Datasheet

LT3571
11
3571fa
MONIN
MON APD
PWM
0.5pF
3571 F04
LT3571
0.1μF
4.99k
1k
PMBT3904
–V
HI
–V
LO
–
+
LT1815
MEASURE
HERE
OUT
4.99k
2.5V
APD Current Monitor Transient Response
Measurement
The transient response of the APD current monitor is a
key performance characteristic. It is essentially a func-
tion of the input step-signal levels, since the small signal
bandwidth increases with the input signal. At greater than
10µA, the LT3571 APD current mirror typically has several
hundred nanoseconds response time. To measure such
fast transient response, any capacitor at the APD and the
MON pin should be removed. Figure 4 shows a suggested
transient response test setup. Choose V
L
and V
H
, cor-
responding to the input step current levels, respectively.
At the MON pin, a wideband transimpedence amplifi er
is implemented using the LT1815. Operating in a shunt
confi guration, the amplifi er buffers the MON output current
and dramatically reduces the effective output impedence
at the OUT node. Note that there is an inversion and a DC
offset present when this measurement technique is used.
A regular oscilloscope probe can then be used to capture
the fast transient response at the OUT node.
APD Bias Voltage Temperature Compensation
Typically, the APD reverse bias voltage has a positive tem-
perature coeffi cient. The APD pin voltage can be adjusted
with temperature via the CTRL pin. One simple solution is
to form a resistor divider from the V
REF
pin to the CTRL
pin, as shown in Figure 5. By carefully choosing the resis-
tor values, a temperature coeffi cient can be applied to the
APD reverse bias voltage. A more complicated and precise
way to set the APD temperature coeffi cient involves a
transistor network as shown in the “5V to 50V APD Bias
Power Supply with Temperature Compensation”. Please
consult with factory for this type applications.
Figure 4. Transient Response Measurement Set-Up
Figure 5. Setting Temperature Compensation
APPLICATIONS INFORMATION
V
REF
CTRL
R1
NTC
3571 F05
LT3571
R2