Datasheet

Data Sheet AD5737
Rev. C | Page 39 of 44
Using Slew Rate Control
Using slew rate control can greatly reduce the current require-
ments of the AV
CC
supply, as shown in Figure 60.
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
10067-187
Figure 60. AI
CC
Current vs. Time for 24 mA Step Through 1 kΩ Load
with Slew Rate Control
When using slew rate control, it is important to remember that
the output cannot slew faster than the dc-to-dc converter. The
dc-to-dc converter slews slowest at higher currents through
large loads (for example, 1 kΩ). The slew rate is also dependent
on the configuration of the dc-to-dc converter. Two examples of
the dc-to-dc converter output slew are shown in Figure 58 and
Figure 59. (V
BOOST
corresponds to the output voltage of the
dc-to-dc converter.)
EXTERNAL PMOS MODE
The AD5737 can also be used with an external PMOS transistor
per channel, as shown in Figure 61. This mode can be used to
limit the on-chip power dissipation of the AD5737, although this
mode does not reduce the power dissipation of the total system.
The IGATEx functionality is not typically required when using
the dynamic power control feature; therefore, Figure 61 shows
the configuration of the device for a fixed V
BOOST_x
supply.
In this configuration, the SW
x
pin is left floating, and the GNDSW
x
pin is grounded. The V
BOOST_x
pin is connected to a minimum
supply of 7.4 V and a maximum supply of 33 V. This supply can
be sized according to the maximum load required to be driven.
The IGATEx functionality works by holding the gate of the
external PMOS transistor at (V
BOOST_x
− 5 V). This means that
the majority of the channel’s power dissipation takes place in
the external PMOS transistor.
The external PMOS transistor should be selected to tolerate a
V
DS
voltage of at least the V
BOOST_x
voltage, as well as to handle
the power dissipation required. The external PMOS transistor
typically has minimal effect on the current output performance.
10067-190
R1
R2 R3
R
SET_A
R
LOAD
GNDSW
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 61. Configuration of Channel A Using IGATEx