M9999-072205 8 July 2005
MIC4420/4429 Micrel, Inc.
Table 1: MIC4429 Maximum
Operating Frequency
V
S
Max Frequency
18V 500kHz
15V 700kHz
10V 1.6MHz
Conditions: 1. DIP Package (θ
JA
= 130°C/W)
2. T
A
= 25°C
3. C
L
= 2500pF
Input Stage
The input voltage level of the 4429 changes the quiescent
supply current. The N channel MOSFET input stage tran
-
sistor drives a 450µA current source load. With a logic “1”
input, the maximum quiescent supply current is 450µA.
Logic “0” input level signals reduce quiescent current to
55µA maximum.
The MIC4420/4429 input is designed to provide 300mV 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
4 .5V to 18V operating supply voltage range. Input current
is less than 10µA over this range.
The MIC4429 can be directly driven by the TL494,
SG1526/1527, SG1524, TSC170, MIC38HC42 and similar
switch mode power supply integrated circuits. By offloading
the power-driving duties to the MIC4420/4429, 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 propagation delay
for T
D2
will increase to as much as 400ns at room tem-
perature. The input currents can be as high as 30mA p-p
(6.4mA
RMS
) with the input, 6 V greater than the supply
voltage. No damage will occur to MIC4420/4429 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. Care
should be taken so that the input does not go more than 5
volts below the negative rail.
Power Dissipation
CMOS circuits usually permit the user to ignore power dis-
sipation. 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 cur
-
rent to destroy the device. The MIC4420/4429 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.
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 volt
-
ages when driving a 2500pF 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 MSOP package, from the
data sheet, is 250°C/W. In a 25°C ambient, then, using a
maximum junction temperature of 150°C, this package will
dissipate 500mW.
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)
Figure 4. Switching Time Degradation Due to
Negative Feedback
MIC4429
1
8
6,
7
5
4
+18 V
0.1µF
0.1µF
TE K C U R R E N T
PROB E 6 302
2,500 pF
POL Y CAR BON ATE
5.0V
0 V
18 V
0 V
WIMA
MKS-2
1 µF
LOGIC
GROUND
POWER
GROUND
6 AMPS
300 mV
PC TRACE RESISTANCE = 0.05Ω
