Datasheet

Data Sheet AD5735
Rev. C | Page 43 of 48
DC-to-DC Converter External Schottky Diode Selection
The AD5735 requires an external Schottky diode for correct
operation. Ensure that the Schottky diode is rated to handle the
maximum reverse breakdown voltage expected in operation
and that the maximum junction temperature of the diode is not
exceeded. The average current of the diode is approximately
equal to the I
LOAD
current. Diodes with larger forward voltage
drops result in a decrease in efficiency.
DC-to-DC Converter Compensation Capacitors
Because the dc-to-dc converter operates in discontinuous conduc-
tion mode, the uncompensated transfer function is essentially a
single-pole transfer function. The pole frequency of the transfer
function is determined by the output capacitance, input and output
voltage, and output load of the dc-to-dc converter. The AD5735
uses an external capacitor in conjunction with an internal 150 kΩ
resistor to compensate the regulator loop.
Alternatively, an external compensation resistor can be used in
series with the compensation capacitor by setting the DC-DC
comp bit in the dc-to-dc control register (see Table 28). In this
case, a resistor of ~50 kΩ is recommended. The advantages of this
configuration are described in the AI
CC
Supply Requirements—
Slewing section. For typical applications, a 10 nF dc-to-dc com-
pensation capacitor is recommended.
DC-to-DC Converter Input and Output Capacitor
Selection
The output capacitor affects the ripple voltage of the dc-to-dc
converter and 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 and the equivalent series
resistance (ESR) of the capacitor. For typical applications, a
ceramic capacitor of 4.7 µF is recommended. Larger capacitors
or parallel capacitors improve the ripple at the expense of
reduced slew rate. Larger capacitors also affect the current
requirements of the AV
CC
supply while slewing (see the AI
CC
Supply Requirements—Slewing section). The capacitance at
the output of the dc-to-dc converter should be >3 µF under all
operating conditions.
The input capacitor provides much of the dynamic current
required for the dc-to-dc converter and should be a low ESR
component. For the AD5735, a low ESR tantalum or ceramic
capacitor of 10 µF is recommended for typical applications.
Ceramic capacitors must be chosen carefully because they can
exhibit a large sensitivity to dc bias voltages and temperature.
X5R or X7R dielectrics are preferred because these capacitors
remain stable over wider operating voltage and temperature
ranges. Care must be taken if selecting a tantalum capacitor to
ensure a low ESR value.
AI
CC
SUPPLY REQUIREMENTS—STATIC
The dc-to-dc converter is designed to supply a V
BOOST_x
voltage of
V
BOOST_x
= I
OUT
× R
LOAD
+ Headroom (2)
See Figure 51 for a plot of headroom supplied vs. output
current. Therefore, for a fixed load and output voltage, the
output current of the dc-to-dc converter can be calculated
by the following formula:
CC
V
BOOSTOUT
CC
CC
AVη
VI
AV
Efficiency
OutPower
AI
BOOST
×
×
=
×
=
(3)
where:
I
OUT
is the output current from I
OUT_x
in amperes.
η
V
BOOST
is the efficiency at V
BOOST_x
as a fraction (see Figure 53
and Figure 54).
AI
CC
SUPPLY REQUIREMENTS—SLEWING
The AI
CC
current requirement while slewing is greater than in
static operation because the output power increases to charge
the output capacitance of the dc-to-dc converter. This transient
current can be quite large (see Figure 79), although the methods
described in the Reducing AI
CC
Current Requirements section
can reduce the requirements on the AV
CC
supply.
If not enough AI
CC
current can be provided, the AV
CC
voltage
drops. Due to this AV
CC
drop, the AI
CC
current required for
slewing increases further, causing the voltage at AV
CC
to drop
further (see Equation 3). In this case, the V
BOOST_x
voltage and,
therefore, the output voltage, may never reach their intended
values. Because the AV
CC
voltage is common to all channels, this
voltage drop may also affect other channels.
0
5
10
15
20
25
30
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0.5 1.0
1.5
2.0 2.5
I
OUT_x
CURRENT (mA)/V
BOOST_x
VO
LT
AGE (V)
AI
CC
CURRENT (A)
TIME (ms)
AI
CC
I
OUT
V
BOOST
0mA TO 24mA RANGE
1kΩ LOAD
f
SW
= 410kHz
INDUCTOR = 10µH (XAL4040-103)
T
A
= 25°C
09961-184
Figure 79. AI
CC
Current vs. Time for 24 mA Step Through 1 kΩ Load
with Internal Compensation Resistor