Load Management Devices: Simplifying Design and Ensuring Protection for Portable Applications

The rapid growth of portable consumer products such as digital still cameras, MP3 players, digital video cameras, cell phone handsets, palm top computers, and personal digital assistants has created a competitive landscape for equipment manufacturers. These manufacturers must satisfy the insatiable consumer demand for higher performance, smaller size form factors, and competitive pricing while quickly getting new and exciting products to the market well ahead of the competition. Adding to the design complexity is the convergence of these functions into one product – smart phones with cameras, MP3 players, USB data interfaces, PDA functions as an example. This trend has also given rise to a number of external peripherals that connect through an expansion card slot, serial data bus, or pluggable power interface. The hardware and software teams designing these latest generation products must minimize power consumption (to increase battery life), reduce component count and board space, and ensure product functionality/quality- all while getting products to market faster then ever dreamed possible even three to five years ago.

To further complicate the situation, it is often critical to ensure operation of the end products when these products interface with external accessories that are manufactured and marketed by a wide variety of global third party companies. These accessories include chargers, communication cards, data entry / keyboard connections, flash memory cards, printers, wired headsets, ear buds, and communication cables. Designers must consider that these external ports and card slots are connected to sensitive integrated circuits embedded inside of these consumer products. A single faulty connector, the incorrect sequencing of voltages when a cable is connected or an internal power bus is activated, or undesired excessive current drain can damage or degrade the performance of the end product resulting in field quality issues. This can lead to unhappy customers and from the manufacturer standpoint, a loss in market share. This article explores the protection possibilities, selection, and implementation of new load management and protection products to avoid these issues and to ensure that the manufacturer’s end product is robust enough to meet the rigors of the external environment. A typical load management application is shown below in Figure 1.

Figure 1: Typical Load switch Application

Note that in order to implement a standard load switch application several discrete components and PMOS device are required in addition to an available logic or enable signal to control the turn-on of the device. The solution above doesn’t take into account any protection or control functions or software required to ensure the system operates properly due to fault conditions previously discussed.

Below is an example of device that provides a variety of protection and control features integrated into one device to simplify product design and reduce space and component count. The combination of protection for under-voltage, over-current, in-rush current, short circuit, or over-temperature conditions is important to designers. Next generation products need to integrate all of these functions in one small SMD device with all of necessary control functions to both satisfy design concerns and to make these devices easy to use for the consumer.

From a design standpoint, incorporating these protection and control features into one IC simplifies design, reduces part count, ensures reliability and conformance to industry standards and also meets consumer expectations for latest technologies and features.

Over Current Protection

As portable products continue to evolve and integrate denser levels of complexity and functionality, there arises a variety of applications where load currents must be limited to protect the internal power bus, external battery, or external device or accessory that is plugged into the device. Due to the nature of these connections, it is often not possible for internal processor or other hardware to be aware of a load or device that is creating excessive current or operation. These types of external devices often have an intended power (often current) drain that is considered “normal” operation, but there are fault conditions or power interfaces that must limited to protect internal batteries from over-discharge, over-charge or other conditions that create undesired behavior or performance. An example of integrated solution providing a variety of control/protection features, notably the current limit protection is the IntelliMAXTM FPF210X and FPF200X load management products. Both devices include integrated current limit with fast response time for nominal and short-circuit conditions to prevent damage when fault conditions due occur. The minimal, nominal, and maximum currents are noted in the table below as well.

In selecting a device, the designer must consider the nominal, minimum and maximum current in the desired application to make sure that the product’s performance is within range of desired operation. For example, if the intended nominal load current is less than 400mA, then the designer would consider the FPF2110 device, which trips at a minimum of 400mA as specified by the product datasheet. The designer would also need to verify that the absolute maximum level of over-current protection of 800 mA would not cause damage or undesired operation in the system.

Critical to the operation of the current-limiting function is the response time under short circuit or nominal over-current conditions.. The devices typically have a short circuit response time, which is ultra-fast (in the 20nS range for FPF210X devices) to prevent damage to the system and a fast response for nominal over-current conditions (typically 3uS for FPF210X devices). By selecting a device, which provides the adequate current limiting protection and properly responds to short circuit conditions, designers can be assured of proper system behavior.

UVLO (Under-Voltage Lockout)
Another important design consideration is how the system behaves in conditions where the system supply voltage – whether powered by battery or low-voltage DC source – can dip to levels that may cause undesired operations for other critical ICs: DSPs, application or media processors, or other hardware devices. Integrated into all IntelliMAX FPF200X and FPF210X devices are control circuitry that provides under-voltage lockout protection to ensure a level of protection against supply voltages that haven’t stabilized or batteries, which are below useable output voltage. This simple function ensures that the switch is disabled when the input voltage drops below the specified threshold. For these devices this threshold is specified as 1.5V min, 1.6 nominal, 1.7V max. Equally important, the design includes built-in hysteresis (typically 47 mV) as the input voltage is increasing that ensures that the device doesn’t turn off intermittently near the threshold voltage. If the ON pin is enabled and the VIN signal is rising towards the threshold, it will turn on @ VTH and not turn off again until the input passes below threshold minus the hysteresis. The designer must therefore insure that the target active operating region for the switch (in the active state) is above the threshold and hysteresis of the UVLO conditioning specifications.

In-Rush Current Limiting

Another challenge power management designers are facing is the handling of in-rush currents or overshoot of power signals as switches are enabled due to capacitive loads. In the past, designers have had to make tradeoffs between the capacitor values to minimize noise or EMI present on power supplies generated by switching or digital circuits and the management of turn-on and in-rush currents. With the latest IntelliMAX load management, in-rush limiting is built into the devices to control the turn-on of the integrated PMOS switch that minimizes the in-rush current and overshoot without requiring adjustment of external capacitors. The control section of the IC allows for the gradual turn-on of the PMOS output device over typical time of 25 uS that prevents excessive in-rush. The amount of in-rush that results is dependent upon the specific system conditions (VIN, VOUT, Inominal, Imax) and must be tested in the application for verification. An example of the in-rush limiting is shown in the plot below where nominal current of 200mA is switched through an FPF210X device. Overshoot of less then 30% is measured with a load of 10ohms (simulated short condition) and COUT = 1.0 uF. By reducing in-rush current, system designers can decrease power loss, system damage or undesired behavior due to transients and stress of other components in the system.

Thermal Limiting/Protection
In order to protect the system from internal or external over-temperature conditions, IntelliMAX devices include thermal shutdown control that allows for shutdown (with hysteresis) when the device temperature approaches the junction temperature. During an over-temperature condition, the FLAG B pin is asserted and the device is shut-down. The device will re-activate when the device cools below the “Return from Shutdown” threshold (including the hysteresis of 10 C) noted below. The hysteresis prevents the device from shutting down intermittently near the shutdown threshold.

The thermal protection function is a key feature for integrated switch functions, especially in portable or DC-powered systems that need to protect from excessive heating due to internal or external environmental conditions.

Reverse Current Protection
Two key factors have driven a wider range of consumer and enterprise products towards portability: a) the wide variety of low-cost advanced battery technologies and b) the rapid integration provided by leading suppliers of core chipsets for portable products. These factors combined with the adoption of pluggable power sources and USB ports by portable products has placed further requirements power management designs as power must be managed into the product (for charging or external supply) and out of the product (to power external accessories). Historically, power designs would need to incorporate a variety of active and passive components including PMOS switches, poly-fuses and other components to manage uni-directional power flow. The application diagram below illustrates how this can be achieved in a single integrated load management device.

Fault Management
While taking current limiting, under-voltage condition or thermal shutdown into account in a power management design, it is often a considerable challenge for a designer to manage all of the design tradeoffs and handle the fault conditions and interactions with the rest of system – i.e. processor/software. The IntelliMAX products have been specifically designed to minimize component count, reduce design complexity, and reduce burden of system software via high level of functional integration. Each device in the IntelliMAX series provides several options within device family to handle fault conditions. As seen in the table below, the user can choose auto-restart, latch off at current limit, or hard limit to allow for designers to select the appropriate device to meet system requirements without requiring extra hardware or software/system modifications.

For example, as the device enters current limiting for an FPF2103 or 2107, it will stay in constant current mode until the current level falls below the current threshold. For an FPF2102 or FPF2106, the device will latch off after the 10mS blanking time, the fault flag is asserted HI and the device stays off until the ON pin is toggled by the system. Finally for an FPF2100/01 or FPF2104/5 the devices will auto-restart after the blanking time of 10mS (FLAG B is activated) for 160mS auto-restart time unless the fault condition is removed. This is also true of the FPF200X product family as reflected in the datasheet for those devices as well.

Summary
From the designer’s perspective, load management devices with the highest level of integrated functionality and power management features are pivotal in addressing the following issues:
•Simplifying power management designs to reduce component count and complexity.
•Offering designers a wider choice of hardware options to match the desired operation and application.
•Providing a wide range of operating voltage 1.8V up to 5.5V operating, 6V ABS max.
•Combining advanced control and protection functions needed to for latest generation technologies and architectures: UVLO, ESD protection, Current Limiting, Thermal Shutdown.
•Integrated PMOS switch with in-rush limiting control circuitry to reduce transients and power drain.

The integration of key building block functions enables solutions that facilitate the design of the leadership products demanded by consumers.