Equipment Manufacturers and consumers are now very conscious of energy conservation, particularly for standby power, also called vampire power, or phantom power. It is the electricity consumed by electronic equipment when switched off or in a standby mode. The typical power loss per equipment is low (from 1 to 25 W) but when multiplied by the billions of equipment in houses and commercial buildings, standby losses represent a significant fraction of total world electricity use. There are two ways to minimize standby power: better power transformers and efficient load switch ICs that enable/disable computer functions.
The ac adapter (external) power supply used with many computer peripherals is one standby power culprit. Even when their ac power is on and their dc load is off, they consume power due to transformer leakage. Applications typically affected include consumer set-top boxes, computer peripherals, etc. This can usually be improved by using better transformers and associated circuit design.
The other form of standby power is associated with computer systems that do not require all functions to be active all the time. Even though the particular function may not be necessary, it can still consume power. Power could be reduced by turning off unnecessary functions that aren't required for a specific operation. Among the computer functions that may be disabled at some point in time are graphics processors, flash memory, disk drives, I/Os, and FPGAs. A relatively simple solution is a smart and efficient load switch that turns off power for these functions when they are not needed and then restores power when it is required.
A recently introduced smart load switch is Fairchild's Intellimax™ family of ICs that combat standby power challenges. A control signal turns the load switch on or off. When the load switch turns on it applies power to the load, enabling it. When the switch turns off it draws <1µA when disabling the associated function. That is, the load switch draws a negligible amount of current when not being used, which means a minimum amount of standby power. It also helps that its small footprint and integrated feature set minimize space requirements. The IntelliMAX ICs can also minimize power consumption and extend battery life in portable systems. These new ICs include the FPF1103, FPF1104, FPF1107 and FPF1108 monolithic IC load switches. With capacitive loads, integrated slew-rate control prevents inrush current glitches from supply rails that are common in power applications.
SAVES LCD PANEL POWER
A typical load switch implementation could be the control of power applied to an LCD panel. The LCD power could be turned on for viewing for 15 seconds, then turned off by a load switch to conserve power. In addition, a load switch can turn on and off various power rails for other functions. The Intellimax switch provides this load switching efficiently, because its off-state current is much less than the current normally drawn by the circuit it can disable.
Figure 1 shows a simplified schematic of the four load switches. In operation, the load switch accepts an external enable (ON) signal, and connects or disconnects a power source to a given load. The IntelliMAX load switches are considered high-side load switches that consist of up to five functional components. They include:
- Pass transistor, typically an enhancement-mode p-channel MOSFET
- Control logic that accepts an input and drives the gate of the pass transistor to switch it ON or OFF.
- Turn-on slew rate control
- Optional output discharge MOSFET
- ESD input protection with a Schottky diode
The pass transistor's most important parameter is its resistance when turned on. This resistance, RDS(ON), affects power consumption characteristics of the pass transistor.
Input voltage for the new load switches, VIN, accepts 1.2V to 4V to satisfy the needs of low-voltage applications. The active-high ON pin accepts low-voltage CMOS logic inputs from GPIOs (general purpose I/O) and embedded processors. The table lists the key characteristics of the new load switch family.
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Unlike discrete and existing load switch solutions, the FPF110x family of advanced slew-rate load switches offer over a 30 percent reduction in RDS(ON) and integrated analog switch control functionality - allowing direct load switch interconnection into most microprocessors for software-transparent operations - all in a 1x1mm WL-CSP package.
Based on its use of power CMOS silicon processing, the FPF110x series eliminates the need for additional input capacitors, and offers robust protection through integrated slew rate control. Their slew rate options of 65µs and 130µs meet individual customer needs and provide system stability over the entire operating range to avoid voltage sags and current spikes. Additionally, ESD input protection (with a Schottky diode) of 4kV reduces failures during manufacturing and the false readings caused by adverse ESD events. The FPF1104 and FPF1108 offer an additional integrated NMOS device for load discharge of output capacitance in off-state cycles.
INPUT/OUTPUT CAPACITORS
Although an IntelliMAX™ switch doesn't require an input capacitor, you can reduce inrush current effect with a 0.1µF ceramic capacitor, CIN, mounted close to the VIN pin (see Fig. 2.). A higher value of CIN can be used to further reduce the voltage drop experienced as the switch turns on into a large capacitive load.
The lntelliMAX switches will work without an output capacitor, but if parasitic board inductance forces VOUT below GND when switching off, connect a 0.1µF capacitor, COUT, between VOUT and GND.
Device output fall time can be calculated based on the RC time constant of external components as follows:
tF = RL × CL × 2.2
where:
tF = 90% to 10% fall time in µS
RL = Output load in ohms
CL = Output capacitor in pF
The same equation works for a device with a pull-down output resistor. RL is replaced by a parallel connected pull-down and an external output resistor combination, as follows:
Where:
RPD = 65Ω
If there is no resistive output load, the lnteIIiMAX™ switch has no pull-down output resistor so it may not discharge the output voltage. In that case, the output voltage drop depends mainly on external device leaks.
The ON pin controls the state of the switch. Activating ON continuously holds the switch in the on-state, as there is no fault. ON is active high and has a low threshold, making it capable of interfacing with low-voltage signals. The ON pin is compatible with standard GPIO logic threshold. It can be used with any microcontroller with 1.2-V, 1.8-V, 2.5-V or 3.3-V GPIOs.
To achieve the best performance using the IntelliMAX switch, designers should keep all traces as short as possible. For maximum effectiveness, the input and output capacitors should be placed close to the device to minimize the effects that parasitic trace inductances may have on normal and short-circuit operation. Using wide traces for VIN, VOUT, and GND helps minimize the parasitic electrical effects, along with minimizing the case-to-ambient thermal impedance.
