MIC4416/4417

Micrel, Inc.

MIC4416/4417

8

May 2005

Application Information

The MIC4416/7 is designed to provide high peak current for

charging and discharging capacitive loads. The 1.2A peak

value is a nominal value determined under specific condi-

tions. This nominal value is used to compare its relative size

to other low-side MOSFET drivers. The MIC4416/7 is not

designed to directly switch 1.2A continuous loads.

Supply Bypass

Capacitors from VS to GND are 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 tantalum capacitor is suitable for many

applications. Low-ESR (equivalent series resistance) metal-

ized film capacitors may also be suitable. An additional 0.1µF

ceramic capacitor is suggested in parallel with the larger

capacitor to control high-frequency transients.

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 auto-

matic test equipment). Avoid instantaneously applying volt-

age, capable of very high peak current, directly to or near

tantalum capacitors without additional current limiting. Nor-

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

trace resistance is usually adequate for normal product

usage.

Circuit Layout

Avoid long power supply and ground traces. They exhibit

inductance that can cause voltage transients (inductive kick).

Even with resistive loads, inductive transients can sometimes

exceed the ratings of the MOSFET and the driver.

When a load is switched off, supply lead inductance forces

current to continue flowing—resulting in a positive voltage

spike. Inductance in the ground (return) lead to the supply

has similar effects, except the voltage spike is negative.

Switching transitions momentarily draw current from VS to

GND. This combines with supply lead inductance to create

voltage transients at turn on and turnoff.

Transients can also result in slower apparent rise or fall times

when driver’s ground shifts with respect to the control input.

Minimize the length of supply and ground traces or use

ground and power planes when possible. Bypass capacitors

should be placed as close as practical to the driver.

MOSFET Selection

Standard MOSFET

A standard N-channel power MOSFET is fully enhanced with

a gate-to-source voltage of approximately 10V and has an

absolute maximum gate-to-source voltage of ±20V.

The MIC4416/7’s on-state output is approximately equal to

the supply voltage. The lowest usable voltage depends upon

the behavior of the MOSFET.

VS

CTL

G

GND

MIC4416

4.7µF

+8V to +18V

1

32

4

Logic

Input

*Gate enhancement voltage

+15V

Standard

MOSFET

† International Rectifier

100m

0.1µF

Try a

15

Ω, 15W

or

1k, 1/4W

resistor

Figure 1. Using a Standard MOSFET

Logic-Level MOSFET

Logic-level N-channel power MOSFETs are fully enhanced

with a gate-to-source voltage of approximately 5V and have

an absolute maximum gate-to-source voltage of ±10V. They

are less common and generally more expensive.

The MIC4416/7 can drive a logic-level MOSFET if the supply

voltage, including transients, does not exceed the maximum

MOSFET gate-to-source rating (10V).

VS

CTL

G

GND

MIC4416

+4.5V to 10V*

1

32

4

Logic

Input

*Gate enhancement voltage

(must not exceed 10V)

+5V

Logic-Level

MOSFET

† International Rectifier

28m

4.7µF

0.1µF

Try a

3

Ω, 10W

or

100

Ω, 1/4W

resistor

Figure 2. Using a Logic-Level MOSFET

At low voltages, the MIC4416/7’s internal P- and N-channel

MOSFET’s on-resistance will increase and slow the output

rise time. Refer to “Typical Characteristics” graphs.

Inductive Loads

On

Off

VS

CTL

G

GND

MIC4416

1

32

4

Schottky

Diode

4.7µF

0.1µF

Figure 3. Switching an Inductive Load

Switching off an inductive load in a low-side application forces

the MOSFET drain higher than the supply voltage (as the

inductor resists changes to current). To prevent exceeding

the MOSFET’s drain-to-gate and drain-to-source ratings, a

Schottky diode should be connected across the inductive

load.