Dec 2001 Fiber Optic Communication Systems Benefit from Tiny, Low Noise Avalanche Photodiode Bias Supply

DESIGN IDEAS
Fiber Optic Communication Systems
Benefit from Tiny, Low Noise
Avalanche Photodiode Bias Supply
by Michael Negrete
C6
0.15µF
50V
C5
0.15µF
50V
D1
L1
10µH
C1
4.7µF
6.3V
VOUT
30V TO 90V
47k
C3
1µF
50V
D2
C2
1µF
50V
VIN
SW
D6
LT1930A
SHDN
R6
1k
R2*
35.7k
R1*
35.7k
R4*
5.49k
C8
0.15µF
C9
0.15µF
FB
GND
LTC1663 DAC
(SOT-23)
C7
0.1µF
100V
D3
+
VIN
2.6V TO 6.3V
C4
1µF
50V
D4
0V TO 4.5V
D7
*1% METAL FILM RESISTORS
R3*
1.24k
Figure 1. LT1930A-based boost regulator produces 30V to 90V for
avalanche photodiode bias supplies with only 200µVP-P noise.
PWM operation keeps output noise
low and easy to filter.
Figure 1 shows a high voltage, low
noise APD bias supply that works
from an input range of 2.6V to 6.3V.
The DAC, driven from a processor,
adjusts the output from 30V to 90V to
compensate for temperature dependent APD gain fluctuations. The
VOUT
100µV/DIV
AC-COUPLED
200ns/DIV
Figure 2. 50V Avalanche photodiode bias shows 200µVP-P ripple and noise,
improving fiber optic receiver sensitivity.
28
R5
100Ω
D5
C1: TAIYO YUDEN JMK212BJ475MG
(408) 573-4150
C2 TO C4: MURATA GRM42-2X7R105K050
(814) 237-1431
C7: TAIYO YUDEN HMK316BJ104ML
C8, C9: VISHAY 695D154X9050AZ
(408) 241-4588
D1 TO D7: CENTRAL SEMI CMDSH2-3TR
(516) 435-1110
L1: MURATA LQH3C100
+
Avalanche photodiodes (APDs) are
the photo detector of choice for longhaul fiber optic communication
systems because of their high sensitivity and high internal gain. An
important characteristic of APDs is
that their internal gain is optimal
when there is a high voltage reverse
bias (30V to 90V) across the APD.
Nevertheless, the high gain is all for
naught if the sensitivity of the APD is
compromised by a noisy bias supply.
Traditionally, such low noise bias
supplies required custom circuits that
brought with them another problem:
large space requirements. Linear
Technology’s LT1930A 2.2MHz stepup DC/DC converter in a 5-lead
SOT-23 package solves these APD
bias voltage problems and does so in
a compact package suitable for most
fiber optic applications.
The LT1930A, a capacitor-diode
tripler and an external DAC provide a
bias voltage of up to 90V, allowing
easy temperature compensation (via
the DAC) to optimize internal gain. By
running the IC at a switching frequency of 2.2MHz, one can use tiny,
low cost capacitors and inductors to
keep the circuit footprint under 0.5in2.
The LT1930A’s constant frequency
LT1930A includes a 35V switch making it capable of producing 105V
output through a capacitor-diode
tripler.
To eliminate noise from the internal reference and error amplifier, two
0.15µF tantalum feedback capacitors are used in series. A series
connection ensures a sufficient voltage rating of the feedback capacitance.
Ceramic feedback capacitors have a
piezoelectric response to temperature
and low frequency vibrations under
1kHz, which is amplified by the
LT1930A internal error amplifier.
These should not be used unless noise
in that bandwidth is acceptable. To
protect the switch pin from negative
voltage swings, a clamping diode is
tied to ground. An identical diode is
placed at the feedback (FB) pin, along
with a 1k resistor to protect the part
from a sudden short in the load,
which would force the feedback
Linear Technology Magazine • December 2001
DESIGN IDEAS
capacitor’s negative side to the negative value of the output voltage. All
other capacitors can be ceramic,
which are small and capable of handling the high voltages of the regulator.
Figure 2 shows the AC coupled
noise of a 50V output with a 5V input.
The switching noise is less than
200µVP-P, allowing greater sensitivity
and dynamic range than most APD
LT1930A solution not only provides
the cleanest output in the industry
for APDs, but also achieves this in a
fraction of the space required by other
solutions.
bias solutions. Oscilloscope measurement bandwidth is 100Hz to 10MHz,
all probe cables are coaxial and special attention is given to grounding.1
Conclusion
The LT1930A exceeds all of the
stringent demands of an APD reversebias voltage, eliminating the need for
custom APD bias supplies. The
1. Discussion of low noise measurement issues
is available in “A Monolithic Switching Regulator with 100µV Output Noise,” Linear
Technology Corporation, Application Note 70
by Jim Williams.
LTC1911, continued from page 26
Applications
Figure 5 shows an application for the
LTC1911-1.8V. Here the SS/SHDN
pin has been connected to the input
supply, thus disabling the soft-start
function. In this application the output will come up immediately when
the supply is applied. This application is good for users who are not
worried about slight transients on
the input supply caused by the IC
turning on, and don’t need the shutdown feature. Here, shutdown is
effectively achieved by removing the
input supply.
Figure 6 shows an application for
the LTC1911-1.5V. Here the SS/
SHDN pin is connected to a soft-start
capacitor and an open drain device.
This application allows the user full
access to the shutdown function as
well as soft-start to limit the inrush
current at power on or coming out of
LTC1851, continued from page 8
shutdown modes and retain the stored
program. The user can run a programmed sequence, interrupt and
take direct control of the MUX or shut
the converter down and then return
to the programmed sequence. Any
edge of M1 or M0 will reset the counter
and/or pointer so that Scan Mode
always starts at MUX address 000
LTC1911-1.5
2.7V TO 5.5V INPUT
1-CELL Li-Ion
OR
3-CELL NiMH
10µF*
1µF*
1
2
3
4
VIN
SS/SHDN
C2+
C1+
C2–
GND
VOUT
8
7
6
5
C1–
10nF
1µF*
VOUT = 1.5V
IOUT = 250mA
2N7002
ON OFF
10µF*
*CERAMIC CAPACITOR
Figure 6. DC/DC converter with shutdown and soft-start
shutdown. The open drain device can
be omitted for users who only wish to
limit the inrush current, but do not
need the shutdown feature of the
part.
Conclusion
The LTC1911-1.8 and LTC1911-1.5
are well suited for medium to low
power step-down applications with
tight board space requirements. These
low noise step-down DC/DC converters can deliver 250mA of output
and the sequencer always starts at
location 0000.
Conclusion
The LTC1851 has everything you’ve
ever wanted from a Multiplexed ADC.
It has a programmable input MUX
and sample-and-hold that can handle
single-ended, differential, unipolar or
bipolar inputs. It has a flexible reference that offers three internal ranges
current and provide efficient operation over the input voltage range of
2.7V to 5.5V. Both parts come in the
thermally enhanced MSOP-8 packages. The LTC1911 keeps external
components to a minimum, requiring
only four or five inexpensive external
capacitors, helping designers meet
the tightest space requirements.
LTC1911 is a good match for single
cell Li-Ion as well as 3-cell NiMH/
NiCd battery powered applications.
and two options for using an external
reference. The ADC is a low power,
high performance 12-bit, 1.25Msps
converter. The three operational
modes make it easy to use in the
simplest applications but powerful
enough to solve your toughest problems. Stop adapting your inputs to
the ADC and start using an ADC
designed to adapt itself to your inputs
and your application.
Authors can be contacted
at (408) 432-1900
Linear Technology Magazine • December 2001
29
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