ISL55110, ISL55111
13
FN6228.8
January 29, 2015
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Detailed Description
The ISL55110 and ISL55111 are dual high-speed MOSFET
drivers intended for applications requiring accurate pulse
generation and buffering. Target applications include ultrasound,
CCD imaging, automotive piezoelectric distance sensing and
clock generation circuits.
With a wide output voltage range and low ON-resistance, these
devices can drive a variety of resistive and capacitive loads with
fast rise and fall times, allowing high-speed operation with low
skew as required in large CCD array imaging applications.
The ISL55110 and ISL55111 are compatible with 3.3V and 5V
logic families and incorporate tightly controlled input thresholds
to minimize the effect of input rise time on output pulse width.
The ISL55110 has a pair of in-phase drivers while the ISL55111
has two drivers operating in anti-phase. Both channels of the
device have independent inputs to allow external time phasing if
required.
In addition to driving power MOSFETs, the ISL55110 and
ISL55111 are well suited for other applications such as bus,
control signal and clock drivers for large memory arrays on
microprocessor boards, where the load capacitance is large and
low propagation delays are required. Other potential applications
include peripheral power drivers and charge pump voltage
inverters.
Input Stage
The input stage is a high impedance buffer with rise/fall
hysteresis. This means that the inputs will be directly compatible
with both TTL and lower voltage logic over the entire V
DD
range.
The user should treat the inputs as high-speed pins and keep rise
and fall times to <2ns.
Output Stage
The ISL55110 and ISL55111 outputs are high-power CMOS
drivers swinging between ground and V
H
. At V
H
= 12V, the output
impedance of the inverter is typically 3.0Ω. The high peak current
capability of the ISL55110 and ISL55111 enables it to drive a
330pF load to 12V with a rise time of <3.0ns over the full
temperature range. The output swing of the ISL55110 and
ISL55111 comes within <30mV of the V
H
and Ground rails.
Application Notes
Although the ISL55110 and ISL55111 are simply dual level
shifting drivers, there are several areas to which careful attention
must be paid.
Grounding
Since the input and the high current output current paths both
include the ground pin, it is very important to minimize any
common impedance in the ground return. Since the ISL55111
has one inverting input, any common impedance will generate
negative feedback and may degrade the delay times and rise
and fall times. Use a ground plane if possible or use separate
ground returns for the input and output circuits. To minimize any
common inductance in the ground return, separate the input and
output circuit ground returns as close to the ISL55110 and
ISL55111 as possible.
Bypassing
The rapid charging and discharging of the load capacitance
requires very high current spikes from the power supplies. A
parallel combination of capacitors, which have a low impedance
over a wide frequency range should be used. A 4.7µF tantalum
capacitor in parallel with a low inductance 0.1µF capacitor is
usually sufficient bypassing.
Output Damping
Ringing is a common problem in any circuit with very fast rise or
fall times. Such ringing will be aggravated by long inductive lines
with capacitive loads. Techniques to reduce ringing include:
1. Reduce inductance by making printed circuit board traces as
short as possible.
2. Reduce inductance by using a ground plane or by closely
coupling the output lines to their return paths.
3. Use small damping resistor in series with the output of the
ISL55110 and ISL55111. Although this reduces ringing, it will
also slightly increase the rise and fall times.
4. Use good bypassing techniques to prevent supply voltage
ringing.
Power Dissipation Calculation
The Power dissipation equation has three components:
1. Quiescent power dissipation.
2. Power dissipation due to internal parasitics.
3. Power dissipation because of the load capacitor.
Power dissipation due to internal parasitics is usually the most
difficult to accurately quantitize. This is primarily due to crowbar
current which is a product of both the high and low drivers
conducting effectively at the same time during driver transitions.
Design goals always target the minimum time for this condition
to exist. Given that how often this occurs is a product of
frequency, crowbar effects can be characterized as internal
capacitance.
Lab tests are conducted with driver outputs disconnected from
any load. With design verification packaging, bond wires are
removed to aid in the characterization process. Based on
laboratory tests and simulation correlation of those results,
Equation 1
defines the ISL55110 and ISL55111 power
dissipation per channel:
• Where 3.3mA is the quiescent current from the V
DD
. This
forms a small portion of the total calculation. When figuring
two channel power consumption, only include this current
once.
• 10pF is the approximate parasitic capacitor (inverters, etc.),
which the V
DD
drives.
• 135pF is the approximate parasitic at the D
OUT
and its buffers.
This includes the effect of the crowbar current.
•C
L
is the load capacitor being driven.
PV
DD
3.3e-3= 10pF V
DD
2
f 135pF VH
2
++f +
(EQ. 1)
CL VH
2
f (Watts/Channel)
