LT3905
12
3905fa
For more information www.linear.com/LT3905
For this strategy, the ILIM_MON resistor is selected so
the ILIM_MON pin reaches 1.248V at the desired APD
regulation current, IAPD
REG
.
R
ILIM _ MON
=
IAPD
Although this loop will effectively limit the output voltage to
that which is needed to regulate the APD current, the fast
overload case must be considered as well. The regulation
of the boost converter output is comparatively slow
with respect to an APD overload condition. Additionally,
substantial energy is stored in the charged output capacitor
that should be prevented from flowing into the APD during
a condition of optical flooding. To protect against this type
of overload, the ILIM_MON pin has a fast current regula
-
tor which limits the output current of the APD pin itself.
This current limiting loop has a threshold 100mV above
the 1.248V boost regulation threshold, and two actions
are taken if this threshold is reached.
First, the output current of the APD pin is limited to the
value corresponding to 1.348V on ILIM_MON. For a
shorted or optically flooded APD, the voltage at the APD
pin will drop to any level necessary to limit the current
to the programmed value. Even for the case of the APD
pin shorted to GND, the current will remain limited to the
programmed value and ILIM_MON will continue to indicate
an accurate measurement of APD current.
The second action that is taken in response to an overload
condition is assertion of the open-drain ILIM flag to indicate
the overload condition to the user. It is asserted once
the ILIM_MON voltage exceeds 1.348V and enters active
limiting, and will remain asserted until the ILIM_MON
voltage has dropped by 50mV, indicating that the device
is no longer in active limiting.
The response of the APD current limiter is shown in
Figure5. When the overload is applied, the APD voltage
drops abruptly while the output voltage decays slowly
due to the regulated current load. As the output voltage
reaches the new operating point, the ILIM indicator is
released and the boost converter begins regulating to
1.248V on ILIM_MON.
To compute the values of R7 and R8, start by setting R8
at least 10 times the value of the pull-up resistor R10.
This will prevent undue loading of the LOS signal itself.
Next, compute the total hysteresis using the voltage divider
generated by R7/R8:
V
H
= V
IN
R7+R8
+ 20mV
This hysteresis is composed of two parts, the hysteresis
when LOS_MON is rising (leaving loss of signal) and that
when LOS_MON is falling (entering loss of signal).
Use the formulas below with the value of external threshold
applied to LOS ADJUST to calculate the new rising and
falling thresholds of LOS_MON when using external
hysteresis. Adjust the LOS_MON resistor if necessary to
achieve a desired threshold with respect to APD current.
V
LOS _MON(FALL)
= V
LOS _ ADJUST
R8
R7+R8
V
LOS _MON(RISE)
= V
LOS _ ADJUST
+ 20mV +
R7•
V
IN
– V
LOS _ ADJUST
R7+R8
Current Limit Monitor
The smallest output current is the ILIM_MON output, at
5% of the APD current. This output is used for regulating
the boost converter to provide a fixed APD current, as well
as detecting and limiting of overload conditions.
When the voltage at ILIM_MON reaches the first threshold
of 1.248V, an auxiliary error amplifier limits the boost
converter output voltage. In this manner, the LT3905 can
regulate a fixed APD current with a single resistor at the
ILIM_MON pin. In this mode, use the FB function to set
a maximum output voltage limit. The FB resistor divider
should be set for an output voltage higher than the expected
output voltage required for the APD and monitor drop. The
ILIM_MON error amplifier is the dominant regulation path,
and the FB error amplifier will only take over to limit the
output voltage in the case of a disconnected APD.
applicaTions inForMaTion
