Micrel, Inc.

MIC5019

July 2012

7

Application Information

Supply Bypass

A capacitor from VDD to GND is recommended to

control switching and supply transients. Load current

and supply lead length are some of the factors that affect

capacitor size requirements.

A 4.7μF or 10μF ceramic capacitor, aluminum

electrolytic or tantalum capacitor is suitable for many

applications.

The low ESR (equivalent series resistance) of ceramic

and tantalum capacitors makes them especially

effective,

but

also

makes

them

susceptible

to

uncontrolled inrush current from low impedance voltage

sources (such as NiCd batteries or automatic test

equipment). Avoid applying voltage instantaneously,

capable of high peak current, directly to or near tantalum

capacitors without additional current limiting. Normal

power supply turn-on (slow rise time) or printed circuit

trace resistance is usually adequate for normal product

usage.

MOSFET Selection

The

MIC5019

is

designed

to

drive

N-channel

enhancement type MOSFETs. The gate output (OUT) of

the MIC5019 provides a voltage, referenced to ground,

that is greater than the supply voltage. Refer to the

“Typical Characteristics: Output Voltage vs. Supply

Voltage” graph.

The supply voltage and the MOSFET drain-to-source

voltage drop determine the gate-to-source voltage.

where:

Supply Voltage)

(approx. 0V at low current, or when fully enhanced)

Figure 1. Node Voltages

The performance of the MOSFET is determined by the

gate-to-source voltage. Choose the type of MOSFET

according to the calculated gate-to-source voltage.

Standard MOSFET

Standard MOSFETs are fully enhanced with a gate-to-

source voltage of about 10V. Their absolute maximum

gate-to-source voltage is ±20V.With a 4.5V supply, the

MIC5019 produces a gate output of approximately 15V.

Figure 2 shows how the remaining voltages conform.

The actual drain-to-source voltage drop across an

IRFZ24 is less than 0.1V with a 1A load and 10V

enhancement. Higher current increases the drain-to-

source voltage drop, increasing the gate-to-source

voltage.

Figure 2. Using a Standard MOSFET

The MIC5019 has an internal zener diode that limits the

gate-to-ground voltage to approximately 16V.

Lower supply voltages, such as 3.3V, produce lower

gate output voltages which will not fully enhance

standard MOSFETs. This significantly reduces the

maximum current that can be switched. Always refer to

the MOSFET data sheet to predict the MOSFET’s

performance in specific applications.

Logic-Level MOSFET

Logic-level N-channel MOSFETs are fully enhanced with

a gate-to-source voltage of approximately 5V. Some of

the MOSFET’s may have an absolute maximum gate-to-

source voltage of ±10V (Refer to MOSFET datasheet).