General Instructions

The AAT4650 smart switch is a single-channel PC card (PCMCIA) power switch. It is used to select between two different voltage inputs, each between 2.7V and 5.5V. An internal switch powers the circuit from the higher input voltage. The device's output, V, is controlled for slew rate and current limit, in accordance with PC Card specifications. The current limit response time for a short circuit is typically 1 μs. The internal P-channel MOSFET switches are configured to turn off before turning on; that is, both switches cannot be turned on at the same time. Controlled by a 2-bit parallel interface, the four states V represent V, V, high impedance, or ground. When at ground, V is pulled to ground by a 5k resistor - Tor. Disconnects the drain fault output in the over-current condition. A fault also signals V out of tolerance during power-swing. When an overtemperature condition exists, an internal overtemperature sensor forces V into a high impedance state. As long as I is less than about 500mA, the quiescent current is typically 15μa low. Above this load current, the quiescent current increases to 200µA.

The AAT4650 is available in a lead-free, 8-pin SOP package with a temperature range of -40°C to +85°C.

feature

• 2.7V to 5.5V input voltage range

• 80m (5V) typical radio data system (on)

• Low quiescent current 15µA (typ)

• Reverse blocking switch

•Short circuit protection

•Overheating protection

• Fault flag output

• Temperature range: -40°C to +85°C

• 8-pin SOP package

application

•laptop

• Pocket PC, sub-notebook

• Power multiplexing circuit

typical application

Pin Description

Pin configuration

Typical features

T=25°C unless otherwise noted.

Functional block diagram

Function description

The AAT4650 is a single-channel power switch that can be used in any application that requires dual-supply multiplexing. Typical applications include PC card applications that do not require a 12V power supply, or applications where power is switched, for example, between a 5V operating mode and a 3.3V standby mode. The AAT4650 operates in any combination over an input voltage range of 2.7V to 5.5V and automatically shuts down its internal circuitry, no matter which input voltage is higher. Two identical low-impedance P-channel MOSFETs are used as a power multiplexing circuit, with one common drain as the V output and the other independent source as the V and V inputs. The 2-bit parallel interface determines the state of the multiplexer: V=V, V=V, V (resistive pull-down to ground), or V high impedance. When the state is set to one of the two inputs, the multiplexGNDDS(on) control circuit will slowly transition the V output to the new voltage level, protecting the upstream supply from sudden load transients. When V selects a resistor pull-down, the V output is quickly discharged through the resistor pull-down. The AAT4650 always acts as an electronic fuse if the load current exceeds the current limit threshold. During an energized short circuit, the current will gradually increase until the current limit is reached. In the event of a sudden short circuit at the output, the current limit will respond within 1µs to isolate and protect the upstream supply from a load short circuit. In most applications, a short circuit to V will not affect the upstream supply due to the fast response time, so system functionality will not be affected. In an overcurrent condition, the open-drain fault flag output will signal an event. The fault output is also active during output voltage transitions and becomes inactive once the output is within regulation.

application information

input capacitor

A 1µF or larger capacitor is generally recommended for C. A C capacitor is not required for basic operation; however, it helps prevent load transients from affecting upstream circuits. The location of C should be as close as possible to the device's VIN. Ceramic, tantalum, or aluminum electrolytic capacitors can be selected for C. There is no specific capacitor equivalent series resistance (ESR) requirement for C. However, for higher current operation, ceramic capacitors are recommended for C due to their inherent ability to withstand input current surges from low impedance sources, such as batteries in portable equipment, than tantalum capacitors.

output capacitor

A 0.1µF or larger capacitor is typically required between V and GND. Likewise, for output capacitors, there are no specific capacitor ESR requirements. If required, C can be increased to accommodate any load transient conditions.

Parallel interface/disconnect before making

A 2-bit parallel interface determines the state of the V output. Logic levels are compatible with CMOS or TTL logic. The logic low value must be less than 0.8V and the logic high value must be greater than 2.4V. If the state of an interface pin changes rapidly from 3V to 5V directly (or vice versa), the internal disconnect before closing the circuit prevents current from flowing back from one input supply to the other. In addition, the body connection of the internal P-channel MOSFET switch is always set to the highest potential of V, V, or V, which prevents any body diode conduction, power backflow, or possible device damage.

fault output

During overcurrent or output transition conditions, the fault output is pulled to ground by an open-drain N-channel MOSFET. It should be pulled to the reference supply of the controller IC through a nominal 100k resistor.

Voltage regulation

The 5V power supply tolerance specified by the PC card specification is ±5%. Of these, a typical power supply will drop by less than 2%, while a PCB trace will drop by 1%. This leaves 2% of the AAT4650 as a PC card switch. In PC card applications, the maximum allowable current of the AAT4650 is determined by voltage regulation rather than thermal factors, and is set by the current limit or maximum R of the P-channel MOSFET. The maximum R at 85°C is calculated by applying the RDS(ON) temperature coefficient to the maximum room temperature R:

-or-

The maximum current is equal to the 2% tolerance of the 5V supply (100mV) on the AAT4650 divided by

RDS (on) (max). or:

For a 3.3V supply in PC card applications, the conditions are a bit looser, allowing a voltage regulation drop equal to 300mV. Using 2% power and 1% PCB tracking regulation, the PC card switch can have a 200 volt drop. so:

Since 1.5A is the rated current limit, the AAT4650 will reach the current limit before reaching IMAX3.

In the SOP-8 package, thermal issues are not an issue as the package thermal resistance JA is only 120°C/W. At any given ambient temperature (TA), the maximum power dissipation of the package can be determined by the following equation:

The constants for the AAT4650 are the maximum junction temperature, T=125°C, and the package thermal resistance, =120°C/W. Worst case conditions are calculated at maximum operating temperature, where T=85°C. Typical conditions are calculated under normal ambient conditions, where T=25°C. At T=85°C, P=333mW. At T=25°C, P=833mW.

The maximum current is given by the following formula:

For the AAT4650 at 85°C, I=1.65A, which is greater than the value of the internal minimum current limit specification.

overcurrent and

Over temperature protection

Because many AAT4650 applications provide power to peripherals, it is designed to protect its host device from failures of these peripherals through slew rate control, current limiting, and thermal limiting. The current limit and thermal limit of the AAT4650 acts as an immediate and reliable electronic fuse without any increase in R for this function. Other solutions, such as multiple fuses, do not protect the host power supply and system from misuse or peripheral short circuits; they only protect against fire. The high-speed current limit and thermal limit of the AAT4650 not only protects against fire, but also isolates the power supply and the entire system from any activity on external ports and reports mis-DS (on) through a fault signal.

Overcurrent and overtemperature exist simultaneously. Once an overcurrent condition exists, the current supplied by the AAT4650 to the load is limited within the overcurrent threshold. This causes a voltage drop across the AAT4650, resulting in excessive power dissipation and increased package temperature. When the mold begins to heat up, the overheat circuit is activated. If the temperature reaches the maximum level, the AAT4650 automatically turns off the P-channel mosfet. When they are off, the overheat circuit remains active. After the temperature cooled by about 10°C, the P-channel mosfet turned back on. In this way, the AAT4650 is thermally cycled on and off until the short is removed. Normal operation will resume automatically when the short circuit is removed.

To conserve power, the full high-speed overcurrent circuit is not activated until a lower current threshold (about 500mA) is exceeded in the power supply. When the load current exceeds this rough threshold, the quiescent current of the AAT4650 increases from 15µA to 200µA. The high-speed overcurrent circuit works by limiting the current linearly when the current reaches the limit. Because of the current limit, the voltage starts to drop above V, and the current limit varies in magnitude, usually decreasing as the V voltage drops to 0V.

switching voltage

The AAT4650 satisfies the PC card standard for converting the V output by providing a ground path for V and a high impedance state. The PC Card protocol for determining low voltage operation is to first power the peripheral with 5V, then poll for 3.3V operation. When transitioning from 5V to 3.3V, V must discharge to less than 0.8V to provide a hard reset. The resistor ground state (CTL1=0, CTL0=0) will accommodate this. The ground state will also guarantee that the V voltage is discharged within the specified time (100ms). Cocos IslandsCocos IslandsCocos IslandsCocos Islands

A printed circuit board

Layout suggestion

For proper thermal management, to minimize PCB trace resistance, and to take advantage of the low R of the AAT4650, some board layout rules should be followed: V, V, and V should be routed using wider paths than normal traces; two V The pins (pins 6 and 7) should be connected to the same wide PCB trace; the ground should be connected to the ground plane. For best performance, C and C should be placed close to the packing pins.

Timing diagram

Typical PC Card Application Circuit

Evaluation Board Layout

The AAT4650 evaluation board layout follows the printed circuit board layout recommendations for a good application layout.

NOTE: The board layout shown is not to scale.

1. Sample stock is normally held on all part numbers listed in bold.

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[1]. Stresses in excess of the Absolute Maximum Ratings may cause permanent damage to the device. Functional operation under conditions other than specified operating conditions is not implied.

[2]. The Human Body Model is a 100pF capacitor that discharges into each pin through a 1.5k resistor.

[3]. Mounted on FR4 board.