New Generation of Load Switch ICs Cut Standby power

Apr 1, 2010 12:00 PM
SAM DAVIS, Editor in Chief

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.

Table: IntelliMAX load switch characteristics

PART NUMBER	1.8VIN SWITCH RDS(ON)	INPUT BUFFER	OUTPUT DISCHARGE	ON PIN ACTIVITY	SLEW RATE (µS)
FPF1103	55 mΩ	CMOS	No	High	65
FPF1104	55 mΩ	CMOS	65Ω MOSFET	High	65
FPF1107	55 mΩ	CMOS	No	High	130
FPF1108	55 mΩ	CMOS	65Ω MOSFET	High	130

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