MIC5018
Micrel
across an IRFZ24 is less than 0.1V with a 1A load and 10V
enhancement. Higher current increases the drain-to-source
voltage drop, increasing the gate-to-source voltage.
+5V
Application Information
Supply Bypass
A capacitor from VS to GND is recommended to control
switching and supply transients. Load current and supply
lead length are some of the factors that affect capacitor
size requirements.
A 4.7碌F or 10碌F aluminum electrolytic or tantalum capacitor
is suitable for many applications.
The low ESR (equivalent series resistance) of tantalum
capacitors makes them especially effective, but also makes
them susceptible to uncontrolled inrush current from low
impedance voltage sources (such as NiCd batteries or auto-
matic test equipment). Avoid instantaneously applying volt-
age, capable of high peak current, directly to or near tantalum
capacitors without additional current limiting. Normal power
supply turn-on (slow rise time) or printed circuit trace resis-
tance is usually adequate for normal product usage.
MOSFET Selection
The MIC5018 is designed to drive N-channel enhancement-
type MOSFETs. The gate output (G) of the MIC5018 pro-
vides a voltage, referenced to ground, that is greater than the
supply voltage. Refer to the 鈥淭ypical Characteristics: Gate
Output Voltage vs. Supply Voltage鈥?graph.
The supply voltage and the MOSFET drain-to-source
voltage drop determine the gate-to-source voltage.
V
GS
= V
G
鈥?(V
SUPPLY
鈥?V
DS
)
where:
V
GS
= gate-to-source voltage (enhancement)
V
G
= gate voltage (from graph)
V
SUPPLY
= supply voltage
V
DS
= drain-to-source voltage (approx. 0V at
low current, or when fully enhanced)
V
SUPPLY
4.7碌F
MIC5018
2
4
VS
CTL
G
GND
3
1
15V
10V
IRFZ24*
approx. 0V
Logic
High
Voltages are approximate
* International Rectifier
standard MOSFET
5V
To demonstrate
this circuit, try a
2
鈩?/div>
, 20W
load resistor .
Figure 2. Using a Standard MOSFET
The MIC5018 has an internal zener diode that limits the gate-
to-ground voltage to approximately 16V.
Lower supply voltages, such as 3.3V, produce lower gate
output voltages which will not fully enhance standard
MOSFETs. This significantly reduces the maximum current
that can be switched. Always refer to the MOSFET data sheet
to predict the MOSFET鈥檚 performance in specific applica-
tions.
Logic-Level MOSFET
Logic-level N-channel MOSFETs are fully enhanced with a
gate-to-source voltage of approximately 5V and generally
have an absolute maximum gate-to-source voltage of
鹵10V.
+3.3V
4.7碌F
MIC5018
2
4
VS
CTL
G
GND
3
1
9V
5.7V
Logic
High
Load
Load
IRLZ44*
approx. 0V
Voltages are approximate
* International Rectifier
logic-level MOSFET
3.3V
To demonstrate
this circuit, try
5
鈩?/div>
, 5W or
47
鈩?/div>
, 1/4W
load resistors.
MIC5018
2
4
VS
CTL
G
GND
3
1
V
G
G
D
V
DS
S
Figure 3. Using a Logic-Level MOSFET
Refer to figure 3 for an example showing nominal voltages.
The maximum gate-to-source voltage rating of a logic-level
MOSFET can be exceeded if a higher supply voltage is used.
An external zener diode can clamp the gate-to-source volt-
age as shown in figure 4. The zener voltage, plus its
tolerance, must not exceed the absolute maximum gate
voltage of the MOSFET.
V
SUPPLY
V
GS
V
LOAD
Figure 1. Voltages
The performance of the MOSFET is determined by the gate-
to-source voltage. Choose the type of MOSFET according to
the calculated gate-to-source voltage.
Standard MOSFET
Standard MOSFETs are fully enhanced with a gate-to-source
voltage of about 10V. Their absolute maximum gate-to-
source voltage is
鹵20V.
With a 5V supply, the MIC5018 produces a gate output of
approximately 15V. Figure 2 shows how the remaining
voltages conform. The actual drain-to-source voltage drop
Load
MIC5018
2
4
VS
CTL
G
GND
3
1
Logic-level
N-channel
MOSFET
5V < V
Z
< 10V
Protects gate of
logic-level MOSFET
Figure 4. Gate-to-Source Protection
5-160
Load
1997
MICREL
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2
3
4
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