ltc1599bin-pbf Linear Technology Corporation, ltc1599bin-pbf Datasheet - Page 14

• Manufacturer Part Number: ltc1599bin-pbf
• Description: Ltc1599 - 16-bit Byte Wide, Low Glitch Multiplying Dac With 4-quadrant Resistors
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC1599
The output current of the DAC is converted to a voltage via
U3 (LT1112), producing 0V to – 2.5V at Pin 1 of U3. The
resulting current in Q1 is determined by two elements of
resistor array, R
maintained at 0V by the action of U1B.
In applications that do not require 16-bit resolution and
accuracy, the LTC1599 can be replaced by the 14-bit
parallel LTC1591. Furthermore, the resistor array can be
substituted with discrete resistors, and Q2 could be re-
placed by a high gain bipolar PNP; for example, an FZT600
from Zetex.
No trim is provided a shown, as it is expected that software
control is preferable. The output range of 4mA to 20mA is
defined by software, as the full output range is nominally
0mA to 24mA.
U1 is a rail-to-rail amplifier that can operate on suppy
voltages up to 36V. This defines the maximum voltage on
the loop power. If higher loop voltages are required, a
separate low power amplifier at U1A, powered by a zener
regulated supply and referenced to loop power, would
allow voltages up to the breakdown voltages of Q1 and Q2.
14
LT1460-5
MLBYTE
INPUTS
CLVL
12
4
14 TO 18,
DATA
21 TO 23
CLR
WR
LD
8
6
13
IF 2.5V REF USED CONNECT
DIRECTLY TO REF
MLBYTE
WR
4
N1
R1
12 11
0.1µF
R1
U
LD
(3mA max). The emitter of Q1 is
CLR CLVL
LTC1599
R
3
24
COM
U2
INFORMATION
R2
U
5
6
+
–
10
1/2 LT1112
2
R2
Figure 5. 16-Bit Current Loop Controller for Industrial Applications
W
7
1
REF
5V
V
24V
CC
20
0.1µF
U
R
5
R
0.1µF
OFS
OFS
16-BIT DAC
R
15
10
FB
2
7
DGND
R
6
R
R
FB
N1
N1
I
OUT2S
I
In the example shown, the use of a dual op amp requires
a zener clamp to protect the gate of the MOS power
transistor. If a separate shunt-regulated supply is pro-
vided for the amplifier replacing U1A, the gate clamp (Z1)
is not required.
As shown, this topology uses the LTC1599’s internal
divider (R1 and R2) to reduce the reference from 5V to
2.5V. If a 2.5V reference is used, it can be connected
directly to REF (Pin 1). Alternatively, if the op amp is
powered such that it has –10V output capability, the
divider and amplifier prior to the REF input are not required
and R
trim. The two R
changed in this case.
Note that the output of the current transmitter shows a
network that is intended to provide a first line of defense
against ESD and prevent oscillation (1000pF and 10Ω)
that could otherwise occur in the power MOSFET if lead
inductance were more than a few inches. C1 should be as
close as possible to Q2. Using MOSFETs that have higher
threshold voltages may require changing Z1 in order to
allow full current output.
OUT1
I
2
3
OUT2F
19
1/2 LT1366
–
+
9
U1A
7
OFS
8
8
4
2
3
100pF
can be used for other purposes such as offset
C3
33pF
–
1/2 LT1112
+
LOOP POWER
C2
1
U3
N1
R3
R4
1k
1k
resistors at the emitter of Q1 must be
6.2V
1
Z1
R
N1
1
= 400Ω × 8 RESISTOR ARRAY
14
R
3
N1
5
6
13
16
4
+
1/2 LT1366
–
9
12
U1B
5
R
N1
11
6
Q2
Si9407AEX
8
R
N1
7
R6
1k
C1
1000pF
10Ω
R5
Q1
MMBT6429
HFE = 500
I
OUT
sn1599 1599fs