ADR435 AD [Analog Devices], ADR435 Datasheet - Page 42

• Manufacturer Part Number: ADR435
• Description: Quad-Channel, 12-Bit, Serial Input, 4 mA to 20 mA Output
• Manufacturer: AD [Analog Devices]

AD5735
DC-TO-DC CONVERTERS
The
These are used to provide dynamic control of the V
voltage for each channel (see Figure 71). Figure 77 shows the
discrete components needed for the dc-to-dc circuitry, and the
following sections describe component selection and operation
of this circuitry.
Table 36. Recommended Components for a DC-to-DC Converter
Symbol
L
C
D
It is recommended that a 10 Ω, 100 nF low-pass RC filter be
placed after C
but reduces the amount of ripple on the V
DC-to-DC Converter Operation
The on-board dc-to-dc converters use a constant frequency, peak
current mode control scheme to step up an AV
to 5.5 V to drive the
are designed to operate in discontinuous conduction mode with
a duty cycle of <90% typical. Discontinuous conduction mode
refers to a mode of operation where the inductor current goes
to zero for an appreciable percentage of the switching cycle. The
dc-to-dc converters are nonsynchronous; that is, they require an
external Schottky diode.
DC-to-DC Converter Output Voltage
When a channel current output is enabled, the converter regulates
the V
whichever is greater (see Figure 51 for a plot of headroom
supplied vs. output current). In voltage output mode with the
output disabled, the converter regulates the V
15 V (±5%). In current output mode with the output disabled,
the converter regulates the V
Within a channel, the V
V
stages can be tied together (see the Voltage and Current Output
Pins on the Same Terminal section).
DC-to-DC Converter Settling Time
In current output mode, the settling time for a step greater than
~1 V (I
dc converter. The exception to this is when the required voltage at
the I
Figure 47 shows a typical plot of the output settling time. This
plot is for a 1 kΩ load. The settling time for smaller loads is faster.
The settling time for current steps less than 24 mA is also faster.
DCDC
DCDC
DCDC
BOOST_x
AD5735
OUT_x
BOOST_x
AV
OUT
CC
supply; therefore, the outputs of the I
≥10µF
pin plus the compliance voltage is below 7.4 V (±5%).
C
× R
Component
XAL4040-103
GRM32ER71H475KA88L
PMEG3010BEA
IN
supply to 7.4 V (±5%) or (I
contains four independent dc-to-dc converters.
DCDC
LOAD
. This filter consumes a small amount of power
) is dominated by the settling time of the dc-to-
L
10µH
DCDC
AD5735
Figure 77. DC-to-DC Circuit
SW
OUT_x
x
D
C
4.7µF
BOOST_x
and I
DCDC
DCDC
output channel. These converters
OUT_x
supply to 7.4 V (±5%).
R
FILTER
10Ω
Value
10 µH
4.7 µF
0.285 V
OUT
stages share a common
× R
BOOST_x
F
LOAD
OUT_x
C
0.1µF
BOOST_x
FILTER
CC
V
supply.
BOOST_x
Manufacturer
Coilcraft®
Murata
NXP
input of 4.5 V
+ Headroom),
BOOST_x
and V
supply to
supply
OUT_x
Rev. B | Page 42 of 48
DC-to-DC Converter V
The maximum V
register (23 V, 24.5 V, 27 V, or 29.5 V; see Table 28). When the
maximum voltage is reached, the dc-to-dc converter is disabled,
and the V
V
reenabled, and the voltage ramps up again to V
required. This operation is shown in Figure 78.
As shown in Figure 78, the DC-DCx bit in the status register
is asserted when the
is deasserted when the voltage decays to V
DC-to-DC Converter On-Board Switch
The
current is monitored on a pulse-by-pulse basis and is limited
to 0.8 A peak current.
DC-to-DC Converter Switching Frequency and Phase
The
selected from the dc-to-dc control register (see Table 28). The
phasing of the channels can also be adjusted so that the dc-to-dc
converters can clock on different edges. For typical applications,
a 410 kHz frequency is recommended. At light loads (low output
current and small load resistor), the dc-to-dc converter enters a
pulse-skipping mode to minimize switching power dissipation.
DC-to-DC Converter Inductor Selection
For typical 4 mA to 20 mA applications, a 10 µH inductor (such
as the XAL4040-103 from Coilcraft), combined with a switching
frequency of 410 kHz, allows up to 24 mA to be driven into a
load resistance of up to 1 kΩ with an AV
5.5 V. It is important to ensure that the inductor can handle the
peak current without saturating, especially at the maximum
ambient temperature. If the inductor enters saturation mode,
efficiency decreases. The inductance value also drops during
saturation and may result in the dc-to-dc converter circuit not
being able to supply the required output power.
BOOST_x
AD5735
AD5735
29.6
29.5
29.4
29.3
29.2
29.1
29.0
28.9
28.8
28.7
28.6
voltage decays by ~0.4 V, the dc-to-dc converter is
0
BOOST_x
DC-DCx BIT = 1
DC-DCx BIT = 0
V
DC-DCx BIT
MAX
contains a 0.425 Ω internal switch. The switch
dc-to-dc converter switching frequency can be
0.5
Figure 78. Operation on Reaching V
voltage is allowed to decay by ~0.4 V. After the
BOOST_x
AD5735
1.0
voltage is set in the dc-to-dc control
MAX
1.5
Functionality
ramps up to the V
TIME (ms)
DC-DC MaxV BITS = 29.5V
0mA TO 24mA RANGE, 24mA OUTPUT
OUTPUT UNLOADED
f
T
2.0
SW
A
= 25°C
= 410kHz
2.5
CC
MAX
supply of 4.5 V to
3.0
MAX
− ~0.4 V.
Data Sheet
MAX
MAX
, if still
3.5
value but
4.0