MIC5013

Micrel, Inc.

MIC5013

8

July 2005

† Suppliers of Kelvin-sensed power resistors:

Dale Electronics, Inc., 2064 12th Ave., Columbus, NE 68601. Tel: (402) 564-3131

International Resistive Co., P.O. Box 1860, Boone, NC 28607-1860. Tel: (704) 264-8861

Kelvin, 14724 Ventura Blvd., Ste. 1003, Sherman Oaks, CA 91403-3501. Tel: (818) 990-1192

RCD Components, Inc., 520 E. Industrial Pk. Dr., Manchester, NH 03103. Tel: (603) 669-0054

Ultronix, Inc., P.O. Box 1090, Grand Junction, CO 81502. Tel: (303) 242-0810

Fault

V+

Gate

1

2

3

4

8

MIC5013

Gnd

7

6

5

Thresh

Sense

Source

Input

IRCZ44

(S=2590,

R=11mΩ)

10µF

22Ω

20kΩ

LOAD

Control Input

=15V

V+

+

V

LOAD

SO U R C E

KELVIN

S E N S E

S

R

S R

R I –

R =

L

V

TRI P

V

TRI P

S

R =

–1000

2200

V

TRI P

TH

For this example:

=20A (trip current)

I

L

=100mV

V

TRI P

inductive switching transients. The MIC5013 supply should

be limited to 15V in low-side topologies; otherwise, a large

current will be forced through the gate clamp zener.

Low-side drivers constructed with the MIC501X family are

also fast; the MOSFET gate is driven to near supply imme-

diately when commanded ON. Typical circuits achieve 10V

enhancement in 10µs or less on a 12 to 15V supply.

High-Side Driver with Current Shunt (Figure 3). The

comparator input pins (source and sense) float with the

triggering of the over-current shutdown circuit with open

or inductive loads. R1 is sized for a current flow of 1mA,

while R2 contributes a drop of 100mV. The shunt voltage

should be 200 to 500mV at the trip point. The example of

Figure 3 gives a 10A trip current when the output is near

supply. The trip point is somewhat reduced when the output

is at ground as the voltage drop across R1 (and therefore

R2) is zero.

High-side drivers implemented with MIC5013 drivers are

self-protectedagainstinductiveswitchingtransients.During

turn-off an inductive load will force the MOSFET source 5V

or more below ground, while the driver holds the gate at

groundpotential.TheMOSFETisforcedintoconduction,and

it dissipates the energy stored in the load inductance. The

MIC5013 source and sense pins (3 and 4) are designed to

withstand this negative excursion without damage. External

clamp diodes are unnecessary.

10W. Worthy of special mention are Kelvin-sensed, “four-

(see next page). Kelvin-sensed resistors eliminate errors

caused by lead and terminal resistances, and simplify

product assembly. 10% tolerance is normally adequate, and

with shunt potentials of 200mV thermocouple effects are

insignificant. Temperature coefficient is important; a linear,

500 ppm/°C change will contribute as much as 10% shift in

the over-current trip point. Most power resistors designed

for current shunt service drift less than 100 ppm/°C.

Low-Side Driver with Current Sensing MOSFET (Figure

4). Several manufacturers now supply power MOSFETs in

which a small sampling of the total load current is diverted

to a “sense” pin. One additional pin, called “Kelvin source,”

is included to eliminate the effects of resistance in the

sourcebondwires.Current-sensingMOSFETsarespecified

with a sensing ratio “S” which describes the relationship

between the on-resistance of the sense connection and

the body resistance “R” of the main source pin. Current

sensing MOSFETs eliminate the current shunt required by

standard MOSFETs.

The design equations for a low-side driver using a current

sensing MOSFET are shown in Figure 4. “S” is specified

on the MOSFET’s datasheet, and “R” must be measured

become negative. Substituting a MOSFET with higher on-

to 200mV is suggested. Although the load supply is limited

only by MOSFET ratings, the MIC5013 supply should be

limited to 15V to prevent damage to the gate clamp zener.

Output clamping is necessary for inductive loads.

“R” is the body resistance of the MOSFET, excluding bond

includes bond resistances. A Kelvin-connected ohmmeter

Figure 4. Low-Side Driver with

Current-Sensing MOSFET