DN273 - Fiber Optic Communication Systems Benefit from Tiny, Low Noise Avalanche Photodiode Bias Supply

Fiber Optic Communication Systems Benefit from Tiny, Low
Noise Avalanche Photodiode Bias Supply – Design Note 273
Michael Negrete
and does so in a compact package suitable for most
fiber optic applications.
Avalanche photodiodes (APDs) are the photo detector of
choice for long-haul 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.
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 PWM operation keeps
output noise low and easy to filter.
Traditionally, such low noise bias supplies required
custom circuits that brought with them another problem: large space requirements. Linear Technology’s
LT®1930A 2.2MHz step-up DC/DC converter in a 5-lead
SOT-23 package solves these APD bias voltage problems
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R5
100Ω
D5
C4
1μF
50V
D4
C6
0.15μF
50V
C5
0.15μF
50V
D1
L1
10μH
C2
1μF
50V
VIN
SW
D6
LT1930A
SHDN
LTC1663 DAC
(SOT-23)
R6
1k
R2*
35.7k
R4*
5.49k
C8
0.15μF
C9
0.15μF
R1*
35.7k
FB
GND
0V TO 4.5V
D7
*1% METAL FILM RESISTORS
C1: TAIYO YUDEN JMK212BJ475MG
C2 TO C4: MURATA GRM42-2X7R105K050
C7: TAIYO YUDEN HMK316BJ104ML
C8, C9: VISHAY 695D154X9050AZ
D1 TO D7: CENTRAL SEMI CMDSH2-3TR
L1: MURATA LQH3C100
R3*
1.24k
DN273 F01
Figure 1. LT1930A-Based Boost Regulator Produces 30V to 90V
for Avalanche Photodiode Bias Supplies with Only 200μVP-P Noise
12/01/273_conv
47k
C3
1μF
50V
D2
+
C1
4.7μF
6.3V
VOUT
30V TO 90V
D3
+
VIN
2.6V TO 6.3V
C7
0.1μF
100V
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 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 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 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 reverse-bias voltage, eliminating the need for
custom APD bias supplies. The 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.
1Discussion 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.
VOUT
100μV/DIV
AC-COUPLED
200ns/DIV
DN273 F02
Figure 2. 50V Avalanche Photodiode Bias Shows 200μVP-P
Ripple and Noise, Improving Fiber 0ptic Receiver Sensitivity
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn273f_conv LT/TP 1201 371.5K • PRINTED IN THE USA
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