Principle and application of bidirectional overvoltage and overcurrent protection device NCP370

0 Introduction

NCP370 is an integrated bidirectional protection device with overvoltage, overcurrent protection and reverse control functions produced by ON Semiconductor. The device can provide positive protection up to +28V and negative protection as low as -28V, thereby improving the protection of the front end of portable devices. NCP370 adopts an innovative architecture to provide OCP protection for external accessories connected to the bottom connector and powered by lithium-ion batteries inside the device without the need for external OCP devices. In addition, NCP370 also integrates a low on-resistance (Ron) N-channel MOSFET, which can effectively support the 1.3A direct charging current required by lithium-ion batteries in portable devices and further reduce system costs.

1 Features and functions of NCP370

1.1 Main features of NCP370

NCP370 is a bidirectional protection device. Through the "reverse" mode provided by the device, the reverse current of the battery of the portable device can be stabilized, and external devices such as FM transceivers, audio or flash connected to the bottom connector of the portable device can be powered. Therefore, NCP370 can provide positive and negative (+/-) overvoltage protection (OVP) for mobile phones, digital cameras, MP3/4, personal digital assistants (PDA) and GPS systems, and overcurrent protection (OCP) for portable accessories such as FM transceivers, load speakers/buzzers and flash modules.

The main features of the NCP370 overvoltage and overcurrent protector are as follows:

◇With overvoltage protection up to +28 V and undervoltage protection as low as -28 V;

◇With reverse charge control pin /REV and forward charge control pin /DIR;

◇With over-current protection and thermal shutdown function;

◇ Contains low RDS(on) N-channel MOS transistor, with a typical RDS(on) value of 130 mΩ;

◇With overvoltage lockout (OVLO) and undervoltage lockout (OV-LO) functions;

◇With soft start function;

◇With alarm output pin/FLAG;

◇Comply with IEC61000-4-2 (Level 4) standard, ordinary insulation voltage is 8 kV, air insulation voltage is 15 kV;

◇Use 3×3 mm TLLGA-12 lead-free package, which complies with RoHS lead-free packaging requirements.

1.2 Pin Functions of NCP370

NCP370 adopts 3 mm × 3 mm × 0.55 mm LLGA-12 lead-free package. Figure 1 shows its pin arrangement. The functions of each pin are as follows:

IN: Power input pin, which should be hard-wired to the common power supply and connected to GND using a 1 μF or larger low-ESR ceramic capacitor for power supply filtering;

GND: ground terminal;

RES: Reserved input terminal, can be connected to GND when not in use;

Ilim: Current limiting output terminal. When the battery is used to power the peripherals, the NCP370 will generate a bandgap voltage reference inside and use it to limit the current, and then use the internal N-MOSFET to achieve the purpose of current limiting. If a capacitor with a tolerance of 1% is connected to this pin, the accuracy of the overcurrent threshold will be the highest;

REV: Reverse charge control input terminal, which needs to be used in conjunction with the DIR pin. When the battery is connected to the OUT terminal, the N-channel MOS tube inside the NCP370 is turned on, and the device enters the reverse mode. At this time, the overcurrent protection circuit inside the device works. When the reverse mode of the NCP370 fails, the current entering the OUT terminal will be greatly reduced to avoid battery discharge.

DIR: Forward mode input terminal, which needs to be used in conjunction with REV. When the wall adapter AC-DC is connected to the IN terminal, the N-channel MOS tube inside the NCP370 is turned on and the circuit enters the forward mode. When this terminal and REV are both high, the NCP370 enters the shutdown mode, at which point the output will be disconnected from the input. However, the state of the DIR terminal will not affect the fault detection of FLAG.

FLAG, fault indication output terminal, NCP370 also allows external circuits to judge faults. When the input voltage is higher than the OVLO threshold or lower than the UVLO threshold, the charging current from the battery to the peripheral exceeds the threshold, or the internal temperature is higher than the shutdown current threshold, this terminal is set low. Since the FLAG terminal is an open-drain output, a pull-up resistor (minimum 10 kΩ) should be added between this port and Vbat.

OUT: Voltage output terminal. When "no input fault" is detected, this pin will output along with the IN terminal. When the input voltage is lower than the undervoltage threshold, or higher than the overvoltage threshold, or a thermal shutdown occurs, this output terminal will be disconnected from Vin. Generally, in reverse mode, NCP370 will be powered through the OUT terminal.

NC: Floating terminal;

PAD1: This pin is usually used to dissipate heat for the internal MOSFET. When used, it should be soldered to an independent PCB area without any potential contact.

2 Working Principle

NCP370 provides two working modes: forward mode and reverse mode, which can be set through the logic pins of the device. The first one refers to the working mode in which the wall adapter is used to provide power to the system. In this mode, the forward protection voltage and reverse protection voltage of NCP370 can reach +28 V and -28 V respectively. In addition, when the system input voltage exceeds the overvoltage threshold (OV-LO) or is lower than the undervoltage threshold (UVLO), the wall adapter (or AC/DC charger) will automatically disconnect. Usually during the power-on period of NCP370, if Vin exceeds the undervoltage threshold, Vout will be delayed for another 30ms.

The second is the reverse mode, which is a charging mode in which the system battery or booster provides power to the external device. In this mode, the peripheral must be connected to the input of the NCP370 (the system bottom connector), and the battery should be connected to its output. At the same time, the overcurrent protection circuit inside the NCP370 will also be activated, which can prevent peripheral failure and battery discharge. In addition, the NCP370 also has a reverse fault indication output (FLAG), which can be used to alarm the system when a fault occurs. If a 1μF or larger capacitor is bypassed at its input, the NCP370 can also perform ESD protection (15 kV air discharge).

During the power-on period of NCP370, if DIR is low and REV is high, and the input voltage is higher than UVLO, then the output voltage will still last for 30 ms. When the output signal rises, the FLAG signal will also last for 30 ms.

Connect the peripheral to the IN terminal, and the NCP370 can enter the reverse mode. At this time, the battery inside the peripheral can be powered by the OUT terminal of the NCP370. In the reverse mode, the REV terminal of the NCP370 must be set low, so that when the peripheral has an overvoltage, undervoltage or overcurrent fault, the overvoltage, undervoltage or overcurrent protection circuit inside the NCP370 will work, and its FLAG will also send an alarm signal to the system. Figure 2 shows the internal functional block diagram of the NCP370.

2.1 Undervoltage Lockout Circuit (UVLO)

The NCP370M chip has an integrated undervoltage lockout circuit that ensures that the AC/DC (or wall charger) is turned on normally under any conditions. During the positive rise of Vin, if the voltage is lower than the UVLO threshold, the output and input of the NCP370 are disconnected, and the FLAG output is set low. It should be noted that the undervoltage lockout circuit has a 60 mV hysteresis, which can be used to improve transient interference. When the NCP370 operates in reverse mode (REV<=0.55 V, DIR>=1.2 V), its internal UVLO and OVLO comparators do not work.

2.2 Overvoltage lockout circuit

The overvoltage lockout circuit inside the NCP370 can be used to prevent overvoltage damage to the Vout end system. Once the input voltage of the NCP370 is higher than the OVLO threshold, its output will fail and FLAG will output a low level. The overvoltage lockout circuit also has a hysteresis of 80 mV to improve the immunity to voltage transients. In addition, the OVLO threshold of the NCP370 can also be set manually.

2.3 System voltage lock

When DIR is low and REF is low, the system (output) can be powered by the wall adapter (input). At this time, the second overvoltage comparator inside the NCP370 will work, but its RDS(on) will be higher at this time, and the current is close to 10mA. In fact, during the test product (under 25℃ conditions), its input voltage can be higher (the typical value of OVLO is 8.27V).

2.4 Fault alarm output

NCP370 contains a FLAG alarm pin. Once NCP370 detects that the input voltage is higher than the OVLO threshold, lower than the U-VLO threshold, or there are faults such as overcurrent or overvoltage, the FLAG pin will be set low and notify the microcontroller to take appropriate measures. When the input voltage returns to the normal range, FIAG will be set high after a delay, and then Vout will resume output. Its working sequence is shown in Figure 3.

Since FLAG is an open-drain output, a pull-up resistor (minimum 10 kΩ) must be connected to the power supply. When the temperature of NCP370 is too high, its FLAG will be set low and the output will be disconnected from the input. The Vin status can be reflected at any time through the FLAG pin, even when the device is turned off (DIR=0). In this way, in reverse mode, the microcontroller can also correctly handle faults such as overvoltage, overcurrent and thermal shutdown according to the state change of its FLAG pin.

2.5 DIR input

By forcibly pulling the DIR terminal of NCP370 low and pulling the DEV terminal high, NCP370 can enter the forward charging state. When the DIR terminal is set high, the OUT terminal will be disconnected from the IN terminal. The specific charging state settings are listed in Table 1. In addition, DIR will also be affected by OVLO or U-VLO faults (FLAG is valid).

3 Typical Applications

NCP370 has a voltage drop protection function, which can protect the circuit from the reverse voltage at the IN terminal. When reverse voltage appears, the output of NCP370 will be disconnected from the input terminal.

Figure 4 shows a typical application circuit of NCP370.

By forcing the DIR terminal of NCP370 to high (>1.2 V) and the DEV terminal to low (<0.55 V), NCP370 can enter the reverse power supply state, at which time the power supply in the system can supply power to the peripherals placed on the bottom connector. The NCP370 is powered by a battery with a range of 2.5 to 5.5 V, and the OCP and thermal shutdown modes also work at this time.

When the current (Ilim) on the peripheral resistor Rlim is higher than Iocp, the current flowing through its internal N-channel MOS tube will be limited, so that NCP370 can provide reverse overcurrent protection from the battery to the peripheral. Since the internal resistance of NCP370 is in series with Ilim, its OCP is maximum when the Ilim pin is directly connected to GND, and when Rlim is connected between them, its OCP will decrease. The values ​​of OCP are listed in Table 2. When the current is higher than OCP, its internal NMOS will be turned off, and /FIAG will be set low. At the same time, NCP370 will notify the microcontroller to handle the fault and prohibit the reverse charging mode.

Through the thermal shutdown protection circuit inside the NCP370, the internal MOSFET can be turned off when the temperature is too high, so as to cool the device in time. The maximum thermal shutdown value allowed by the NCP370 is 150℃. If the temperature exceeds this value, the FLAG will be set low and the MCU will be notified. However, since the NCP370 chip also has a thermal hysteresis of 30℃, the MOSFET inside the chip can only be turned on when the temperature is lower than 120℃; when it is higher than 120℃, the NCP370 will always be in the thermal shutdown state. After that, the fault is eliminated and the chip can resume normal operation. The NCP370 integrates two low RDS(on) N-channel MOS tubes at the OUT end, which can be used to protect the external system at the OUT end from overvoltage, negative voltage and reverse current. Usually, this RDS(on) characteristic of the N-channel MOS tube will introduce a small amount of loss at the OUT end.

4 Conclusion

NCP370 is the latest important device launched by ON Semiconductor for portable application system products. This device can provide 28 V positive and negative overvoltage and overcurrent protection (OVP) for mobile phones, digital cameras, MP3/4, personal digital assistants (PDA) and GPS systems. In addition, NCP370 also provides a "reverse" mode to achieve a steady flow of reverse current in the battery of portable devices, so it can be widely used in the power supply of FM transceivers and audio and flash function accessory circuits on the bottom connector of portable devices.