Datasheet

AD5735 Data Sheet
Rev. C | Page 42 of 48
DC-TO-DC CONVERTERS
The AD5735 contains four independent dc-to-dc converters.
These are used to provide dynamic control of the V
BOOST_x
supply
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.
A
V
CC
L
DCDC
D
DCDC
C
DCDC
4.7µF
C
FI
L
TER
0.1µF
R
FI
L
TER
C
IN
SW
x
V
BOOST_x
≥10µF
10Ω
10µH
09961-077
Figure 77. DC-to-DC Circuit
Table 36. Recommended Components for a DC-to-DC Converter
Symbol Component Value Manufacturer
L
DCDC
XAL4040-103 10 µH Coilcraft®
C
DCDC
GRM32ER71H475KA88L 4.7 µF Murata
D
DCDC
PMEG3010BEA 0.285 V
F
NXP
It is recommended that a 10 Ω, 100 nF low-pass RC filter be
placed after C
DCDC
. This filter consumes a small amount of power
but reduces the amount of ripple on the V
BOOST_x
supply.
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
CC
input of 4.5 V
to 5.5 V to drive the AD5735 output channel. These converters
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
BOOST_x
supply to 7.4 V (±5%) or (I
OUT
× R
LOAD
+ Headroom),
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
BOOST_x
supply to
15 V (±5%). In current output mode with the output disabled,
the converter regulates the V
BOOST_x
supply to 7.4 V (±5%).
Within a channel, the V
OUT_x
and I
OUT_x
stages share a common
V
BOOST_x
supply; therefore, the outputs of the I
OUT_x
and V
OUT_x
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
OUT
× R
LOAD
) is dominated by the settling time of the dc-to-
dc converter. The exception to this is when the required voltage at
the I
OUT_x
pin plus the compliance voltage is below 7.4 V (±5%).
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.
DC-to-DC Converter V
MAX
Functionality
The maximum V
BOOST_x
voltage is set in the dc-to-dc control
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
BOOST_x
voltage is allowed to decay by ~0.4 V. After the
V
BOOST_x
voltage decays by ~0.4 V, the dc-to-dc converter is
reenabled, and the voltage ramps up again to V
MAX
, if still
required. This operation is shown in Figure 78.
28.6
28.7
28.8
28.9
29.0
29.1
29.2
29.3
29.4
29.5
29.6
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
V
BOOST_x
VOLTAGE (V)
TIME (ms)
V
MAX
0mA TO 24mA RANGE, 24mA OUTPUT
OUTPUT UNLOADED
DC-DC MaxV BITS = 29.5V
DC-DCx BIT
DC-DCx BIT = 0
DC-DCx BIT = 1
f
SW
= 410kHz
T
A
= 25°C
09961-183
Figure 78. Operation on Reaching V
MAX
As shown in Figure 78, the DC-DCx bit in the status register
is asserted when the AD5735 ramps up to the V
MAX
value but
is deasserted when the voltage decays to V
MAX
− ~0.4 V.
DC-to-DC Converter On-Board Switch
The AD5735 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 AD5735 dc-to-dc converter switching frequency can be
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
CC
supply of 4.5 V to
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.