Datasheet

Data Sheet AD5757
Rev. D | Page 39 of 44
0
4
12
8
16
24
20
28
32
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
AI
CC
CURRENT (A)
0mA TO 24mA RANGE
1kΩ LOAD
f
SW
= 410kHz
INDUCTOR = 10µH (XAL4040-103)
T
A
= 25°C
0 0.5 1.0 1.5 2.0 2.5
I
OUT_x
CURRENT (mA)/V
BOOST_x
VOLTAGE (V)
TIME (ms)
AI
CC
I
OUT
V
BOOST
09225-185
Figure 59. AI
CC
Current vs. Time for 24 mA Step Through 1 kΩ Load
with External 51 kΩ Compensation Resistor
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
AI
CC
CURRENT (A)
0mA TO 24mA RANGE
500Ω LOAD
f
SW
= 410kHz
INDUCTOR = 10µH (XAL4040-103)
T
A
= 25°C
0
4
12
8
16
24
20
28
32
0 0.5 1.0 1.5 2.0 2.5
I
OUT_x
CURRENT (mA)/ V
BOOST_x
VOLTAGE (V)
TIME (ms)
AI
CC
I
OUT
V
BOOST
09225-186
Figure 60. AI
CC
Current vs. Time for 24 mA Step Through 500 Ω Load
with External 51 kΩ Compensation Resistor
Using slew rate control can greatly reduce the AV
CC
supplies cur-
rent requirements, as shown in Figure 61. When using slew rate
control, attention should be paid to the fact that the output cannot
slew faster than the dc-to-dc converter. The dc-to-dc converter
slews slowest at higher currents through large (for example, 1 kΩ)
loads. This slew rate is also dependent on the configuration of
the dc-to-dc converter. Two examples of the dc-to-dc converter’s
output slew are shown in Figure 59 and Figure 60 (V
BOOST
corre-
sponds to the dc-to-dc converter’s output voltage).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
AI
CC
CURRENT (A)
0mA TO 24mA RANGE
1kΩ LOAD
f
SW
= 410kHz
INDUCTOR = 10µH (XAL4040-103)
T
A
= 25°C
0
4
12
8
16
24
20
28
32
01 2345 6
I
OUT_x
CURRENT (mA)/V
BOOST_x
VOLTAGE (V)
TIME (ms)
AI
CC
I
OUT
V
BOOST
09225-187
Figure 61. AI
CC
Current vs. Time for 24 mA Step Through 1 kΩ Load
with Slew Rate Control
EXTERNAL PMOS MODE
The AD5757 can also be used with an external PMOS transistor
per channel, as shown in Figure 62. This mode can be used to
limit the on-chip power dissipation of the AD5757, though this
will not reduce the power dissipation of the total system. The
IGATE functionality is not typically required when using the
dynamic power control feature so Figure 62 shows the configura-
tion of the device for a fixed V
BOOST_x
supply.
In this configuration the SW
x
pin are left floating and the
GNDSW
x
pin is grounded. The V
BOOST_x
pin is connected to a
minimum supply of 7.5 V and a maximum supply of 33 V. This
supply can be sized according to the maximum load required to
be driven.
The IGATE functionality works by holding the gate of the
external PMOS transistor at (V
BOOST_x
− 5 V). This means that
the majority of the channels power dissipation will take place in
this external PMOS transistor.
The external PMOS transistor should be chosen tolerate a V
DS
voltage of at least −V
BOOST_x
, as well as to handle the power
dissipation required. This external PMOS transistor typically
has minimal effect on the current output performance.
09225-190
R1
R2 R3
R
SET_A
R
LOAD
SWGND
A
V
BOOST_A
(V
BOOST_A
–5V)
SW
A
CHARTA
IGATEA
CURRENT OUTPUT
DAC CHANNEL A
(LEFT FLOATING)
I
OUT_A
DAC A
5.0V
A
V
CC
Figure 62. Configuration off a Particular Channel Using IGATE