MIC5014/5015

Micrel

5-142

1997

the short is removed, feedback to the input pin insures that the

MIC5014 will turn back on. This output can also be level

shifted and sent to an I/O port of a microcontroller for intelli-

gent control.

Current Shunts (R

use at R

to 50m

Ω, at 2 to 10W. If a precise overcurrent trip point is

not necessary, then a nonprecision resistor or even a mea-

sured PCB trace can serve as R

resistor values with such resistors is temperature coefficient;

the designer should be aware that a linear, 500 ppm/

°C

change will contribute as much as 10% shift in the overcurrent

trip point. If this is not acceptable, a power resistor designed

for current shunt service (drifts less than 100 ppm/

°C), or a

† Suppliers of Precision Power Resistors:

Dale Electronics, Inc., 2064 12th Ave., Columbus, NE 68601. (402) 565-

3131

International Resistive Co., P.O. Box 1860, Boone,NC 28607-1860.

(704) 264-8861

Isotek Corp., 566 Wilbur Ave. Swansea, MA 02777. (508) 673-2900

Kelvin, 14724 Ventura Blvd., Ste. 1003, Sherman Oaks, CA 91403-3501.

(818) 990-1192

RCD Components, Inc., 520 E. Industrial Pk. Dr., Manchester, NH 03103.

(603) 669-0054

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

High Side Driver With Delayed Current Sense (Figure 5)

Delay of the overcurrent detection to accomodate high inrush

loads such as incandescent or halogen lamps can be accom-

plished by adding an LM3905 timer as a one shot to provide

an open collector pulldown for the comparator output such

that the control input of the MIC5015 stays low for a preset

amount of time without interference from the current sense

circuitry. Note that an MIC5015 must be used in this applica-

tion (figure 5), as an inverting control input is necessary. The

delay time is set by the RC time constant of the external

components on pins 3 and 4 of the timer; in this case, 6ms was

chosen.

An LM3905 timer was used instead of a 555 as it provides a

clean transition, and is almost impossible to make oscillate.

Good bypassing and noise immunity is essential in this circuit

to prevent spurious op amp oscillations.

Low Side Driver (Figure 2) A key advantage of this topology,

as previously mentioned, is speed. The MOSFET gate is

driven to near supply immediately when the MIC5014/15 is

turned on. Typical circuits reach full enhancement in 50

µs or

less with a 15V supply.

Bootstrapped High Side Driver (Figure 3) The turn-on time of

a high side driver can be improved to faster than 40

µs by

bootstrapping the supply with the MOSFET source. The

Schottky barrier diode prevents the supply pin from dropping

more than 200mV below the drain supply and improves turn-

on time. Since the supply current in the “off” state is only a

small leakage, the 100nF bypass capacitor tends to remain

charged for several seconds after the MIC5014/15 is turned

off. Faster speeds can be obtained at the expense of supply

voltage (the overvoltage shutdown will turn the part off when

the bootstrapping action pulls the supply pin above 35V) by

using a larger capacitor at the junction of the two 1N4001

diodes. In a PWM application (this circuit can be used for either

PWM’ed or continuously energized loads), the chip supply is

sustained at a higher potential than the system supply, which

improves switching time.

Figure 3. Bootstrapped Hgh-Side Driver

High Side Driver With Current Sense (Figure 4) Although no

current sense function is included on the MIC5014/15 devices,

a simple current sense function can be realized via the addition

of one more active component; an LM301A op amp used as

the negative rail is tied to ground. This op amp was chosen as

it can withstand having input transients that swing below the

negative rail, and has common mode range almost to the

positive rail.

The inverting side of this comparator is tied to a voltage divider

is tied to the node between the drain of the FET and the sense

resistor. If the overcurrent trip point is not exceeded , this node

comparator will be high which feeds the control input of the

MIC5014 (polarities should be reversed if the MIC5015 is

used). One the overcurrent trip point has been reached, the

comparator will go low, which shuts off the MIC5014. When the

MIC5014

1

2

3

4

8

7

6

5

NC

Gate

Gnd

Source

Input

V+

NC

NC

10µF

R4

1k

Ω

R

S

0.06

Ω

12V

On

R1

1k

Ω

R2

120k

Ω

LM301A

2.2k

Ω

= 1.7A

MIC5015

Control Input

OFF

ON

1

2

3

4

8

7

6

5

+2.75V to +30V

NC

Gate

Gnd

Source

Input

V+

NC

NC

1µF

100nF

1N4001 (2)

1N5817

1RF540

Figure 4. High Side Driver with Overcurrent Shutdown