Micrel, Inc.
Input Stage
The input voltage level of the MIC4421A changes the
quiescent supply current. The N-Channel MOSFET input
stage transistor drives a 320µA current source load. With
a logic “1” input, the quiescent supply current is typically
500µA. Logic “0” input level signals reduce quiescent
current to 80µA typical.
The MIC4421A/4422A input is designed to provide
600mV of hysteresis. This provides clean transitions,
reduces noise sensitivity, and minimizes output stage
current spiking when changing states. Input voltage
threshold level is approximately 1.5V, making the device
TTL compatible over the full temperature and operating
supply voltage ranges. Input current is less than ±10µA.
The MIC4421A can be directly driven by the TL494,
SG1526/1527, SG1524, TSC170, MIC38C42, and
similar switch mode power supply integrated circuits. By
off loading the power-driving duties to the MIC4421A/
4422A, 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 (6.4mARMS) with the input.
No damage will occur to MIC4421A/4422A 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
MIC4421A/4422A on the other hand, can source or sink
several amperes and drive large capacitive loads at high
frequency. The package power 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.
V
IN
+18V
MIC4421A/4422A
WIMA
MKS-2
1µF
+5.0V
1
8
6, 7
+18V
TEK Current
Probe 6302
0V
2500pF
Polycarbonate
MIC4421A
0V
0.1µF
5
4
0.1µF
Logic
Ground
300mV
Power
Ground
6 Amps
PC Trace
Figure 7. Switching Time Due to Negative Feedback
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 84.6°C/W. In a 25°C
ambient, then, using a maximum junction temperature of
150°C, this package will dissipate 1478mW.
Accurate power dissipation numbers can be obtained by
summing the three sources of power dissipation in the
device:
•
•
Load Power Dissipation (PL)
Quiescent power dissipation (PQ)
•
Transition power dissipation (PT)
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).
June 2007
9
M9999-062707
MIC [ MIC GROUP RECTIFIERS ]
