MIC5016/5017
driven to near supply immediately when the MIC5016/17 is
turned on. Typical circuits reach full enhancement in 50µs 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 40µs 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 MIC5016/17 is turned
off. Faster switching 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.
+2.75V to +30V
1N5817
10µF
1/2 MIC5016
V+
R
S
0.06Ω
R4
1kΩ
Gate
LM301A
R2
120kΩ
2.2kΩ
R1
1kΩ
Micrel
will go low, which shuts off the MIC5016. When the short is
removed, feedback to the input pin insures that the MIC5016
will turn back on. This output can also be level shifted and sent
to an I/O port of a microcontroller for intelligent control.
Current Shunts
(R
S
). Low valued resistors are necessary for
use at R
S
. Resistors are available with values ranging from 1
to 50mΩ, at 2 to10W. If a precise overcurrent trip point is not
necessary, then a nonprecision resistor or even a measured
PCB trace can serve as R
S
. The major cause of drift in resistor
values with such resistors is temperature coefficient; the
designer should be aware that a linear, 500ppm/°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 100ppm/°C), or a Kelvin-sensed
resistor may be used.
†
12V
On
I
TRIP
= V
TRIP
/R
S
= 1.7A
V
TRIP
= R1/(R1+R2)
1N4001 (2)
100nF
1µF
V+
Control Input
ON
OFF
Input
Source
Gnd
Input
Source
Gnd
Gate
IRF540
Figure 4. High Side Driver with Overcurrent Shutdown
† 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
Figure 3. Bootstrapped High-Side Driver
High Side Driver With Current Sense
(Figure 4) Although no
current sense function is included on the MIC5016/17 de-
vices, a simple current sense function can be realized via the
addition of one more active component; an LM301A op amp
used as a comparator. The positive rail of the op amp is tied
to V
+
, and 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
which sets the voltage to V
+
– V
TRIP
. The noninverting side
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
will always be above V
+
– V
TRIP
, and the output of the compara-
tor will be high which feeds the control input of the MIC5016
(polarities should be reversed if the MIC5017 is used). Once
the overcurrent trip point has been reached, the comparator
October 1998
Load
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 MIC5017 stays low for a preset
amount of time without interference from the current sense
circuitry. Note that an MIC5017 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.
5-151
Load
1/2 MIC5016
5
MICREL [ MICREL SEMICONDUCTOR ]
