General Instructions

The AOZ1282CI is a high-efficiency, simple-to-use, 1.2A buck regulator that is flexible enough to be used in a variety of applications. The AOZ1282CI operates over an input voltage range of 4.5V to 36V and delivers up to 1.2A of continuous output current. The output voltage is adjustable to 0.8V, and the fixed switching frequency of 450kHz reduces inductor size.

feature

4.5V to 36V operating input voltage range; 420mΩ internal NMOS; up to 95% efficiency; internally compensated; 1.2A continuous output current; fixed 450kHz PWM operation; internal soft-start; output voltage adjustable down to 0.8V; cycle-by-cycle current limit; short circuit protection; thermal shutdown; small SOT33-6L.

application

Point-of-load DC/DC conversion; set-top boxes and cable modems; DVD drives and HDDs; LCD monitors and TVs; telecommunications/networking/datacom equipment.

typical application

Ordering Information

block diagram

Typical performance characteristics

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

Detailed description

The AOZ1282CI is a current mode buck regulator with an integrated high side NMOS switch. It operates from an input voltage range of 4.5V to 36V and delivers up to 1.2A of load current. Features include enable control, undervoltage lockout, internal soft-start, output overvoltage protection, overcurrent protection, and thermal shutdown.

AOZ1282CI is available in SOT33-6L package.

Enable and Soft Start

The AOZ1282CI has an internal soft-start function to limit inrush current and ensure a smooth rise of the output voltage to the regulated voltage. The soft-start process begins when the input voltage rises above UVLO and the voltage on the EN pin is high. During soft-start, the output voltage gradually changes to the regulated voltage within a typical 400 microseconds. The 400 microsecond soft-start time is set internally.

The EN pin of AOZ1282CI is active high. If the enable function is not used, connect the EN pin to the VIN. Pulling it to ground will disable the AOZ1282CI. Don't leave it on. The voltage on the EN pin must be higher than 1.2 V to enable the AOZ1282CI. When the voltage on the EN pin is below 0.4V, the AOZ1282CI is disabled.

steady state operation

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

AOZ1282CI integrates an internal NMOS as a high-end 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 is compared to the sum current signal 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 freewheeling for output through an external Schottky diode.

On-off level

The AOZ1282CI switching frequency is fixed and set by the internal oscillator. The switching frequency is internally set to 450kHz.

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 R1 and R2. Typically, the design starts with choosing a fixed value of R2 and then calculating the required R1 using the formula below.

Table 1 lists some standard values of R1 and R2 for the most commonly used output voltage values.

The combination of R1 and R2 should be large enough to avoid drawing too much current from the output, which will cause power loss.

Protection features

AOZ1282CI 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.

The cycle-by-cycle current limit threshold is set to a normal value of 1.9A. 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. Cycle-by-cycle current limit protection directly limits the inductor peak 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 AOZ1282CI 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. As a result, the torque converter shuts down and hiccups. Once the short-circuit condition disappears, the drive will start with a soft start. In short-circuit protection mode, the average inductor current is greatly reduced.

Under Voltage Lockout (UVLO)

The UVLO circuit monitors the input voltage. When the input voltage exceeds 2.9V, the inverter starts to work. When the input voltage is lower than 2.3V, the inverter will stop switching.

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 NMOS are turned off. When the junction temperature drops to 110°C, the regulator automatically restarts under the control of the soft-start circuit.

application information

The basic AOZ1282CI application circuit is shown in Figure 1. Component selection is described below.

input capacitor

The input capacitor must be connected to the VIN pin and PGND pin of the AOZ1282CI 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 plus the 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 we let m equal 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, 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.

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 C is the output capacitor value and ESR 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 formula is:

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.

Thermal Management and Layout Considerations

In the AOZ1282CI 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, goes to the output capacitor and load, to the anode of the Schottky diode, to the cathode of the Schottky diode. 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 strongly recommended to use a ground plane to connect the input capacitor, output capacitor and PGND pin of the AOZ1282CI.

In the AOZ1282CI buck regulator circuit, the main power dissipation components are the AOZ1282CI, Schottky diode and output inductor. The total power consumption of the converter circuit can be measured by subtracting the output power from the input power.

Schottky power dissipation can be approximated as:

where VFW_Schottky is the Schottky diode forward voltage drop.

The power dissipation of the inductor can be approximated by calculating the output current of the inductor and the DCR.

The actual junction temperature can be calculated from the power dissipation in the AOZ1282CI and the thermal impedance from junction to ambient.

The maximum junction temperature of the AOZ1282CI is 150°C, which limits the maximum load current capability.

The thermal performance of the AOZ1282CI 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.

For optimum electrical and thermal performance, some layout tips are listed below.

1. The input capacitor should be placed as close as possible to the VIN pin and the ground pin.

2. The inductor should be placed as close as possible to the LX pin and the output capacitor.

3. Make the Schottky diode connection between the LX pin and the GND pin as short and wide as possible.

4. Place the feedback resistor and compensation components as close to the chip as possible.

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

6. Inject the largest copper area on the VIN pin, LX pin, and especially the GND pin to help dissipate heat.

7. Place a copper plane on all unused board areas and connect that plane to a stable DC node such as VIN, GND, or VOUT.

Package size, SOT33-6

notes:

1. The package size does not include mold flash and gate burr. Die flash on the non-lead side should be less than 5 mils.

2. Dimension "L" is measured in the instrument plane.

3. Tolerance ±0.100 mm (4 mil) unless otherwise specified.

4. Connected from JEDEC MO-178C and MO-193C.

5. The control size is millimeters. Converted inch sizes are not necessarily accurate.

Tape and Reel Dimensions, SOT33-6

part mark

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