Features

3V to 26V working input voltage range

240 mΩ internal nmos

High efficiency: up to 95 high 95 %

Internal compensation

1.2 A continuous output current

fixed 1.5 Mille pulse width modulation operation

[

[[

123] Internal soft start

output voltage can be adjusted to 0.8V

cyclical current limit

Short-circuit protection

heat shutdown

small SOT2 3-6L

Application

load point DC/ DC conversion

set -top box

DVD driver and hard disk

LCD display and TV

#8226 ; Cable modem

Telecom/network/data communication equipment

generally explained 1.2 A antihypertensive regulator has sufficient flexibility and can be optimized for a variety of applications. AOZ1280 works within the input voltage range of 3V to 26V and provides continuous output current as high as 1.2A. The output voltage can be adjusted to 0.8V, and the frequency of the fixed 1.5MHz PWM switch reduces the inductance size.

AOZ1280 is encapsulated with SOT33-6L, and the rated working environment temperature range is between -40 ° C to+85 ° C.

Typical Application

Typical performance features

Figure in Figure 1 circuit. Unless there are other regulations, VIN 12 V, VOUT 3.3 V, L 4.7 μH, C1 10 μF, C2 22 μF, TA 25 ° C.

Efficiency

Detailed description

Aoz1280 is a current mode mode The antihypertensive regulator with an integrated high -voltage side NMOS switch. Its operating voltage range from 3V to 26V and provides a load current of up to 1.2A. Function includes: enableControl, owed voltage atresia, internal soft start, output overvoltage protection, over -current protection, and heat off.

AOZ1280 has SOT33-6L packaging.

Enable and soft start

AOZ1280 has an internal soft startup function to limit the impact current and ensure that the output voltage rises smoothly to the adjustment voltage. When the input voltage rises to the voltage higher than the UVLO and the voltage level on the EN pin is high, the soft startup process begins. During the soft startup process, the output voltage usually becomes a adjustable voltage within 400 μs. 400 μs Soft start time is set internally.

The EN pin of AOZ1280 is in a high activation state. If the connection is not used, enter it to the vehicle recognition number. Pull EN to the ground will be disabled aoz1280. Don't let the door open. The voltage on the EN pin must be higher than 1.2 V to enable AOZ1280. When the voltage on the EN pin is lower than 0.4 V, the AOZ1280 is disabled.

steady -state operation

Under steady -state conditions, the converter works in a fixed frequency and continuous transmission mode (CCM).

AOZ1280 integrates an internal NMOS as a high -end switch. The inductor current is detected by the voltage of the drain to the high -voltage side power MOSFET source. The output voltage is reduced by the external pressure dealer at the FB pin. The difference between the voltage and reference voltage of the FB tube foot through the internal cross -guidance error. At the PWM comparator input terminal, compares the current signal of the error voltage to the sum of the amount of compensation signal with the inductance current signal with an inductive current signal. If the current signal is less than an error voltage, the internal high -voltage side switch is connected. The inductive current from the input through the inductance flow to the output. When the current signal exceeds the error voltage, the high -voltage side switch is broken. The inductor current is free to rotate the output through the external Schottky diode.

Switching frequency

AOZ1280 switching frequency is fixed and set by internal oscillator. The internal settings of the switch are set to 1.5 MM.

Output voltage programming

The output voltage can be set to the FB pin and resistance voltage network by feedback. In the application circuit shown in FIG. 1. The resistance separation network includes R1 and R2. Generally, the design is to start with a fixed R2 value, and then use the following formula to calculate the required R1.

Table 1 lists some of the most common output voltage values R1 and R2.

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

Protection feature

AOZ1280 has multiple protection functions to prevent the system circuit from damaging in abnormal conditions.

PassingCurrent Protection (OCP)

The inductive induction current signal is also used for over -current protection.

The weekly current limit threshold is set to 2 a normal value. When the load current reaches the current limitation threshold, the weekly current limit circuit is turned off immediately off the high -voltage side switch to terminate the current duty occupation ratio. The sensor current stopped rising. Plug -in -limiting protection directly restricts the peak of inductance. Due to the limitation of the peak inductance current, the average inductor current is also limited. When a weekly current limitation circuit is triggered, the output voltage decreases with the decrease of the duty cycle.

AOZ1280 has internal short circuit protection to prevent catastrophic failures under the 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 transformer was closed and snoring. After the short -circuit failure is eliminated, the inverter will start with soft start. In short -circuit protection mode, the average current of inductance is greatly reduced.

IOU locking (UVLO)

UVLO circuit monitor input voltage. When the input voltage exceeds 2.9V, the inverter starts to work. When the input voltage is lower than 2.3V, the converter will stop the switch.

Hot protection

Internal temperature sensor monitoring joint temperature. When the knot temperature exceeds 150 ° C, the sensor turns off the internal control circuit and the high -voltage side NMOS. When the knot temperature drops to 100 ° C, the regulator will automatically restart under the control of the soft startup circuit.

Application information

The basic AOZ1280 application circuit is shown in Figure 1. The selection of parts is as follows.

Input capacitor

Input capacitors must be connected to the V pins and GND pins of AOZ1280 to maintain a stable input voltage and filter out the pulse input current. The rated voltage of the input capacitor must be greater than the maximum input voltage plus ripple voltage.

The input ripple voltage can be approximately similar to the following formula:

Because the input current of the BUCK converter is not continuous, when selecting a capacitor, the input capacitor is entered. The current stress is another question to be considered. For the antihypertensive circuit, the average root value of the input capacitance current can be calculated through the following formulas:

If we make M equal to the conversion ratio:

]

Calculate the relationship between the average cubes of the input capacitor and the voltage conversion rate, as shown in Figure 2. It can be seen that when V is half of V, C's current stress is the most. CIN's maximum current stress is 0.5x IO.

For reliable operation and best performance, the rated current of the capacitor must be higher than IAT under the worst working conditions. Ceramic capacitors due to their low ESRAnd high -grained wave current rated values are prioritized as input capacitors. Depending on the application circuit, other low ESR 钽 electrical containers or aluminum electrolytic capacitors can be used. When choosing a ceramic capacitor, the X5R or X7R dielectric ceramic capacitor becomes the first choice for its better temperature and voltage characteristics.

Note that the ripple current rated value of the capacitor manufacturer is based on a fixed service life. The actual design requirements may require further reduction.

sensor

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

The peak of the inductor current is:

[ 123]

High inductor provides low -induced ripple current, but requires a larger dimension to avoid saturation. The low -grained wave current reduces the iron heart loss of the inductors, and also reduces the balance of the square root current through the induction and switch. This will reduce the conduction loss.

When selecting an inductor, it should be ensured that it can process the peak current at the highest working temperature without saturation.

The highest current in the antihypertensive circuit. The conduction loss on the inductor needs to check whether it meets the thermal efficiency requirements.

CoilCraft, Elytone, and Murata offers different shapes and styles of surface stickers. The shielding inductance volume is small, the radiation EMI noise is small, but the cost is higher than the non -shielding inductance. Selecting depends on electromagnetic interference requirements, prices and size.

Output capacitor

Select the output capacitor according to DC output voltage, output ripple voltage specifications, and ripple current rated values.

The rated voltage specifications of the selected output capacitor must be higher than the maximum expected output voltage (including ripples). Long -term reliability needs to be relegated.

Output ripple voltage specifications are another important factor in selecting output capacitors. In the BUCK converter circuit, the output ripple voltage is determined by inductance value, switching frequency, output capacitance value and ESR. Can be calculated through the following formula:

In the formula, the CO is the output capacitor value, and ESRCO is an equivalent connected resistance capacitor output.

When a low ESR ceramic capacitor is used as the output capacitor, the impedance of the capacitor at the switch frequency is dominated. Output ripples are mainly caused by capacitor values and inductive ripples. The calculation of the output ripple voltage can be simplified to:

When the ESR impedance of the switch frequency dominates, the output ripple voltage is mainly determined by the capacitor ESR and inductive ripple current. The calculation of the output ripple voltage can be further simplified as:

[12]3] For the lower output ripple voltage with a lower operating temperature range, the X5R or X7R medium -type ceramics or other low ESR 钽 electrical containers or aluminum electrolytic capacitors can also be used as output capacitors.

In the buck converter, the output capacitor current is continuous. The equity of the output capacitor is determined by an inductive peak ripple current. The calculation method is as follows:

Generally, due to the low current stress, the ripple current rated value of the output capacitor is a small problem. When the choice of voltage reduction inductance is small and the electromotive ripple current is large, the output capacitor will produce stress.

Selecting the Schottky diode

When the high -voltage side NMOS switch is turned off, the external free rotation diode provides current to the inductance. In order to reduce losses caused by forward voltage drops and diode recovery, the Shawkki diode is recommended. The maximum reverse voltage rated value of the Schottky diode should be greater than the maximum input voltage, and the current rated value should be greater than the maximum load current.

Thermal management and layout considerations

In AOZ1280 antihypertensive regulator circuit, the high pulse current flows through two circuits. The first circuit starts from the input capacitor, to the vehicle identification number pins, to the LX pin, to the filtering inductance, to the output capacitor and load, and then return to the input capacitance by grounding. When the high -voltage side switch is turned on, the current flows in the first circuit. The second loop starts from the inductance, to the output capacitance and load, to the anode of the Schottky diode, to the cathode of the Schottky diode. When the low -voltage side diode is opened, the current flows in the second loop.

In the design of the PCB layout, the area of the minimized two circuits can reduce the noise of the circuit and improve efficiency. It is strongly recommended to use ground plane connecting input capacitors, output capacitors and AOZ1280 ground pins.

In the AOZ1280 antihypertensive voltage regulator circuit, the main power dissipation components are AOZ1280, Shawitki diode and output inductance. The total power consumption of the converter circuit can be measured by subtracting the output power by input power.

Schartky's power consumption can be approximately:

Among them, vfw_schottky is the positive direction Pressure drop.

The power loss of the inductance can be calculated by the output current of the inductance and DCR.

The actual knot temperature can be calculated through the power consumption in AOZ1280 and the thermal impedance of the environment.

The maximum knot temperature of AOZ1280 is 150 ° C, which limits the maximum load current capacity.

The thermal performance of AOZ1280 is greatly affected by the PCB layout. During the design process, users should be extra carefulTo ensure that integrated circuits work under the recommended environmental conditions.

In order to obtain the best electrical and thermal performance, some layout tips are listed below.

1. Input a capacitor should be as close to the vehicle identification number pins and ground pins.

2. The sensor should be close to the LX pin and output capacitor as much as possible.

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

4. The feedback resistance and compensation element should be as close to the chip as much as possible.

5. Keep a sensitive signal trajectory away from the LX pin.

6. Inject the largest copper area into the vehicle identification number pins, LX pins, especially the GND pins to help heat dissipation.

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

Packaging size, SOT33-6

Note:

1, the size of the bag does not include mold flying edges and pouring Bond in the mouth. The non -lead -side mold flash should be less than 5 mils.

2. The size L " measured in the instrument plane.

3