Let's briefly talk about the chips commonly used in mobile phones - OVP chips

What is? OVP

OVP（Over Voltage Protection） Overvoltage protection , When the voltage exceeds a certain value , It plays a protective role in the later stage circuit . Avoid excessive input voltage , Cause damage to the subsequent circuit .

There are many ways of overvoltage protection , However, it is used in mobile phones , It's often the use of OVP chip . Affordable , And the threshold is controllable , Yes PCB The layout saves space .

As an important chip in mobile phones , When charging , Avoid accidents VBUS Overvoltage , And then damage the function of the subsequent chip .

Why use OVP chip

In reality , Overvoltage caused by mobile phone charging , There are often the following scenes .

1. During on-board charging , At the moment when the car starts , There is often surge voltage , If it is not suppressed, there will be harm to the mobile phone .

2. In some underdeveloped countries , Grid instability can also lead to VBUS Input exception .

3. Use some inferior chargers , It is also one of the reasons for overpressure .

OVP manufacturer

The current market OVP Are relatively mature , Foreign manufacturers have ：Fairchild,Maxim. Domestic WAYON, Ai Wei ,ETEK etc. .

Example

For example, a small series was used in a project OVP chip ：ET9540CL, It comes from ETEK.

This is its internal schematic diagram ：

From the block diagram ,OVP The output voltage of , In essence, it is internal NMOS Tubular Uds. The maximum output voltage is NMOS Tubular DS Withstand voltage value .OVP The output of is disconnected , Nature is NMOS Disconnection and conduction of pipe .NMOS The opening and disconnection of pipes are mainly controlled by Vgs decision . However Vgs With Vin Change by change . This also has Vin Input range of .

Important parameter

Surge： Surge protection . There are some OVP Internal integration TVS, Therefore, it has certain surge protection function . General OVP The chip can resist 100V perhaps 120V Surge voltage , The biggest resistance of this chip 80V Surge voltage .

Vin： Input voltage , namely Vin Acceptable range of input voltage .

Vout： Output voltage , namely OVP Range of output voltage .

Rdson： Internal to the chip NMOS On resistance of . The lower the resistance, the better . Especially in the case of high current charging , It can reduce the pressure drop loss .

VIN_UVLO： namely OVP Minimum working voltage . When the input voltage reaches VIN_UVLO when ,OVP Turn on and start working , Its output has voltage .

VIN_OVLO： namely OVP Maximum working voltage . When the input voltage exceeds VIN_OVLO when ,OVP Disconnect the output of the stage after stopping , Its output has no voltage . It is controllable .

Isw, Input current , majority OVP chip , Input current MAX The value is 5V Even larger .

TOFF OVP Disconnect response time , It is often 50ns about . namely OVP The time between the input voltage exceeding the threshold and the output voltage breaking is 50ns.

Circuit design

If the default UovLo simply OVLO Pin ground .

If you modify the threshold voltage by yourself , As shown in the figure . Through adjustment R1 And R2 Value , To adjust OVLO value .

The input and output voltages in the figure above , The image can be displayed with an oscilloscope .

In terms of design , The capacitance at the input is also very particular . It is suggested to strictly follow datesheet It is required to select the tolerance . At the same time, the voltage withstand value , If there is TVS, Then the withstand voltage value should be greater than TVS The clamping voltage of . without TVS, It is better to choose the withstand voltage value as 50V, The capacity value is greater than or equal to 0.1uf The capacitance of . At the same time, the capacitance of the output , It is suggested to strictly follow datesheet choice , The withstand voltage value is greater than OVP The threshold voltage of . In previous projects , Most of us choose 0402 Encapsulated capacitor , Its withstand voltage value is also greater than that of the same type 0201 The capacitance of .