MIC4451/4452
Input Stage
The input voltage level of the MIC4451 changes the quies-
cent supply current. The N channel MOSFET input stage
transistor drives a 320µA current source load. With a logic “1”
input, the maximum quiescent supply current is 400µA. Logic
“0” input level signals reduce quiescent current to 80µA
typical.
The MIC4451/4452 input is designed to provide 200mV of
hysteresis. This provides clean transitions, reduces noise
sensitivity, and minimizes output stage current spiking when
changing states. Input voltage threshold level is approxi-
mately 1.5V, making the device TTL compatible over the full
temperature and operating supply voltage ranges. Input
current is less than
±10µA.
The MIC4451 can be directly driven by the TL494, SG1526/
1527, SG1524, TSC170, MIC38C42, and similar switch
mode power supply integrated circuits. By offloading the
power-driving duties to the MIC4451/4452, the power supply
controller can operate at lower dissipation. This can improve
performance and reliability.
The input can be greater than the V
S
supply, however, current
will flow into the input lead. The input currents can be as high
as 30mA p-p (6.4mA
RMS
) with the input. No damage will
occur to MIC4451/4452 however, and it will not latch.
The input appears as a 7pF capacitance and does not change
even if the input is driven from an AC source. While the device
will operate and no damage will occur up to 25V below the
negative rail, input current will increase up to 1mA/V due to
the clamping action of the input, ESD diode, and 1kΩ resistor.
Power Dissipation
CMOS circuits usually permit the user to ignore power
dissipation. Logic families such as 4000 and 74C have
outputs which can only supply a few milliamperes of current,
and even shorting outputs to ground will not force enough
current to destroy the device. The MIC4451/4452 on the other
hand, can source or sink several amperes and drive large
capacitive loads at high frequency. The package power
+18
Micrel
dissipation limit can easily be exceeded. Therefore, some
attention should be given to power dissipation when driving
low impedance loads and/or operating at high frequency.
The supply current vs. frequency and supply current vs
capacitive load characteristic curves aid in determining power
dissipation calculations. Table 1 lists the maximum safe
operating frequency for several power supply voltages when
driving a 10,000pF load. More accurate power dissipation
figures can be obtained by summing the three dissipation
sources.
Given the power dissipation in the device, and the thermal
resistance of the package, junction operating temperature for
any ambient is easy to calculate. For example, the thermal
resistance of the 8-pin plastic DIP package, from the data
sheet, is 130°C/W. In a 25°C ambient, then, using a maximum
junction temperature of 125°C, this package will dissipate
960mW.
Accurate power dissipation numbers can be obtained by
summing the three sources of power dissipation in the device:
• Load Power Dissipation (P
L
)
• Quiescent power dissipation (P
Q
)
• Transition power dissipation (P
T
)
Calculation of load power dissipation differs depending on
whether the load is capacitive, resistive or inductive.
Resistive Load Power Dissipation
Dissipation caused by a resistive load can be calculated as:
P
L
= I
2
R
O
D
where:
I = the current drawn by the load
R
O
= the output resistance of the driver when the output is
high, at the power supply voltage used. (See data
sheet)
D = fraction of time the load is conducting (duty cycle)
Capacitive Load Power Dissipation
Dissipation caused by a capacitive load is simply the energy
placed in, or removed from, the load capacitance by the
WIMA
MKS-2
1
µF
5
5.0V
1
8
MIC4451
6, 7
TEK CURRENT
PROBE 6302
18 V
Table 1: MIC4451 Maximum
Operating Frequency
V
S
Max Frequency
0V
0V
0.1µF
4
5
0.1µF
2,500 pF
POLYCARBONATE
18V
15V
10V
5V
Conditions: 1.
θ
JA
= 150°C/W
2. T
A
= 25°C
3. C
L
= 10,000pF
220kHz
300kHz
640kHz
2MHz
LOGIC
GROUND
300 mV
POWER
GROUND
12 AMPS
PC TRACE RESISTANCE = 0.05Ω
Figure 5. Switching Time Degradation Due to
Negative Feedback
April 1998
5-77
MICREL [ MICREL SEMICONDUCTOR ]
