SG2644 Microsemi Corporation, SG2644 Datasheet - Page 5

• Manufacturer Part Number: SG2644
• Description: Dual High Speed Driver
• Manufacturer: Microsemi Corporation

9/91 Rev 1.2 6/97
Copyright
APPLICATION INFORMATION
POWER DISSIPATION
The SG1644, while more energy-efficient than earlier gold-doped
driver IC’s, can still dissipate considerable power because of its
high peak current capability at high frequencies. Total power
dissipation in any specific application will be the sum of the DC or
steady-state power dissipation, and the AC dissipation caused by
driving capacitive loads.
The DC power dissipation is given by:
P
where I
dependent.
The AC power dissipation is proportional to the switching fre-
quency, the load capacitance, and the square of the output
voltage. In most applications, the driver is constantly changing
state, and the AC contribution becomes dominant when the
frequency exceeds 100-200KHz.
The SG1644 driver family is available in a variety of packages to
accommodate a wide range of operating temperatures and power
dissipation requirements. The Absolute Maximums section of the
data sheet includes two graphs to aid the designer in choosing an
appropriate package for his design.
The designer should first determine the actual power dissipation
of the driver by referring to the curves in the data sheet relating
operating current to supply voltage, switching frequency, and
capacitive load. These curves were generated from data taken on
actual devices. The designer can then refer to the Absolute
Maximum Thermal Dissipation curves to choose a package type,
and to determine if heat-sinking is required.
DESIGN EXAMPLE
Given: Two 2500pF loads must be driven push-pull from a +15 volt
supply at 100KHz. The application is a commercial one in which
the maximum ambient temperature is +50°C, and cost is impor-
tant.
under these conditions will be 18mA, and the power dissipation
will be 15volts x 18mA, or 270mW.
that the M package, which is an 8-pin plastic DIP with a copper
lead frame, has more than enough thermal conductance from
junction to ambient to support operation at an ambient tempera-
ture of +50°C. The SG36446M driver would be specified for this
application.
SUPPLY BYPASSING
Since the SG1644 can deliver peak currents above 3amps under
some load conditions, adequate supply bypassing is essential for
proper operation. Two capacitors in parallel are recommended to
guarantee low supply impedance over a wide bandwidth: a 0.1µF
ceramic disk capacitor for high frequencies, and a 4.7µF solid
2. From the ambient thermal characteristic curve, it can be seen
DC
1. From Figure 11, the average driver current consumption
= +V
1997
CC
CC
is a function of the driver state, and hence is duty-cycle
· I
CC
[1]
5
tantalum capacitor for energy storage. In military applications, a
CK05 or CK06 ceramic operator with a CSR-13 tantalum capaci-
tor is an effective combination. For commercial applications, any
low-inductance ceramic disk capacitor teamed with a Sprague
150D or equivalent low ESR capacitor will work well.
capacitors must be located as close as physically possible to the
V
less than 0.5 inches.
GROUNDING CONSIDERATIONS
The ability of the SG1644 to deliver high peak currents into
capacitive loads can cause undesirable negative transients on
the logic and power grounds. To avoid this, a low inductance
ground path should be considered for each output to return the
high peak currents back to it’s own ground point. A ground plane
is recommended for best performance. If space for a ground
plane is not available, make the paths as short and as wide as
possible. The logic ground can be returned to the supply bypass
capacitor and be connected at one point to the power grounds.
LOGIC INTERFACE
The logic input of the 1644 is designed to accept standard DC-
coupled 5 volt logic swings, with no speed-up capacitors required.
If the input signal voltage exceeds 6 volts, the input pin must be
protected against the excessive voltage in the HIGH state. Either
a high speed blocking diode must be used, or a resistive divider
to attenuate the logic swing is necessary.
LAYOUT FOR HIGH SPEED
The SG1644 can generate relatively large voltage excursions
with rise and fall times around 20-30 nanoseconds with light
capacitive loads. A Fourier analysis of these time domain signals
will indicate strong energy components at frequencies much
higher than the basic switching frequency. These high frequen-
cies can induce ringing on an otherwise ideal pulse if sufficient
inductance occurs in the signal path (either the positive signal
trace or the ground return). Overshoot on the rising edge is
undesirable
the gate oxide of a power MOSFET. Trailing edge undershoot is
dangerous because the negative voltage excursion can forward-
bias the parasitic PN substrate diode of the driver, potentially
causing erratic operation or outright failure.
Ringing can be reduced or eliminated by minimizing signal path
inductance, and by using a damping resistor between the drive
output and the capacitive load. Inductance can be reduced by
keeping trace lengths short, trace widths wide, and by using 2oz.
copper if possible. The resistor value for critical damping can be
calculated from:
R
where L is the total signal line inductance, and C
capacitance. Values between 10 and 100ohms are usually
sufficient. Inexpensive carbon composition resistors are best
because they have excellent high frequency characteristics.
They should be located as close as possible to the gate terminal
of the power MOSFET.
CC
D
= 2 L/C
pin, with combined lead and pc board trace lengths held to
L
[2]
because the excess drive voltage could rupture
SG1644/SG2644/SG3644
11861 Western Avenue
L
INFINITY Microelectronics Inc.
[phone]
Garden Grove, CA 92841
L
FAX: [phone]
is the load
The