AD5757 Analog Devices, AD5757 Datasheet - Page 37

• Manufacturer Part Number: AD5757
• Description: Quad Channel, 16-Bit, Serial Input, 4-20mA Output DAC, Dynamic Power Control, HART Connectivity
• Manufacturer: Analog Devices

Specifications of AD5757

• Resolution (bits): 16bit
• Dac Update Rate: 60kSPS
• Dac Settling Time: 15µs
• Max Pos Supply (v): +33V
• Single-supply: No
• Dac Type: Current 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 AD5757 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 31 for a plot of headroom
supplied vs. output current). When the output is disabled, the
converter regulates the V
DC-to-DC Converter Settling Time
The settling time for a step greater than ~1 V (I
dominated by the settling time of the dc-to-dc converter. The
exception to this is when the required voltage at the I
the compliance voltage is below 7.4 V (±5%). A typical plot of the
output settling time can be found in Figure 26. 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 24). 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 57.
DCDC
DCDC
DCDC
BOOST_x
BOOST_x
BOOST_x
DCDC
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
supply to 7.4 V (±5%) or (I
voltage is allowed to decay by ~0.4 V. After the
BOOST_x
voltage is set in the dc-to-dc control
MAX
BOOST_x
Functionality
supply to 7.4 V (±5%).
OUT
BOOST_x
Value
10 μH
4.7 μF
0.38 V
× R
LOAD
supply.
F
OUT
+ Headroom),
Manufacturer
Coilcraft®
Murata
NXP
MAX
OUT_x
× R
CC
, if still
LOAD
input of
pin plus
) is
Rev. B | Page 37 of 44
As shown in Figure 57, the DC-DCx bit in the status register
asserts when the AD5757 is ramping to the V
deasserts when the voltage is decaying to V
DC-to-DC Converter On-Board Switch
The AD5757 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 AD5757 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 24). 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 AD5757 requires an external Schottky for correct
operation. 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
current. 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
V
DC_DC BIT
DC-DCx BIT = 1
DC-DCx BIT = 0
MAX
0.5
Figure 57. Operation on Reaching V
1.0
1.5
TIME (ms)
0mA TO 24mA RANGE, 24mA OUTPUT
OUTPUT UNLOADED
DC-DCMaxV = 29.5V
f
T
2.0
SW
A
= 25°C
= 410kHz
2.5
CC
MAX
supply of 4.5 V to
MAX
3.0
MAX
− ~0.4 V.
value but
AD5757
3.5
LOAD
4.0