LTC1599 Datasheet, PDF (14/20 Pages) Linear Technology – 16-Bit Byte Wide, Low Glitch Multiplying DAC with 4-Quadrant Resistors

LTC1599
APPLICATIONS INFORMATION
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_N1 (3mA max). The emitter of Q1 is
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.
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_OFS can be used for other purposes such as offset
trim. The two R_N1 resistors at the emitter of Q1 must be
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.
IF 2.5V REF USED CONNECT
DIRECTLY TO REF
12
6
LT1460-5
4
0.1µF
5₊
7
1/2 LT1112
6_–
24V
0.1µF
5V
0.1µF
LOOP POWER
2
R_N1
15
2–
10
C2
8 _100pF
7
R_N1
U1A
3 ¹+^{/2 LT1366}
1
4
R3
1k
Z1
R4 ^6.2V
1k
3456
R_N1
14 13 12 11
Q2
R5
Si9407AEX 10Ω
I_OUT
C1
1000pF
4
3
2
R1
R_COM R2
8
DATA
INPUTS
14 TO 18,
21 TO 23
R1
R2
U2
LTC1599
13
MLBYTE
MLBYTE
WR LD CLR CLVL
WR
LD
CLR
CLVL
12 11 24 10
1
20
REF V_CC
5
6
R_OFS
R_FB
R_OFS R_FB
I_{OUT1 7}
16-BIT DAC
I_{OUT2F 8}
C3
33pF
2–
U3
3 ¹+^{/2 LT1112}
I_{OUT2S 9}
DGND ¹⁹
5+
U1B
7
6 ¹â€“^{/2 LT1366}
R6
1k
1 1 ^RN1 16 9 ^RN1 8
R_N1 = 400Ω × 8 RESISTOR ARRAY
Figure 5. 16-Bit Current Loop Controller for Industrial Applications
Q1
MMBT6429
HFE = 500
14