feature

●4.5V to 27V operating input voltage range

70mΩ internal NFET, efficiency: up to 95%

●Internal soft-start; the output voltage can be adjusted down to 0.8V

●2A continuous output current

●Fixed 370kHz PWM operation

●cycle-by-cycle current limit

●Short circuit protection

● Thermal shutdown; small SO-8 package

application

●Point-of-load DC/DC conversion

●Set-top box

DVD drives and hard drives; LCD monitors and televisions

●Cable Modem

●Telecom/Network/Data Communication Equipment

General Instructions

The AOZ1210 is an efficient, easy-to-use 2A buck regulator that is flexible enough to be optimized for a variety of applications. The AOZ1210 operates over an input voltage range of 4.5V to 27V, delivering up to 2A of continuous output current on each buck regulator output. The output voltage can be adjusted down to 0.8V.

typical application

Ordering Information

Pin configuration

Pin Description

block diagram

Typical performance characteristics

The circuit of Figure 1. TA=25°C, VIN=VEN=24V, VOUT=3.3V, unless otherwise specified.

efficiency curve

Detailed description

The AOZ1210 is a current mode buck regulator with an integrated high side NMOS switch. It operates over an input voltage range of 4.5V to 27V and provides up to 2A of load current. The duty cycle can be adjusted from 6% to 85%, allowing a wide range of output voltages. Features include: enable control, power-on reset, input undervoltage lockout, fixed internal soft-start, and thermal shutdown.

AOZ1210 is available in SO-8 package.

Enable and Soft Start

The AOZ1210 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 the soft-start process, the output voltage usually becomes the regulated voltage within 6.8ms, and the 6.8ms soft-start time is set internally.

If the enable function is not used, connect the EN pin to V. Pulling EN to ground disables the AOZ1210. Don't leave the door open. The voltage on the EN pin must be higher than 2.5 V to enable the AOZ1210. When the voltage on the EN pin falls below 0.6V, the AOZ1210 is disabled. If the application circuit requires that the AOZ1210 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 AOZ1210 integrates an internal N-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. Since the N-MOSFET requires a gate voltage higher than the input voltage, a boost capacitor connected between the LX and BST pins drives the gate. When LX is low, the boost capacitor charges. An internal 10Ω switch from LX to GND is used to ensure that LX is pulled to GND even under light load conditions. 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. Compare the error voltage displayed on the COMP pin with the current signal. The current signal is the sum of the inductor current signal and the slope compensation signal at the PWM comparator input. 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 passes freely to the output through the Schottky diode.

On-off level

The AOZ1210 switching frequency is fixed and set by the internal oscillator. The switching frequency is set to 370kHz.

Output voltage programming

The output voltage can be set by feeding back the output to the FB pin and 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 R value and calculating the required R value using the formula below.

Table 1 lists some standard values for the most commonly used output voltages R and R.

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

Protection features

AOZ1210 has multiple protection functions to prevent damage to the system circuit under abnormal conditions.

Over Current Protection (OCP)

The sensed inductor current signal is also used for overcurrent protection. Since the AOZ1210 adopts 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.

The cycle-by-cycle current limit threshold is set internally. When the load current reaches the current limit threshold, the cycle-by-cycle current limit circuit immediately turns off the high-side switch to terminate the current duty cycle. The inductor current stops rising. The circulating current limit protection directly limits the peak inductor current. Due to the limitation of peak inductor current, the average inductor current is also limited. When the cycle-by-cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreases.

The AOZ1210 has internal short-circuit protection to prevent catastrophic failure under output short-circuit conditions. The FB pin voltage is proportional to the output voltage. When the FB pin voltage is lower than 0.2V, the short circuit protection circuit is triggered. To prevent the current limit from disappearing when the comp pin voltage is higher than 2.1V, short circuit protection is also triggered. As a result, the converter is turned off and hiccups at a frequency equal to 1/16 of the normal switching frequency. After the short circuit fault is cleared, the drive will start with a soft start. In short-circuit protection mode, the average current of the inductor is greatly reduced due to its low disturbance frequency.

Power-On Reset (POR)

A power-on reset circuit monitors the input voltage. When the input voltage exceeds 4.3V, the inverter starts to work. When the input voltage is lower than 4.1V, the inverter will stop switching.

Thermal Protection

An internal temperature sensor monitors the connector temperature. When the junction temperature exceeds 145°C, the internal control circuit and the high-side NMOS 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 AOZ1210 application circuit is shown in Figure 1. Component selection is described below.

input capacitor

The input capacitor (C in Figure 1) must be connected to the V and GND pins of the AOZ1210 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 + ripple voltage.

The input ripple voltage can be approximated by:

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 m is equal to the conversion ratio:

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

For reliable operation and optimum performance, the input capacitor must have a current rating higher than I under worst-case operating conditions. Ceramic capacitors are the first choice for input capacitors due to their low ESR and high ripple current ratings. Depending on the application circuit, other low ESR tantalum capacitors or aluminum electrolytic capacitors can also be used. When choosing ceramic capacitors, X5R or X7R type dielectric ceramic capacitors are preferred due to their better temperature and voltage characteristics. Note that capacitor manufacturers' ripple current ratings are based on a certain lifetime. Actual design requirements 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, namely:

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.

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 voltage rating of the selected output capacitor must be higher 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 capacitor value; 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:

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

For lower output ripple voltage over the entire operating temperature range, X5R or X7R dielectric ceramic or other low ESR tantalum or aluminum electrolytic capacitors can also be used as output capacitors.

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.

Schottky Diode Selection

When the high-side NMOS switch is turned off, an external free-wheeling diode supplies current to the inductor. To reduce diode forward voltage drop and recovery losses, Schottky diodes are recommended. The maximum reverse voltage rating of the selected Schottky diode should be greater than the maximum input voltage and the current rating should be greater than the maximum load current.

loop compensation

The AOZ1210 features peak current mode control for ease of use and 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 closed-loop transfer function of the converter. Several different types of compensation networks can be used with the AOZ1210. 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 AOZ1210, the FB pin and the COMP pin are the inverting input and output of the internal transconductance 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 transconductance of the error amplifier, which is 200×10-6A/V, GVEA is the zero point given by the external compensation network of the error amplifier voltage, capacitor CC (C5 in Figure 1) and resistor RC (R1 in Figure 1 ),lie in:

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. The crossover frequency is also known as the converter bandwidth. Generally, higher bandwidth means faster response to load transients. However, due to system stability issues, 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.

Typically, it is recommended to set the bandwidth to be less than 1/10 of the switching frequency. It is recommended to choose a crossover frequency less than 30kHz.

The strategy for choosing RC and CC is to set the cross with RC overfrequency and set the compensator zero to CC. Calculate RC using the selected crossover frequency fC:

where fC is the desired crossover frequency, VFB is 0.8V, GEA is the error amplifier transconductance, which is 200x10-6A/V, and GCS is the transconductance of the current sense circuit, which is 5.64A/V.

Compensation capacitor CC and resistor RC return to zero. This zero is placed close to the gate fp1, but below the 1/5 crossover frequency of the selected pole. CC 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 AOZ1210 buck regulator circuit, high pulse current flows through two circuit loops. The first loop starts with the input capacitor, V pin, LX pin, filter inductor, output capacitor, and load, and returns 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 GND pin of the AOZ1210, to the LX pin of the AZ1210. When the low-side diode 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 recommended to use a ground plane to connect the input capacitor, output capacitor and GND pin of the AOZ1210.

In the AOZ1210 buck regulator circuit, the three main power dissipating components are the AOZ1210, the external diode, 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 approximated by the output current of the inductor and the DCR.

The power dissipation of the diode is:

The actual AOZ1210 junction temperature can be calculated from the power dissipation in the AOZ1210 and the thermal impedance from the junction to ambient.

The maximum junction temperature of the AOZ1210 is 145°C, limiting the maximum load current capability.

The thermal performance of the AOZ1210 is greatly affected by the PCB layout. During the design process, the user should take care to ensure that the integrated circuit operates under the recommended environmental conditions.

For optimum electrical and thermal performance, some layout tips are listed below. Figure 3 is an example of a layout.

1. Do not use thermal connections to the V and ground pins. Inject maximum copper area on the GND pin and V pin to help with heat dissipation.

2. The input capacitors should be placed as close as possible to the V and GND pins.

3. Keep the current trace from the LX pin to L to C to GND as short as possible.

4. Pour copper planes on all unused board areas and connect them to a stable DC node such as V, GND, or at u.

5. Keep sensitive signal traces, such as the trace connecting the FB and COMP pins, away from the LX pin.

Package dimensions

notes:

1. All dimensions are in millimeters.

2. Dimensions include electroplating

3. The package size does not include mold flash and gate burr. Die flash on the non-lead side should be less than 6 mils.

4. Dimension L is measured in the instrument plane.

5. The control size is in millimeters, and the converted inch size is not necessarily accurate.

Tape and Reel Dimensions

AOZ1210 Packaging Identification

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:

A life support device or system is device or 2. A critical component of any part of the living system that (A) is used for surgical implantation of a stent, device or system, which function it cannot perform may be that of the body, or (b) supports or sustains life, and (c) when used in accordance with When used properly, a support device is reasonably expected to render life incapable of performing or the system, or to affect its safety or the labeling provided in the instructions for use, can be effective.