NCP346

http://onsemi.com

6

APPLICATION INFORMATION

−

+

OUT

IN

GND

CNTRL

+

Logic

FET

Driver

LOAD

AC/DC Adapter or

Accessory Charger

Zener

Diode

(optional)

C1

Schottky

Diode

Microprocessor

port

P−CH

NCP346

Zener

Diode

(optional)

Figure 3.

(optional)

(opt.)

MBRM130LT1

NHTS4101PT1

Introduction

In many electronic products, an external AC/DC wall

adapter is used to convert the AC line voltage into a

regulated DC voltage or a current limited source. Line

surges or faults in the adapter may result in overvoltage

events that can damage sensitive electronic components

within the product. This is becoming more critical as the

operating voltages of many integrated circuits have been

lowered due to advances in sub−micron silicon lithography.

In addition, portable products with removable battery packs

pose special problems since the pack can be removed at any

time. If the user removes a pack in the middle of charging,

a large transient voltage spike can occur which can damage

the product. Finally, damage can result if the user plugs in

the wrong adapter into the charging jack. The challenge of

the product designer is to improve the robustness of the

design and avoid situations where the product can be

damaged due to unexpected, but unfortunately, likely events

that will occur as the product is used.

Circuit Overview

To address these problems, the protection system above

has been developed consisting of the NCP346 Overvoltage

Protection IC and a P−channel MOSFET switch such as the

MGSF3441. The NCP346 monitors the input voltage and

will not turn on the MOSFET unless the input voltage is

within a safe operating window that has an upper limit of the

overvoltage detection threshold. A Zener diode can be

placed in parallel to the load to provide for secondary

protection during the brief time that it takes for the NCP346

to detect the overvoltage fault and disconnect the MOSFET.

The decision to use this secondary diode is a function of the

charging currents expected, load capacitance across the

battery, and the desired protection voltage by analyzing the

dV/dT rise that occurs during the brief time it takes to

turn−off the MOSFET. For battery powered applications, a

low−forward voltage Schottky diode such as the

MBRM120LT3 can be placed in series with the MOSFET to

block the body diode of the MOSFET and prevent shorting

the battery out if the input is accidentally shorted to ground.

This provides additional voltage margin at the load since

there is a small forward drop across this diode that reduces

the voltage at the load.

When the protection circuit turns off the MOSFET, there

can be a sudden rise in the input voltage of the device. This

transient can be quite large depending on the impedance of

the supply and the current being drawn from the supply at the

time of an overvoltage event. This inductive spike can be

clamped with a Zener diode from IN to ground. This diode

breakdown voltage should be well above the worst case

supply voltage provided from the AC/DC adapter or

Cigarette Lighter Adapter (CLA), since the Zener is only

intended to clamp the transient. The NCP346 is designed so

withstand transients to 30 V. Since these spikes can be very

narrow in duration, it is important to use a high bandwidth

probe and oscilloscope when prototyping the product to

verify the operation of the circuit under all the transient

conditions. A similar problem can result due to contact

bounce as the DC source is plugged into the product.

For portable products it is normal to have a capacitor to

ground in parallel with the battery. If the product has a

battery pack that is easily removable during charging, this

scenario should be analyzed. Under that situation, the

charging current will go into the capacitor and the voltage

may rise rapidly depending on the capacitor value, the

charging current and the power supply response time.