MIC5013

Micrel

MIC5013

8

July 2000

†

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=11m

Ω)

10µF

22

Ω

20k

Ω

Figure 4. Low-Side Driver with

Current-Sensing MOSFET

LOAD

Control Input

=15V

V

+

+

R

TH

V

LOAD

SOURCE

KELVIN

SENSE

S

R

SR

R I –

R =

L

V

TRIP

V

TRIP

S

R

=

–1000

2200

V

TRIP

TH

For this example:

=20A (trip current)

I

L

=100mV

V

TRIP

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

immediately when commanded ON. Typical circuits achieve

10V enhancement in 10

µs 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

current sensing resistor (R

add a small, additional potential to V

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-protected against inductive switching transients. Dur-

ing turn-off an inductive load will force the MOSFET source

5V or more below ground, while the driver holds the gate at

ground potential. The MOSFET is forced into conduction,

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. Exter-

nal clamp diodes are unnecessary.

Current Shunts (R

for use at R

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

source bond wires. Current-sensing MOSFETs are speci-

fied 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 or

estimated. V

become negative. Substituting a MOSFET with higher on-

resistance, or reducing V

100 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

resistances. R