Features and advantages

#9642; Wide input voltage range: 8 to 50 v

#9642; Integrated low RDS (on) DMOS switch

#9642; 3 A continuous output current

#9642; 24 v

#9642; small packaging with exposed pads

The application includes:

#9642; Printing machine power supply

#9642 Office automation equipment

#9642; POS heat sensitivity, laser, photo and inkjet printer

#9642; tape machine

#9642; industrial application [ 123]

Explanation

A8447

is a 3A high -efficiency general -purpose antihypertensive regulator, which is suitable for various applications. The output voltage can be adjusted between 0.8 and 24 volts, which is basically the resistance distributioner and the reference voltage of the 0.8 volts ± 2%. External components include external clamping diodes, inductance and filter capacitors. The cutting time is determined by the external ground resistance. It work in continuous and discontinuous models to keep the light load adjustment. The internal consumption circuit is used to filter the transients generated by the reverse recovery of the external diode. The typical hidden time is 200ns.

This new device is very suitable for various terminal products, including applications of 8 to 50 V input voltage range, which requires up to 3 A output current, such as uninterrupted power supply, sales point (POS) application, and 24 Or industrial applications of 36 V bus.

Packaging: 8 -needle SOIC, banded heat pads (suffix LJ)

Typical application

[ 123] Function box diagram

Function description

A8447 is a fixed turnover time, current mode control, antihypertensive regulator. The regulator requires an external diode, inductance and filter capacitor. It works under continuous and discontinuous models. The internal consumption circuit is used to filter the transients caused by the reverse recovery of the external diode. The typical hidden time is 200ns.

The resistance value between TSET and GND determines the fixed closing time (see charts in the TOFF part).

Vout. The output voltage can be adjusted between 0.8 and 24 V, which is set by an external resistor. The calculation formula of the voltage is as follows:

Light load adjustment. In order to maintain voltage adjustment under light load conditions, the switching stabilizer enters the cycle jump mode. As the output current decreases, some energy is still stored within the minimum connection time of the power switch. In order to prevent the output voltage from rising, once the regulator reaches the minimum opening time, it will skip the cycle, which effectively makes the shutdown time larger.

Soft start. The internal slope generator and counter allow the output to rise slowly. This limits the maximum demand of external power supply by controlling the surge current required for external capacitors and any DC load charging during startup. Inside, the slope is set to 10 ms nominal rising time. At the time of soft start, the minimum current limit is 3.5 A.

The following conditions are required to trigger soft start:

Vehicle recognition code gt; 6 volts

ENB pins input decrease

[

[ 123] reset the TSD (heat shutdown) event

vbias. In order to improve the overall efficiency of the system, the regulator output VOUT and VBias input to provide working bias current under normal working conditions. During the startup process, the circuit was disconnected from the vehicle identification number. When the VOUT target level is between 3.3 and 5 V, VBIAS should be connected to VOUT. If the output voltage is less than 3.3 V, and then the A8447 can run internally, and pay the fine in efficiency, because the bias current will come from a high -voltage power supply and vehicle identification number. VBIAS can also be powered by external voltage sources. Normal operation does not need to be powered on.

Open/off control. Pull ENB pins from the outside to the ground to enable the device and start the sequence softly. When ENB opens, the switch is disabled and the output is attenuated to 0V.

Protection. In one or more faults below:

Vehicle recognition number lt; 6 v

ENB pin open road

#8226 ; TSD fault

When the device comes out of the TSD failure, it will enter the soft start to limit the surge current.

TOFF. The resistance value between the test tube and the ground determines the fixed closing time. The formula for calculating the TOFF (μs) is:

The RTSET (KΩ) is the value of the resistor. Show results

In the figure below:

RTSET resistance should not be less than 7.65 kΩ ± 2%, to prevent very short turnover time violating the minimum opening of the switch time.

Short -circuit load. If the voltage on the FB pin drops below 0.4V, the regulator will trigger a typical over -current limit of 1.5A to process the regulator outputShort -circuit load. For the low output voltage and short -circuit output at the time of power, the disconnection time is extended to prevent the control of the current limit due to the minimum opening time of the switching equipment.

The extension of the cutting time is based on the value and FB voltage of the TSET multiplier, as shown in the table below:

l1. The inductor must be rated to the total processing total to total processing total. Load current. This value should be selected to keep the ripple current on a reasonable value. The ripple current Iriple can be calculated through the following formulas:

Example:

Give VOUT 5 V, VF 0.55 V, VIN 42 V. ILOAD 0.5 A, 55 ° C power sensor of L 180 μH and RL 0.5Ωrdc, TOFF 7μs, and RDS (open) 1Ω.

Integrated 4:

Institutional formula 3:

The switch frequency FSW can pass the following can pass the following Method estimates:

Interture of Fang Establishment 7:

Interest 6:

Integrated Fangshu 7:

You can choose a higher inductance value to reduce the ripple current. If you need to increase the maximum total current available to the maximum total current of the switch regulator, this may be a choice. The maximum available current ILOAD (MAX) is:

Among them, the ICL (minimum) is 3.5 A, from the electrical characteristic table.

D1. The rated value of Schottky's capture of the diode should be 1.2 times the maximum load current. The rated voltage should be higher than the expected maximum input voltage at all operating conditions. The duty cycle of high input voltage can be close to 100%.

Kut. The main consideration of selecting the output capacitor is the voltage ripple on the output end. For electrolytic output capacitors, it is recommended to use a low ESR type.

The output voltage ripple of the peak is Iriple × ESR. Note that increasing the inductance value can reduce the ripple current. The minimum rated voltage of the capacitor is 10V. However, due to the decrease in voltage ESR, the most economical and effective choice may be that the rated voltage is higher. It is recommended that ESR is less than 100 mΩ.

RTSET selection. Selecting the RTSET value correctly will ensure that the minimum opening time of the switch is not violated and the switch is prevented from skipping the cycle. For the ratio of a given vehicle recognition number to the voucher, the RTSET must be greater than or equal to the value of the curve definition in the diagram in the following RTEST value.

Note. The curve represents the smallest RTSET value. When calculating RTSET, make sure to use VIN (MAX)/Vout (min). The resistance tolerance should also be considered so that under all operating conditions

RTSET value selection*

The condition test on the condition test is as close to the curve as possible.

The RTEST selection table shows the recommended RTSET value based on common operating conditions. For other operating conditions, see the RTSET value selection diagram.

FB resistor selection. The impedance of the FB network should remain low to improve the ability to resist noise. The large value resistance can pick up the noise generated by the inductor, which affects the voltage adjustment of the switch tube.

The recommended print circuit board layout

In order to minimize the impact of the ground jumping and offset problem, there is one important thing is that there must be one Very close to the low impedance ground of the device. This grounding method is called star ground. It is likely to need a floor to meet the heat requirements. The recommended grounding method illustrates how to create low impedance grounding, which will also help remove thermal energy from the device.

The input capacitor must be as close to the vehicle recognition number as much as possible, because in the connection cycle, it is responsible for providing a current to the switch. In the shutdown cycle, the current path is from the negative pole terminal of the COUT CAP, through the diode and inductance, and then reaches the load. Therefore, the COUT and rectifier diode must share the connection of the negative terminal of the CIN capacitor to reduce the ground bouncing when conducting the diode.

The inductor should be as close to the opening node as possible to reduce noise. Due to the limitations of electromagnetic interference, some applications may need to shield the inductance. This will depend on the parameters defined by the application and system, which will carry the regulator.

High -pressure opening node will affect RTSET. If the longer closing time is used, the resistance on the RTSET pin may be quite large. When designing the layout, try to keep RTSET away from inductance and opening nodes. The short trajectory is also conducive to reducing the impact of noise injection. Due to the layout criteria, the TSET foot is located on the other side of the device, away from the open joint node.

The FB resistance network should have lower impedance to avoid interference from the opening nodes. Because the impedance on the FB node can be controlled, it is not so critical to keep the network isolation. The important thing is to keep the grounding trajectory short, so that the ground bounce will not affect the adjustment of the output voltage.

The large star -shaped area of the PCB filling side shown in the figure is the GND node, which supports high current throughput and allows the VOUT node to be as close to A8447 (U1) as possible Essence The thermal conductivity of the A8447 is enhanced directly through its exposed thermal pads directly contacting the small ground area below the A8447. The area is connected to the PC through the heat -passing holeB large copper ground plane without filling the side.

Packaging lj 8 -needle SOIC

Allegro Microsystems, LLC reserves the right to improve performance and reliability at any time, or it deviates from detailed standardized rights, or it has its own standardized standardProduct can be manufactured.Before the order is placed, remind users to confirm that the information it depends is the latest.