Datasheet

LOOP+
LOOP-
DAC161P997
BASE
OUT
COMA
R
X1
C
X1
C
X2
C
X3
C
X4
-+
A(s)
g
m
+
-
I
AUX
R
1
R
2
R
E
G
m
I
DAC
r
o
DAC161P997
www.ti.com SNAS515E –JULY 2011–REVISED OCTOBER 2013
Loop Interface
The DAC161P997 cannot directly interface to the typical 4 - 20 mA loop due to the excessive loop supply
voltage. The loop interface has to provide the means of stepping down the LOOP Supply down to 3.6V. This can
be accomplished with either a linear regulator (LDO) or switching regulator while keeping in mind that the
regulator’s quiescent current will have direct effect on the minimum achievable I
LOOP
(see DC Input-Output
Transfer Function).
The second component of the loop interface is the external NPN transistor (BJT). This device is part of the
control circuit that regulates the transmitter’s output current (I
LOOP
). Since the BJT operates over the wide current
range, spanning at least 4 - 20 mA, it is necessary to degenerate the emitter in order to stabilize transistor’s
transconductance (g
m
). The degeneration resistor of 22Ω is suggested in typical applications. For circuit details,
see Application Circuit Examples.
The NPN BJT should not be replaced with an N-channel FET (Field Effect Transistor) for the following reasons:
discrete FET’s typically have high threshold voltages (VT), in the order of 1.5V to 2V, which is beyond the BASE
output maximum range; discrete FET’s present higher load capacitance which may degrade system stability
margins; and BASE output relies on the BJT’s base current for biasing.
Loop Compliance
The maximum V(LOOP+,LOOP-) potential is limited by the choice of step-down regulator, and the external BJT’s
Collector Emitter breakdown voltage. For minimum V(LOOP+, LOOP−) potential consider Figure 14. Here,
observe that V(LOOP+,LOOP −) ≅ min(V
CE
) + I
LOOP
R
E
+ I
LOOP
R
2
= min(V
CE
) + 0.53V + 0.96V = 3.66V, at I
LOOP
=
24mA. The voltage drop across internal R
2
is specified in ELECTRICAL CHARACTERISTICS.
AC Characteristics
The approximate frequency dependent characteristics of the loop drive circuit can be analyzed using the circuit in
Figure 16:
Figure 16. Capacitances affecting Control Loop
Here it is assumed that the internal amplifier dominates the frequency response of the system, and it has a single
pole response. The BJT’s response, in the bandwidth of the control loop, is assumed to be frequency
independent and is characterized by the transconductance g
m
and the output resistance r
o
.
As in previous sections I
DAC
and I
AUX
represent the filtered output of the ∑Δ modulator and the quiescent current
of the companion devices.
The circuit in Figure 16 can be further simplified by omitting the on-board capacitances, whose effect will be
discussed in Stability, and by combining the amplifier, the external transistor and resistor R
E
into one G
m
block.
The resulting circuit is shown in Figure 17.
By assuming that the BJT’s output resistance (r
o
) is large, the loop current I
LOOP
can be expressed as:
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