AD5755-1x AD [Analog Devices], AD5755-1x Datasheet - Page 30

• Manufacturer Part Number: AD5755-1x
• Description: Quad Channel, 16-Bit, Serial Input, 4-20mA & Voltage Output DAC, Dynamic Power Control, HART Connectivity
• Manufacturer: AD [Analog Devices]

AD5755-1
the following sections describe component selection for this
circuitry.
DC-DC Operation
The on-board DC-DC converters use a constant frequency,
peak current mode control scheme to step-up an AV
the range 2.7 to 5.5v to drive the AD5755-1 output channel.
These are designed to operate in discontinuous conduction
mode (DCM) with a duty cycle < 85%. Discontinuous
conduction mode refers to a mode of operation where the
inductor current goes to zero for an appreciable % of the
switching cycle. The DCDC converters are non synchronous i.e.
they require an external schottky diode.
DC-DC Output Voltage
When a channel current output is enabled the converter
regulates the V
whichever is greater. The maximum V
DC-DC Control Register (25, 27.3, 28.6 or 30V. See Table 26).
In voltage output mode, or in current output mode with the
output disabled, the converter regulates the V
(±8%).
Within a channel the Vout & Iout stages share a common V
supply so that the outputs of the Iout & Vout stages can be tied
together.
DC-DC On-Board Switch
The AD5755-1 contains a 0.5ohm internal switch . The switch
current is monitored on a pulse by pulse basis & is limited to
0.8A peak current.
DC-DC Switching Frequency and Phase
The AD5755-1 DCDC switching frequency can be selected
from the DCDC Control Register to be 250Khz, 400Khz,
649kHz or 812kHz. The phasing of the channels can also be
adjusted so that the DCDCs can clock on different edges (See
Table 26). For typical applications a 250Khz frequency is
recommended. At light loads (low output current & small load
resistor) the DCDC enters a pulse skipping mode to minimize
switching power dissipation.
DC-DC Inductor Selection
For typical 4-20mA applications a 10uH inductor combined
with a switching frequency of 250Khz will allow up to 24mA to
be driven into a load resistance of up to 1kΩ with an AV
supply from 2.7 to 5.5v. The inductor must be able to handle the
AVcc
boost
L
supply to 7.5V or (Iout*Rload+2V),
DCDC
Figure 21. DC-DC Circuit
SW_x
D
DCDC
boost
voltage is set in the
C
DCDC
boost
supply to +15v
Vboost_x
cc
input in
cc
Rev. PrD| Page 30 of 34
boost
peak current without saturating at the maximum ambient
temperature.
If an alternative Inductor/Switching frequency is preferred then
one must ensure that the DCDC continues to operates in DCM
mode and that the inductor current is less than 0.8A.
Where:
I
F
η = efficiency (Assume = 0.8)
DC-DC External schottky selection
The AD5755-1 requires an external schottky for correct
operation. Ensure the schottky is rated to handle the the
maximum reverse breakdown expected in operation & that the
rectifier maximum junction temperature is not exceeded. The
diode average current = Iload current.
DC-DC Compensation Capacitors
As the DCDC operates in DCM the uncompensated transfer
function is essentially a single pole transfer function. The pole
frequency is determined by Cout, Vin, Vout & Iload. The
AD5755-1 uses an external capacitor in conjunction with an
internal 150k resistor to compensate the regulator loop. For
typical 4-20mA applications connect a 10nF capacitor from
each of the COMP
DC-DC Input and Output Capacitor Selection
The output capacitor effects ripple voltage of the DCDC
converter & also indirectly limits the maximum slew rate at
which the channel output current can rise. The ripple voltage is
caused by a combination of the capacitance & ESR (equivalent
series resistance) of the capacitor. For the AD5755-1 a ceramic
capacitor of 4.7µF is recommended for typical applications.
Larger capacitors or paralled capacitors will improve the ripple
at the expense of reduced slew rate.
The input capacitor will provide much of the dynamic current
required for the DCDC converter & should also be a low ESR
component. For the AD5755-1 a ceramic capacitor of 10µF is
recommended for typical applications. Ceramic capacitors must
be chosen carefully as they can exhibit a large sensitivity to DC
bias voltages & temperature. X5R or X7R dielectrics are
preferred as these capacitors remain stable over wider operating
voltage & temperature ranges.
<
2
PEAK max
SW
×
L
=Switching Frequency set in the DCDC Control Register.
I
<
OUT
=Maximum Peak Current (0.8A limit)
V
2
I
IN
max
PEAK
×
min
I
(
OUT
V
2
max
OUT
(
V
max
DCDC_A/_B/_C/_D
2
OUT
Preliminary Technical Data
×
max
×
F
max
V
SW
−
OUT
V
−
CC
V
max
pins to GND.
IN
min
2
min
)
×
)
F
×
SW
η