LINEAR TECHNOLOGY LT3482 Technical data
A Complete Compact APD Bias Solution for a 10Gbits/s
GPON System – Design Note 447
Xin Qi
Introduction
Avalanche photo diode (APD) receiver modules are
widely used in fi ber optic communication systems. An
APD module contains the APD and a signal conditioning
amplifi er, but is not completely self contained. It still
requires signifi cant support circuitry including a high
voltage, low noise power supply and a precision current
monitor to indicate the signal strength. The challenge is
squeezing this support circuitry into applications with
®
limited board spac e. The LT
3482 addresses this challenge
by integrating a monolithic DC/DC step-up converter and
an accurate current monitor. The LT3482 can support
up to a 90V APD bias voltage, and the current monitor
provides better than 10% accuracy over four decades of
dynamic range (250nA to 2.5mA).
Recent communication design efforts increasingly focus
on the 10Gbits/s GPON system, which demands that the
transient response of the APD current monitor is less
than 100ns for an input current step of two decades of
magnitude. A simple compact circuit using the LT3482 is
fast enough to meet this challenging requirement.
An APD Bias Topology with Fast Current Monitor
Transient Response
The circuit in Figure 1 shows the LT3482 confi gured to
produce an output voltage ranging from 20V to 45V from
a 5V source—capable of delivering up to 2mA of load
current. Its operation is straightforward. The LT3482
contains a 48V, 260mA internal switch, which boosts
to one-half the APD output voltage level. This
V
OUT1
voltage is doubled through an internal charge pump to
generate V
integrated. V
. All boost and charge pump diodes are
OUT2
is regulated by the internal voltage
OUT2
reference and the resistor divider made up of R3 and
R4. At this p oint, V
goes through the integrated high
OUT2
side current monitor (MONIN), which produces a current
proportional to the APD current at the MON pin.
The output voltage is available for the APD at the APD
pin. The CTRL pin serves to override the internal reference. By tying this pin above 1.25V, the output voltage
is regulated with the feedback at 1.25V. By externally
setting the CTRL pin to a lower voltage, the feedback
and the output voltage follow accordingly.
The SHDN pin not only enables the converter when 1.5V
or higher is applied, but also provides a soft-start function to control the slew rate of the switch current, thereby
minimizing inrush current. The switching frequency can
be set to 650kHz or 1.1MHz by tying the FSET pin to
ground or to V
, respectively. Fixed frequency opera-
IN
tion allows for an output ripple that is predictable and
easier to fi lter.
To achieve fast tr ansient response, any time-delay compo nent along the signal path should be minimized. Figure 1
shows an APD bias topology with fast current monitor
transient response. Unlike the ultralow noise topology
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
C1
0.1μF
L1
V
5V
GND
CTRL
16V
C3
1μF
16V
100k
C4
R1
6.8μH
TRANSIMPEDANCE
IN
0.1μF
50V
7, 8
SW
11
V
IN
15
f
SET
12
SHDN V
13
CTRL
MON
GND
TO
AMPLIFIER
PUMP
LT3482
36
MONIN
V
OUT2
OUT1
APD
GND GND
C2
0.22μF
100V
4
5
14
FB
2
9101716
Figure 1. Fast Current Monitor Transient Response
ADP Bias Topology
C5
0.22μF
50V
C7
0.22μF
50V
DN447 F01
APD
R2
100Ω
R5
1k
C6
0.01μF
100V
R3
1M
R4
21k
08/08/447
with a fi lter capacitor at the APD pin, the fi lter capacitor
is moved to the MONIN pin of the LT3482. The output
sourcing current from the MON pin is directly fed into a
transimpendance amplifi er.
A typical measured current monitor transient response
consists of the signal generation delay at the APD pin, the
built-in current monitor response time and the measurement delay at the MON pin. Thus, every effort should be
made to reduce signal generation and the measurement
delays.
Figure 2 shows the measurement setup. An NPN transistor in c ommo n base confi guration is use d to generate the
fast current step representing the APD load. A function
generator provides t wo negative bias voltages a t the PWM
node that result in two decades current step at the APD
5V
V
IN
MON
4.99k
2.5V
0.1μF
Figure 2. Fast Transient Response Measurement Setup
0.5pF
–
+
LT3482
4.99k
LT1815
MEASURE
HERE
APD
PMBT3904
1k
PWM
–V
–V
HI
DN447 F02
LO
pin. At the MON pin, a wideband transimpedence amplifi er
is implemented using the LT1815. Operating in a shunt
confi guration, the amplifi er buff ers the MON output current
and dramatically reduces the effective output impedance
at the OUT node. Note that there is an inversion and a DC
offset present when this measurment technique is used.
A regular oscilloscope probe can then be used to capture
the fast transient response at the OUT node.
Figures 3 and 4 show the measured input signal rising
transient response and the measured input signal falling
transient response, respectively, where the input current
levels are 10μA and 1mA. The PWM input signal levels
are selected based on the static measurement results.
The APD current is accurately mirrored by the LT3482
with an attenuation of fi ve and sourced from the MON
pin. With a 2.5V reference voltage, the OUT node voltage
swings between 1.5V ( = 2.5V – 1mA/5 • 4.99k) and 2.49V
( = 2.5V – 10μA/5 • 4.99k) responding to the input signal
step. The measurements demonstrate less than 50ns
transient response time, which exceeds the stringent
speed demand of the 10Gbits/s GPON system.
Conclusion
The LT3482 is a complete space-saving solution to APD
receiver module support circuitry design. It offers more
than just low bias noise and compact solution size; it
also features UltraFast™ current monitor transient speed
that addresses the challenges presented in the 10Gbits/s
GPON system.
UltraFast is a trademark of Linear Technology Corporation.
PWM GND
PWM
I
= 10μA
OUT
APD
tRD < 50ns
50ns/DIV
I
= 1mA
APD
DN447 F03
1V/DIV
500mV/DIV
OUT GND
Figure 3. Transient Response on Input Signal Rising Edge
(10μA to 1mA)
Data Sheet Download
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
(408) 432-1900
FAX: (408) 434-0507 ● www.linear.com
PWM GND
PWM
I
= 1mA
1V/DIV
OUT
500mV/DIV
OUT GND
APD
tFD < 50ns
50ns/DIV
I
APD
= 10μA
DN447 F04
Figure 4. Transient Response on Input Signal Falling
Edge (1mA to 10μA)
For applications help,
call (408) 432-1900, Ext. 3231
dn447f LT/TP 0808 392K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2008
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