lt3571 Linear Technology Corporation, lt3571 Datasheet - Page 11

• Manufacturer Part Number: lt3571
• Description: 75v Dc/dc Converter For Apd Bias
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
APD Current Monitor Transient Response
Measurement
The transient response of the APD current monitor is a key
performance characteristic. It is essentially a function 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
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.
2.5V
4.99k
Figure 4. Transient Response Measurement Set-Up
MONIN
MON
0.1μF
0.5pF
–
+
LT1815
4.99k
LT3571
MEASURE
HERE
L
and V
OUT
H
PWM
APD
, corresponding
PMBT3904
1k
–V
3571 F04
HI
–V
LO
APD Bias Voltage Temperature Compensation
Typically, the APD reverse bias voltage has a positive
temperature coeffi cient. Customers can adjust the APD
pin voltage with temperature via the CTRL pin. One
simple solution is to form a resistor divider from the V
pin to the CTRL pin, as shown in Figure 5. By carefully
choosing the resistor values, customers can easily apply
a temperature coeffi cient 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 5. Setting Temperature Compensation
LT3571
CTRL
V
REF
3571 F05
R1
NTC
R2
LT3571
11
3571f
REF