MIC4423/4424/4425
to the ground pin of the driver directly to the ground terminal
of the load. Do not use a twisted pair where the second wire
in the pair is the output of the other driver, as this will not
provide a complete current path for either driver. Likewise, do
not use a twisted triad with two outputs and a common return
unless both of the loads to be driver are mounted extremely
close to each other, and you can guarantee that they will never
be switching at the same time.
For output leads on a printed circuit, the general rule is to
make them as short and as wide as possible. The lands should
also be treated as transmission lines: i.e. minimize sharp
bends, or narrowings in the land, as these will cause ringing.
For a rough estimate, on a 1.59mm (0.062") thick G-10 PCB
a pair of opposing lands each 2.36mm (0.093") wide translates
to a characteristic impedance of about 50Ω. Half that width
suffices on a 0.787mm (0.031") thick board. For accurate
impedance matching with a MIC4423/24/25 driver, on a
1.59mm (0.062") board a land width of 42.75mm (1.683")
would be required, due to the low impedance of the driver and
(usually) its load. This is obviously impractical under most
circumstances. Generally the tradeoff point between lands
and wires comes when lands narrower than 3.18mm (0.125")
would be required on a 1.59mm (0.062") board.
To obtain minimum delay between the driver and the load, it
is considered best to locate the driver as close as possible to
the load (using adequate bypassing). Using matching
transformers at both ends of a piece of coax, or several
matched lengths of coax between the driver and the load,
works in theory, but is not optimum.
Driving at Controlled Rates
Occasionally there are situations where a controlled rise or fall
time (which may be considerably longer than the normal rise
or fall time of the driver’s output) is desired for a load. In such
cases it is still prudent to employ best possible practice in
terms of bypassing, grounding and PCB layout, and then
reduce the switching speed of the load (NOT the driver) by
adding a noninductive series resistor of appropriate value
between the output of the driver and the load. For situations
where only rise or only fall should be slowed, the resistor can
be paralleled with a fast diode so that switching in the other
direction remains fast. Due to the Schmitt-trigger action of the
driver’s input it is not possible to slow the rate of rise (or fall)
of the driver’s input signal to achieve slowing of the output.
Input Stage
The input stage of the MIC4423/24/25 consists of a single-
MOSFET class A stage with an input capacitance of
≤38pF.
This capacitance represents the maximum load from the
driver that will be seen by its controlling logic. The drain load
on the input MOSFET is a –2mA current source. Thus, the
quiescent current drawn by the driver varies, depending on
the logic state of the input.
Following the input stage is a buffer stage which provides
~400mV of hysteresis for the input, to prevent oscillations
when slowly-changing input signals are used or when noise is
present on the input. Input voltage switching threshold is
Micrel
approximately 1.5V which makes the driver directly compatible
with TTL signals, or with CMOS powered from any supply
voltage between 3V and 15V.
The MIC4423/24/25 drivers can also be driven directly by the
SG1524/25/26/27, TL494/95, TL594/95, NE5560/61/62/68,
TSC170, MIC38C42, and similar switch mode power supply
ICs. By relocating the main switch drive function into the driver
rather than using the somewhat limited drive capabilities of a
PWM IC. The PWM IC runs cooler, which generally improves
its performance and longevity, and the main switches switch
faster, which reduces switching losses and increase system
efficiency.
The input protection circuitry of the MIC4423/24/25, in addition
to providing 2kV or more of ESD protection, also works to
prevent latchup or logic upset due to ringing or voltage spiking
on the logic input terminal. In most CMOS devices when the
logic input rises above the power supply terminal, or descends
below the ground terminal, the device can be destroyed or
rendered inoperable until the power supply is cycled OFF and
ON. The MIC4423/24/25 drivers have been designed to
prevent this. Input voltages excursions as great as 5V below
ground will not alter the operation of the device. Input excursions
above the power supply voltage will result in the excess
voltage being conducted to the power supply terminal of the
IC. Because the excess voltage is simply conducted to the
power terminal, if the input to the driver is left in a high state
when the power supply to the driver is turned off, currents as
high as 30mA can be conducted through the driver from the
input terminal to its power supply terminal. This may overload
the output of whatever is driving the driver, and may cause
other devices that share the driver’s power supply, as well as
the driver, to operate when they are assumed to be off, but it
will not harm the driver itself. Excessive input voltage will also
slow the driver down, and result in much longer internal
propagation delays within the drivers. T
D2
, for example, may
increase to several hundred nanoseconds. In general, while
the driver will accept this sort of misuse without damage,
proper termination of the line feeding the driver so that line
spiking and ringing are minimized, will always result in faster
and more reliable operation of the device, leave less EMI to be
filtered elsewhere, be less stressful to other components in
the circuit, and leave less chance of unintended modes of
operation.
Power Dissipation
CMOS circuits usually permit the user to ignore power
dissipation. Logic families such as 4000 series and 74Cxxx
have outputs which can only source or sink a few milliamps of
current, and even shorting the output of the device to ground
or V
CC
may not damage the device. CMOS drivers, on the
other hand, are intended to source or sink several Amps of
current. This is necessary in order to drive large capacitive
loads at frequencies into the megahertz range. Package
power dissipation of driver ICs can easily be exceeded when
driving large loads at high frequencies. Care must therefore
be paid to device dissipation when operating in this domain.
The Supply Current vs Frequency and Supply Current vs
Load characteristic curves furnished with this data sheet aid
8
January 1999
MIC4423/4424/4425
MICREL [ MICREL SEMICONDUCTOR ]
