AD5755 Analog Devices, AD5755 Datasheet - Page 43

• Manufacturer Part Number: AD5755
• Description: Quad Channel, 16-Bit, Serial Input,
• Manufacturer: Analog Devices

Specifications of AD5755

• Resolution (bits): 16bit
• Dac Update Rate: 91kSPS
• Dac Settling Time: 11µs
• Max Pos Supply (v): +33V
• Single-supply: No
• Dac Type: I or V Out
• Dac Input Format: SPI

Data Sheet
Table 33. Recommended DC-to-DC Components
Symbol
L
C
D
It is recommended to place a 10 Ω, 100 nF low-pass RC filter
after C
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
4.5 V to 5.5 V to drive the AD5755 output channel. These are
designed to operate in discontinuous conduction mode (DCM)
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 52 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
can be tied together.
DC-to-DC Converter Settling Time
When in current output mode, the settling time for a step greater
than ~1V (I
dc-to-dc converter. The exception to this is when the required
voltage at the I
7.4 V (±5%). A typical plot of the output settling time can be
found in Figure 48. 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.
DC-to-DC Converter V
The maximum V
register (23 V, 24.5 V, 27 V, or 29.5 V; see Table 25). On reaching
this maximum voltage, the dc-to-dc converter is disabled, and
the V
V
is reenabled, and the voltage ramps up again to V
required. This operation is shown in Figure 79.
DCDC
DCDC
DCDC
BOOST_x
BOOST_x
BOOST_x
BOOST_x
DCDC
supply so that the outputs of the I
voltage has decayed by ~0.4 V, the dc-to-dc converter
Component
XAL4040-103
GRM32ER71H475KA88L
PMEG3010BEA
. This consumes a small amount of power but
OUT
supply to 7.4 V (±5%) or (I
voltage is allowed to decay by ~0.4 V. After the
OUT_x
× R
BOOST_x
LOAD
pin plus the compliance voltage is below
) is dominated by the settling time of the
OUT_x
voltage is set in the dc-to-dc control
MAX
Functionality
BOOST_x
and I
OUT_x
supply to 7.4 V (±5%).
OUT
BOOST_x
Value
10 μH
4.7 μF
0.38 V
stages share a common
× R
OUT_x
LOAD
F
supply.
BOOST_x
and V
+ Headroom),
Manufacturer
Coilcraft®
Murata
NXP
MAX
CC
supply to
, if still
OUT_x
input of
stages
Rev. A | Page 43 of 52
As can be seen in Figure 79, the DC-DCx bit in the status register
asserts when the AD5755 is ramping to the V
deasserts when the voltage is decaying to V
DC-to-DC Converter On-Board Switch
The AD5755 contains a 0.425 Ω internal switch. The switch
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 AD5755 dc-to-dc converter switching frequency can be
selected from the dc-to-dc control register. The phasing of the
channels can also be adjusted so that the dc-to-dc converter can
clock on different edges (see Table 25). 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 switch-
ing 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 is able to handle
the peak current without saturating, especially at the maximum
ambient temperature. If the inductor enters into saturation mode,
it results in a decrease in efficiency. 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.
DC-to-DC Converter External Schottky Selection
The AD5755 requires an external Schottky for correct opera-
tion. Ensure that the Schottky is rated to handle the maximum
reverse breakdown expected in operation and that the rectifier
maximum junction temperature is not exceeded. The diode
average current is approximately equal to the I
Diodes with larger forward voltage drops result in a decrease in
efficiency.
29.6
29.5
29.4
29.3
29.2
29.1
29.0
28.9
28.8
28.7
28.6
0
DC-DCx BIT = 1
DC-DCx BIT = 0
V
DC_DC BIT
MAX
0.5
Figure 79. Operation on Reaching V
1.0
1.5
TIME (ms)
0mA TO 24mA RANGE, 24mA OUTPUT
OUTPUT UNLOADED
DC-DC MaxV = 29.5V
f
T
2.0
SW
A
= 25°C
= 410kHz
2.5
CC
MAX
3.0
supply of 4.5 V to
MAX
MAX
LOAD
− ~0.4 V.
value, but
3.5
current.
AD5755
4.0