch. It operates over an input voltage range of 4.5V to 18V and provides up to 3A of load current. The duty cycle can be adjusted from 6% to 100%, allowing a wide range of output voltages. Features include enable control, power-on reset, input undervoltage lockout, output overvoltage protection, active high power good state, fixed internal soft-start, and thermal shutdown. AOZ1031A is available in SO-8 package.

Enable and Soft Start

The AOZ1031A has an internal soft-start function that limits the inrush current and ensures that the output voltage rises smoothly to the regulated voltage. The soft-start process begins when the input voltage rises to 4.1V and the voltage on the EN pin is high. During soft-start, the output voltage typically rises to the regulated voltage within 2.2ms, and the 2.2ms soft-start time is internally set.

The EN pin of AOZ1031A is active high. If the enable function is not used, connect the EN pin to the VIN. Pulling it to ground will disable the AOZ1031A. Don't leave it open. The voltage on the EN pin must be higher than 2V to enable the AOZ1031A. When the voltage on the EN pin falls below 0.6V, the AOZ1031A is disabled. If the application circuit requires that the AOZ1031A be disabled, an open drain or open collector circuit should be used to connect to the EN pin.

steady state operation

Under steady-state conditions, the converter operates in fixed frequency and continuous conduction mode (CCM).

The AOZ1031A integrates an internal P-MOSFET as a high-side switch. The inductor current is sensed by amplifying the voltage drop from the drain to the source of the high-side power MOSFET. The output voltage is reduced by an external voltage divider at the FB pin. The difference between the FB pin voltage and the reference voltage is amplified by an internal transconductance error amplifier. At the PWM comparator input, the error voltage displayed on the COMP pin is compared to the current signal that is the sum of the inductor current signal and the slope compensation signal. If the current signal is less than the error voltage, the internal high side switch is turned on. Inductor current flows from the input through the inductor to the output. When the current signal exceeds the error voltage, the high side switch is turned off. The inductor current is free-wheeled for output through the internal low-side N-MOSFET switch. Internal adaptive FET drivers ensure that neither high-side nor low-side switches turn on overlap.

Compared with regulators that use self-excited Schottky diodes, the AOZ1031A uses self-excited NMOSFETs to achieve synchronous rectification. It greatly improves the efficiency of the converter and reduces the power loss of the low-voltage side switch tube.

The AOZ1031A enters discontinuous conduction mode at light loads. At very light loads, several pulses can be skipped between switching cycles, which further improves light-load efficiency.

The AOZ1031A uses a P-channel MOSFET as the high-side switch. It saves the bootstrap capacitance typically seen in circuits using NMOS switches. Allows 100% turn-on of the high-side switch for linear regulation operation. The minimum voltage drop from V to V is the load current times the DC resistance of the MOSFET plus the DC resistance of the buck inductor. Its calculation formula is as follows:

where; VoxMAX is the maximum output voltage; VIN is the input voltage between 4.5V and 18V; IO is the output current from 0A to 3A, and RDS(ON) is the on-resistance of the internal MOSFET. Depending on the input voltage and junction temperature, this value is between 97mΩ and 200mΩ.

On-off level

The AOZ1031A switching frequency is fixed and set by the internal oscillator. Due to device variations, the actual switching frequency can range from 500kHz to 700kHz.

Output voltage programming

The output voltage can be set by feeding the output back to the FB pin using a resistor divider network. in the application circuit shown in Figure 1. The resistor divider network consists of R and R. Typically, a design is started by picking a fixed value of R and calculating the required R1 using the formula below.

Table 1 on the next page lists some standard values for R, R and the most commonly used output voltage values.

The combination of R and R should be large enough to avoid drawing too much current from the output, which would result in power loss.

Since the switch duty cycle can be as high as 100%, the maximum output voltage can be set to the high input voltage minus the voltage drop across the upper PMOS and inductor.

Protection features

AOZ1031A has multiple protection functions to prevent system circuit damage under abnormal conditions.

Over Current Protection (OCP)

The sensed inductor current signal is also used for overcurrent protection. Since the AOZ1031A uses peak current mode control, the COMP pin voltage is proportional to the peak inductor current. The COMP pin voltage is internally limited between 0.4V and 2.5V. The peak current of the inductor is the automatic limit cycle.

When the output is shorted to ground under fault conditions, the inductor current decays very slowly over the switching cycle due to V=0V. To prevent catastrophic failure, the AOZ1031A detects the duration of an overcurrent condition. If an overcurrent condition occurs within a certain period of time, the AOZ1013A shuts down completely for a period of time and then restarts. If the fault persists, then the chip will disconnect again. Once the overcurrent condition disappears, the converter will initiate a soft start.

Power-On Reset (POR)

A power-on reset circuit monitors the input voltage. When the input voltage exceeds 4.1V, the inverter starts to work. When the input voltage drops below 3.7V, the inverter will shut down.

Thermal Protection

An internal temperature sensor monitors the connector temperature. When the junction temperature exceeds 150°C, the internal control circuit and the high-side PMOS are turned off. When the junction temperature drops to 100°C, the regulator will automatically restart under the control of the soft-start circuit.

application information

The basic AOZ1031A application circuit is shown in Figure 1. Component selection is described below.

input capacitor

An input capacitor must be connected to the V pin and PGND pin of the AOZ1031A to maintain a stable input voltage and filter out pulsed input current. The voltage rating of the input capacitor must be greater than the maximum input voltage and ripple voltage. The ripple voltage at the input can be approximated by the following equation:

Since the input current of a buck converter is discontinuous, the current stress on the input capacitor is another consideration when choosing capacitors. For a buck circuit, the rms value of the input capacitor current can be calculated by the following formula:

If we let m equal the conversion ratio:

Calculate the relationship between the input capacitor rms current and voltage slew rate as shown in Figure 2 on the next page. It can be seen that the current stress of C is the largest when V is half of V. The worst current stress on C is 0.5x I.

For reliable operation and optimum performance, the input capacitor current rating must be higher than the worst-case operating conditions of Iat. Ceramic capacitors are the preferred input capacitors because of their low ESR and high current ratings. Other low ESR tantalum capacitors can also be used depending on the application circuit. When choosing ceramic capacitors, X5R or X7R type dielectric ceramic capacitors should be used for better temperature and voltage characteristics. Note that capacitor manufacturers' ripple current ratings are based on a certain lifetime. Actual designs may require further derating.

sensor

The inductor is used to provide a constant current output when it is driven by a switching voltage. For a given input and output voltage, the inductor and switching frequency together determine the inductor ripple current, which is:

The peak inductor current is:

High inductance provides low inductor ripple current, but requires larger size inductors to avoid saturation. Low ripple current reduces inductor core losses. It also reduces the rms current through the inductor and switch, thereby reducing conduction losses. Typically, the peak-to-peak ripple current on the inductor is designed to be 20% to 30% of the output current.

When choosing an inductor, make sure it can handle peak currents without saturation even at the highest operating temperature.

The inductor accepts the highest current in the buck circuit. Conduction losses on inductors need to be checked for compliance with thermal efficiency requirements.

Coilcraft, Elytone and Murata offer surface mount sensors in different shapes and styles. The shielding inductance is small in size, and the radiated electromagnetic interference is small. But they are more expensive than unshielded inductors. The choice depends on EMI requirements, price and size.

output capacitor

Select the output capacitor based on the DC output voltage rating, output ripple voltage specification, and ripple current rating.

The selected output capacitor must have a higher voltage rating than the maximum expected output voltage including ripple. Long-term reliability requires consideration of degradation.

The output ripple voltage specification is another important factor in selecting an output capacitor. In a buck converter circuit, the output ripple voltage is determined by the inductor value, switching frequency, output capacitor value, and ESR. It can be calculated by the following formula:

where; CO is the output capacitance value and ESRCO is the equivalent series resistance of the output capacitor.

When using a low ESR ceramic capacitor as the output capacitor, the impedance of the capacitor at the switching frequency dominates. The output ripple is mainly caused by the capacitor value and the inductor ripple current. The output ripple voltage calculation can be simplified as:

If the impedance of the ESR at the switching frequency dominates, the output ripple voltage is dominated by the capacitor ESR and inductor ripple current. The output ripple voltage calculation can be further simplified as:

To reduce the output ripple voltage over the entire operating temperature range, it is recommended to use X5R or X7R dielectric ceramic or other low ESR tantalum as the output capacitor.

In a buck converter, the output capacitor current is continuous. The rms current of the output capacitor is determined by the peak-to-peak ripple current of the inductor. The calculation method is as follows:

Usually, the ripple current rating of the output capacitor is a lesser concern due to the low current stress. When the buck inductor is chosen to be small and the inductor ripple current is large, the output capacitor will be overstressed.

loop compensation

AOZ1031A adopts peak current mode control, which is easy to use and has fast transient response. Peak current mode control eliminates the bipolar effect of the output L&C filter. This greatly simplifies the design of the compensation loop.

With peak current mode control, the buck power stage can be simplified as a one-pole-one-zero system in the frequency domain. The pole is the dominant pole and can be calculated by the following formula:

The zero is the ESR zero due to the output capacitance and its ESR. Its calculation method is as follows:

where; CO is the output filter capacitor, RL is the load resistance value, and ESRCO is the equivalent series resistance of the output capacitor.

The compensation design actually obtains the desired gain and phase by changing the transfer function of the converter control loop. Several different types of compensation networks can be used with the AOZ1031A. In most cases, a series capacitor and resistor network connected to the COMP pin sets the pole zero and is sufficient for a stable high bandwidth control loop.

In the AOZ1031A, the FB pin and the COMP pin are the inverting input and output of the internal error amplifier. A series R and C compensation network connected to COMP provides one pole and one zero. The rods are:

where; GEA is the error amplifier transconductance, which is 200 x 10-6 A/V, GVEA is the voltage gain of the error amplifier, which is 500v/V, and C2 is the compensation capacitor in Figure 1.

The zero given by the external compensation network capacitor C and resistor R is located at:

In order to design the compensation circuit, the target crossover frequency f must be chosen as the closed loop. The system crossover frequency is where the control loop has unity gain. Crossover is also known as converter bandwidth. Generally, higher bandwidth means faster response to load transients. However, considering the stability of the system, the bandwidth should not be too high. When designing the compensation loop, the stability of the converter under all line and load conditions must be considered.

In general, it is recommended to set the bandwidth equal to or less than 1/10 of the switching frequency. The AOZ1031A operates from 500kHz to 700kHz. It is recommended to choose a crossover frequency less than or equal to 40kHz.

The strategy for choosing R and C is to use R to set the crossover frequency and C to set the compensator zero. Compute R with the chosen crossover frequency f:

where: fC is the desired crossover frequency. For best performance, set fC to be around 1/10 of the switching frequency, VFB to be 0.8V, GEA to be the error amplifier transconductance of 200×10-6a/V, and GCS to be the transconductance of the current sensing circuit, is 6.68a/V.

Compensation capacitor C and resistor R together form zero. This zero is placed close to the dominant pole f, but below 1/5 of the chosen crossover frequency. C can be selected by:

The above equation can also be simplified to:

An easy-to-use application software that aids in designing and simulating compensation loops can be found on .

Thermal Management and Layout Considerations

In the AOZ1031A buck regulator circuit, high pulse current flows through two circuit loops. The first loop starts from the input capacitor, to the VIN pin, to the LX pin, to the filter inductor, to the output capacitor and load, and back to the input capacitor through ground. When the high-side switch is turned on, current flows in the first loop. The second loop starts from the inductor, to the output capacitor and load, to the low-side NMOSFET. When the low-side NMOSFET is turned on, current flows in the second loop.

In the PCB layout, minimizing the area of the two loops can reduce the noise of the circuit and improve the efficiency. It is strongly recommended to use a ground plane to connect the input capacitor, output capacitor and PGND pin of the AOZ1031A.

In the AOZ1031A buck regulator circuit, the main power dissipating components are the AOZ1031A and the output inductor. The total power consumption of the converter circuit can be measured by subtracting the output power from the input power.

The power dissipation of the inductor can be calculated approximately from the output current of the inductor and the DCR.

The actual junction temperature can be calculated using the power dissipation in the AOZ1031A and the thermal impedance from the junction to ambient.

The maximum junction temperature of the AZZ1031 is 150°C, which limits the maximum load current capability.

See the thermal rating curve for the AZZ1031 A's maximum load current at different ambient temperatures.

The thermal performance of the AOZ1031A is greatly affected by the PCB layout. During the design process, the user should take extra care to ensure that the integrated circuit operates under the recommended environmental conditions.

AOZ1031A is a standard SO-8 package. For optimum electrical and thermal performance, some layout tips are listed below. Figure 3 on the next page shows an example PCB layout for the AOZ1031A.

1. The LX pin is connected to the internal PFET and NFET drains. They are the low resistance thermal conduction paths and the noisiest switching nodes. Connect a large copper plane to the LX pin to help dissipate heat.

2. Do not use thermal connections to the VIN and PGND pins. Dump the PGND pin and VIN pin with the largest copper area to help with heat dissipation.

3. The input capacitor should be connected to the VIN pin and PGND pin as much as possible.

4, the preferred ground plane. If a ground plane is not used, separate PGND from AGND and connect them at only one point to avoid PGND pin noise coupling to the AGND pin.

5. Keep the current trace from the LX pin to L to Co to PGND as short as possible.

6. Pour copper planes on all unused board areas and connect them to stable DC nodes such as VIN, GND, or u.

7. Keep sensitive signal traces away from the LX pin.

Tape and Reel Dimensions

This data sheet contains preliminary data; supplemental data may be published at a later date. Alpha and Omega Semiconductors reserve the right to make changes at any time without notice.

life insurance policy

Alpha and Omega Semiconductor products are not authorized for use as critical components in life support devices or systems.

As used in this article:

1. A life support device or system means a device or system that: (a) is intended for surgical implantation in the body, or (b) is intended to support or sustain life, and (c) when properly used in accordance with the instructions for use provided on the label A device or system that fails to perform can reasonably be expected to cause substantial harm to the user.

2. A critical component of any part of a life-saving appliance, device or system, the failure of which can reasonably be expected to cause the failure of the life-saving appliance or system, or to affect its safety or effectiveness.