MAX14808 Datasheet, PDF (27/31 Pages) Maxim Integrated Products – Octal Three-Level/Quad Five-Level High-Voltage 2A Digital Pulsers with T/R Switch

MAX14808 / MAX14809
Octal Three-Level/Quad Five-Level High-Voltage
2A Digital Pulsers with T/R Switch
T/R Switches (MAX14808 Only)
Each channel features a low-power T/R switch. The T/R
switch recovery time after the transmission is less than
1.2Fs. The T/R switches are controlled by the same pulser
digital inputs (see the Truth Tables section). No dedicated
input signals are required to activate/deactivate the T/R
switches. The integrated T/R switches do not require
any special timings and can operate synchronously with
the digital pulser. To minimize the leakage current dur-
ing transmission, it’s recommended to switch off the T/R
switches 3Fs before the beginning of the transmit burst.
Grass-Clipping Diodes
A pair of diodes in antiparallel configuration (referred to
as grass-clipping diodes) is presented at each pulser’s
output. The diodes’ reverse capacitance is extremely
low, allowing a perfect isolation between the receive path
and the actual pulser’s output stage.
Active Damp Circuit
An active damp circuit is integrated between the internal
pulser output node (before grass-clipping diodes) and
GND. The purpose of this circuit is to fully discharge the
pulser output internal node so that the node is not left in
high-impedance condition as soon as the transmit burst
is over. This results in two main advantages:
1) The grass-clipping isolation is more effective.
2) Suppression of any low-frequency oscillation of a
node that could be detrimental for Doppler mode
performances.
Independent (Floating)
Power-Supply Enable (LDO_EN)
The devices feature the LDO_EN control input to enable/
disable the internal FPSs. This allows the usage of external
high-efficiency power supplies to save system power. This
option must be considered only for special applications
requiring extremely low power dissipation. The low power
dissipation of the embedded FPSs already meets power
requirements in most of the cases. Drive LDO_EN low
or leave unconnected to enable the internal FPSs; drive
LDO_EN high to disable the internal FPSs.
Thermal Warning Outputs
The devices feature an open-drain thermal-protection
output (THP). When the internal junction temperature
exceeds +150NC, the devices automatically enter shut-
down mode and THP asserts. The devices reenter
normal operation and the THP deasserts when the die
temperature drops below +130NC.
Power Sequencing
When using the embedded FPSs (LDO_EN = low),
the devices do not require any power-up/power-down
sequence. When external FPSs are used (LDO_EN =
high), the conditions VGP_ > (VEE - 0.6V) and VGN_
< (VCC + 0.6V) must be satisfied during the entire
power-up/power-down transients (see the Electrical
Characteristics tables).
Applications Information
Exposed Pad and Layout Concerns
The devices provide an exposed pad (EP) underneath
the TQFN package for improved thermal performance.
Connect EP to GND externally and do not run traces
under the package to avoid possible short circuits. To
aid heat dissipation, connect EP to a similarly sized pad
on the component side of the PCB. This pad should be
connected through to the solder-side copper by several
plated holes to a large heat-spreading copper area to
conduct heat away from the device.
The devices’ high-speed pulser requires low-inductance
bypass capacitors to their supply inputs. High-speed
PCB trace design practices are recommended. Pay
particular attention to minimize trace lengths and use
suf­ficient trace width to reduce inductance. Use of
surface-mount components is recommended.
Typical Application Circuit
Figure 7 shows the MAX14808 in an octal three-level
pulsing application.
Maxim Integrated
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