Micrel, Inc.

MIC5018

April 2006

6

M9999-042406

(408) 955-1690

Application Information

Supply Bypass

A capacitor from VS 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 aluminum electrolytic or tantalum

capacitor is suitable for many applications.

The low ESR (equivalent series resistance) of 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

instantaneously applying voltage, 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

MIC5018

is

designed

to

drive

N-channel

enhancement type MOSFETs. The gate output (G) of

the MIC5018 provides a voltage, referenced to ground,

that is greater than the supply voltage. Refer to the

“Typical Characteristics: Gate Output Voltage vs. Supply

Voltage” graph.

The supply voltage and the MOSFET drain-to-source

voltage drop determine the gate-to-source voltage.

where:

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

VS

CTL

G

GND

MIC5018

1

23

4

D

S

Figure 1. 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 5V supply, the MIC5018 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.

VS

CTL

G

GND

MIC5018

4.7µF

+5V

1

23

4

Logic

High

10V

approx. 0V

5V

15V

Voltages are approximate

* International Rectifier

standard MOSFET

IRFZ24*

To demonstrate

this circuit, trya

2 , 20W

load resistor.

Figure 2. Using a Standard MOSFET

The MIC5018 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 and

generally have an absolute maximum gate-to-source

voltage of ±10V.

VS

CTL

G

GND

MIC5018

4.7µF

+3.3V

1

23

4

Logic

High

5.7V

approx. 0V

3.3V

9V

Voltages are approximate

* International Rectifier

logic-level MOSFET

IRLZ44*

To demonstrate

this circuit, try

5 , 5W or

47 , 1/4W

load resistors.

Figure 3. Using a Logic-Level MOSFET

Refer to Figure 3 for an example showing nominal

voltages. The maximum gate-to-source voltage rating of

a logic-level MOSFET can be exceeded if a higher