Application Hint 5 Micrel
5
1997 5-213
Introduction
In battery powered applications, such as laptop computers,
power control has a major impact on battery life. For
example, laptop or notebook computers often have a “sleep”
mode, where the hard drive spins down and the display
backlighting turns off while the RAM—containing valuable
user data—is maintained. A microprocessor can easily make
such power management decisions, but implementing the
Logic Controlled Power Switch
Application Hint 5
b y Bob Wolbert
Figure 1. MIC5011 DB-1 Schematic Diagram. Figure 2. MIC5011 DB-1 Low Voltage
Logic Controlled Power Switch
Power Switches
These high-side implementations have historically taken one
of two forms: relays or PNP transistors. Both have draw-
backs in that relatively large drive current is required: neither
can be switched directly from a microprocessor port or
standard logic. Mechanical relays are bulky, expensive, and
have limited lifetimes. Bipolar transistors exhibit a fixed
voltage drop that reduce margins, especially in 5V logic
systems. This voltage drop has a devastating effect on
defining battery end-of-life (per charge cycle).
Ground
Control Input
Zener
(optional) Output Voltage
Supply
Voltage Input
Another method of power switching is the N-Channel DMOS
FET. This FET has no inherent voltage drop, except for the
I x rDS loss, and requires almost no drive power; unfortu-
nately, it does need a gate driving voltage of from 4V to 10V
above the supply voltage in high-side applications. In other
words, it is an
almost
ideal switch.
DMOS FET Advantages vs. PNP
• No fixed voltage drop
• Extremely low drive current requirement
• Larger Safe Operating Region
DMOS FET Advantages vs. Relays
• Non-mechanical (much longer life)
• No contact bounce
• Extremely low drive current requirement
• Smaller Size
• Lighter weight
• Lower cost
hardware for the actual switching can be complicated. "High-
side" switching is required; i.e., the positive supply voltage
must be controlled. Common grounds for busses and shield-
ing limits the possibility of “low side” switching in a standard
negative ground system. This note discusses a logic con-
trolled power switch that simplifies microprocessor driven
high-side supply switching.
MIC5011BM
FET
V
DD (1)
GND
(4)
IN
(4)
S
(3)
G
(5)
MIC5011BM Load
(or SMPS &
Load
*
* Add an (approximately)
10V Gate-to-Source Protection
Zener if V
IN
> 7V
µP On/Off
Input
Battery
4.5V
to
32V
0.1 to 16A
Application Hint 5 Micrel
5-214 1997
The Micrel MIC5010 Family
The MIC5011 and its relatives control the N-Channel DMOS
FET by generating a gate drive control voltage 4V to 10V
above the supply. Its CMOS compatible control input directly
interfaces with microprocessors, and its BCD (Bipolar-CMOS-
DMOS) construction allows nearly zero power drain in the
OFF state. Pairing the MIC5011 with a low cost DMOS FET
gives you a simple, reliable, easy-to-interface method of
power management.
The MIC5011 is designed for this application and features:
• 4.5V to 32V Operation
• Very low OFF power consumption—0.1µA
typical
• No external components required
• Built-in zener clamp for protecting standard
DMOS gates
• Available in small 8-pin surface mount
packages
Figure 3. MIC5011 Block Diagram and Typical
Application
The IRLR024 N-Channel DMOS
FET
The 100mΩ surface mount IRLR024 is employed as the pass
device in this demonstration circuit. This N-Channel DMOS
FET features “Logic Level” gate drive voltages and can pass
over 50A of peak current (limited by power dissipation con-
siderations). Key features include:
• Low ON resistance—100mΩ maximum
• “Logic Level” gate threshold—ON at
VGS=4V; VGS=5V for full enhancement.
• High pass current
• Surface mount package
G D S
Figure 5. IRLR024 Characteristics
One drawback of this “logic level” device is that its sensitive
gate cannot withstand more than 10V of VGS drive. Although
the MIC5011 includes a protective zener clamp, the zener’s
12.5V threshold is inadequate. With supply voltages from
4.5V to 7V, this is not a problem; however, above 7V, either
an external zener clamp must be added to the MIC5011 gate
drive output or else a standard threshold FET should be used.
The Micrel MIC5011 DB-1
Demonstration Unit
This demonstration unit is built on a single sided board using
surface mount techniques. It has been designed to control
4.5V to 7V supplies, but can easily be modified to use 4.5V
to 32V supply voltages. The first thing you will notice from the
schematic, Figure 1, is its simplicity; only two components
are needed. The MIC5011 contains all of the necessary
intelligence and the drive circuitry required by the N-Channel
DMOS FET.
Four lines provide +VCC, Switched-VCC, Control, and Ground.
VCC and Switched-VCC are current carrying lines, so thick,
low resistances traces are necessary. Both Control and
Ground are low current lines, so thin traces are sufficient.
Simply connect VCC to 4.5V to 7V, Switched-VCC to your
load, Control to a logic output, and Ground. When the logic
Figure 4. IRLR024 DMOS FET
100 kHz
OSCILLATOR
CONTROL
LOGIC
CHARGE PUMP
VOLTAGE TRIPLER
MIC5011
500Ω13V
ON
OFF 4
2
687 1
5
3
V
DD
V
DD
10µF
Power
MOSFET
100pF 100pF
Cext1 Cext2
LOAD
+
Application Hint 5 Micrel
5
1997 5-215
level is high (greater than approximately 3.5V), the load will
be energized. The IRLR024 will exhibit less than 100mΩ of
resistance, so voltage drop, hence power loss, with typical
peripherals will be low. Current drain of up to 16A continuous,
64A peak, can be drawn with suitable heatsinking (limit
current to 3A without additional heatsinking). With a low logic
level, the load will be switched off. Total power drain from the
VCC line will be negligible; only approximately 0.1µA (leak-
age current) flows.
Application Notes
Operating Voltages
This circuit, as designed, controls 4.5V to 7V digital supply
voltages. If higher voltages must be switched, one of two
modifications must be made. To switch widely varying
supplies in the 4.5V to 32V range, use an approximately 7.5V
zener clamp, such as the MLL4693 or equivalent, across the
gate and source of the FET. If your application switches 7V
to 32V, replace the “logic level” FET with a standard gate N-
Channel DMOS FET, such as the IRF540, BUZ1LS2, or the
SMP60N05. Regardless of the FET employed, the MIC5011
allows power control from a standard CMOS-level logic
signal.
TO-220 Package FETs
The MIC5011-DB1 demonstration board also allows using
a standard TO-220 package FET. Connect the gate and
source to the zener diode pads, and solder the tab (drain)
to the drain heatsink pad. Remove the center lead drain
connection. The TO-220 tab will extend from the top of the
board a short distance.
Faster Switching
If switching time is critical, adding a 1000pF capacitor from
pins 6-7 on the MIC5011 will help. Another 1000pF capacitor
from pins 7-8 will further accelerate switching time, but by a
smaller margin.
Dual Independent Switches
When two separate circuits require switching, the MIC5012
Dual High Side FET Driver provides two independent drivers
in a single 14-pin DIP or 16-pin surface mount package.
Over Current Protection
Replace the MIC5011 with the MIC5013 to enable over
current protection with fault detection and signalling. See the
MIC5013 datasheet for further information and suggested
component values.
Parts List
• MIC5011BM Surface mount MOSFET driver
• IRLR024 Surface mount DMOS FET
• MLL4693 Surface mount 7.5V zener diode
(optional)
Additional Notes
Although the MIC5011 datasheet specifically states that a
minimum of 7V of supply voltage is required for high-side
driving, the introduction of “logic level” N-Channel DMOS
FETs requiring only 4V to 5V VGS for full ON operation
enables this minimum operating voltage to be lowered. The
MIC5011 provides gate enhancement with supply voltages
down to below 3.5V. Variations in the control voltage thresh-
old, however, restrict low voltage operations to somewhat
less than 4.5V (for lower voltage devices, please contact the
factory).
Figure 6. MIC5011 DB-1 Board Layout Silk ScreenSolder MaskComponent Side
