Datasheet

DS1886
SFP and PON ONU Controller
with Digital LDD Interface
25Maxim Integrated
Figure 8. Recommended Shunt Reference and RC Filter for
DAC Output
Delta-Sigma Output and Reference
One delta-sigma output (DAC) is provided. This provides
a 10-bit resolution output. The maximum voltage output is
set by the input REFIN. An inexpensive shunt reference is
recommended to generate the voltage applied to REFIN,
as shown in Figure 8. The output includes the ability to
compensate the APD bias for temperature as given by
the following formula:
DAC_INT = TINDEX[6:0] + DAC OFFSET
If INV_DAC = 0, then DAC[9:0] = DAC_INT/DACFS x
V
REFIN
.
If INV_DAC = 1, then DAC[9:0] = [3FF - (DAC_INT/
DACFS)] x V
REFIN
.
where:
1) INV_DAC is at A2h Table 02h, Register 8Dh, bit 7.
2) TINDEX is at A2h Table 02h, Register 81h.
3) DAC OFFSET is an 8-bit value, representing the 8
MSBs of a 10-bit value. The two LSBs are 0.
4) DACFS (A2h Table 02h, Register 88h) is an 8-bit
value, representing the 8 MSBs of a 10-bit value. The
two LSBs are 0.
5) DAC is a 10-bit value.
6) The DAC[9:0] is clamped at DACFS.
7) DAC_INT is an internal signal.
The delta-sigma output uses pulse-density modulation.
It provides much lower output ripple than a standard
digital PWM output given the same clock rate and filter
components. An RC filter is required on the DAC output
as suggested in Figure 8. The external RC filter compo-
nents are chosen based on ripple requirements, output
load, delta sigma frequency, and desired response time.
Before t
INIT
, the DAC output is high impedance.
The reference input, REFIN, is the supply voltage for the
DAC’s output buffer. The voltage source connected to
REFIN must be able to support the edge rate require-
ments of the delta sigma outputs. In a typical application,
a 0.1uF capacitor should be connected between REFIN
and ground.
The DS1886’s delta-sigma output is 10 bits. For illustra-
tive purposes, a 3-bit example is provided in Figure 9.
Figure 9. Delta-Sigma Output
O
1
2
3
4
5
6
7
DS1886
REFIN
2.5V
0.1µF
0201
ZTL431A
SOT23
1µF
0402
CONNECT TO
CONTROL INPUT
ON DC-DC
39.2kΩ
0201
DAC
68.1kΩ
0201
1kΩ
0201
20kΩ
0201
V
CC