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LT3571

3571fa

MONIN

MON

APD

PWM

0.5pF

3571 F04

LT3571

0.1μF

4.99k

PMBT3904

–VHI

–VLO

–

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 VL and VH, cor-

responding to the input step current levels, respectively.

At the MON pin, a wideband transimpedence ampliﬁer

is implemented using the LT1815. Operating in a shunt

conﬁguration, the ampliﬁer 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 coefﬁcient. The APD pin voltage can be adjusted

with temperature via the CTRL pin. One simple solution is

to form a resistor divider from the VREF pin to the CTRL

pin, as shown in Figure 5. By carefully choosing the resis-

tor values, a temperature coefﬁcient can be applied to the

APD reverse bias voltage. A more complicated and precise

way to set the APD temperature coefﬁcient 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

VREF

CTRL

NTC

3571 F05

LT3571

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