MAX15031
The MAX15031 includes a versatile current monitor
intended for monitoring the APD, PIN, or varactor diode
DC current in fiber and other applications. The
MAX15031 features more than three decades of
dynamic current ranging from 500nA to 4mA and pro-
vides an output current accurately proportional to the
APD current at MOUT.
The MAX15031 also features a shutdown logic input to
disable the device and reduce its standby current to
2μA (max).
Fixed-Frequency PWM Controller
The heart of the MAX15031 current-mode PWM con-
troller is a BiCMOS multiple-input comparator that
simultaneously processes the output-error signal and
switch current signal. The main PWM comparator uses
direct summing, lacking a traditional error amplifier and
its associated phase shift. The direct summing configu-
ration approaches ideal cycle-by-cycle control over the
output voltage since there is no conventional error
amplifier in the feedback path.
The device operates in PWM mode using a fixed-fre-
quency, current-mode operation. The current-mode fre-
quency loop regulates the peak inductor current as a
function of the output error signal.
The current-mode PWM controller is intended for DCM
(discontinuous conduction mode) operation. No internal
slope compensation is added to the current signal.
Charge Pump
At low supply voltages (2.7V to 5.5V), internal charge-
pump circuitry and an external 10nF ceramic capacitor
connected between CP and CN double the available inter-
nal supply voltage to drive the internal switch efficiently.
In the 5.5V to 11V supply voltage range, the charge
pump is not required. In this configuration, disable the
charge pump by connecting CP to IN and leaving CN
unconnected.
Monitor Current Limit (RLIM)
The current limit of the current monitor is programmable
from 1mA to 5mA. Connect a resistor from RLIM to
ground to program the current-limit threshold up to 5mA.
The current monitor mirrors the current out of APD with
a 1:10 ratio, and the MOUT current can be converted to
a voltage signal by connecting a resistor from MOUT to
SGND.
The APD current-monitor range is from 500nA to 4mA,
and the MOUT current-mirror output accuracy is ±10%
from 500nA to 1mA of APD current and ±3.5% from
1mA to 4mA of APD current.
Clamping the Monitor
Output Voltage (CLAMP)
CLAMP provides a means for diode clamping the volt-
age at MOUT; thus V
MOUT
is limited to (V
CLAMP
+
0.6V). CLAMP can be connected to either an external
supply or BIAS. CLAMP can be left unconnected if volt-
age clamping is not required.
Adjusting the Boost Converter
Output Voltage (FB/CNTRL)
The boost converter output voltage can be set by con-
necting FB to a resistor-divider from V
OUT
to ground.
The set-point feedback reference is the 1.245V (typ)
internal reference voltage when V
CNTRL
> 1.5V and is
equal to the CNTRL voltage when V
CNTRL
< 1.25V.
To change the converter output on the fly, apply a volt-
age lower than 1.25V (typ) to the CNTRL input and
adjust the CNTRL voltage, which is the reference input
of the error amplifier when V
CNTRL
< 1.25V (see the
Functional Diagram
). This feature can be used to adjust
the APD voltage based on the APD mirror current,
which compensates for the APD avalanche gain varia-
tion with temperature and manufacturing process. As
shown in Figure 4, the voltage signal proportional to the
MOUT current is connected to the ADC (analog to digi-
tal) input of the APD module, which then controls the
reference voltage of the boost converter error amplifier
through a DAC (digital to analog) block connected to
the CNTRL input. The BIAS voltage and, therefore, the
APD current, are controlled based on the MOUT mirror
current, forming a negative feedback loop.
Shutdown (
SHDN
)
The MAX15031 features an active-low shutdown input
(SHDN). Pull SHDN low to enter shutdown. During shut-
down, the supply current drops to 2μA (30μA from
BIAS) (max). However, the output remains connected to
the input through the inductor and the output diode,
holding the output voltage to one diode drop below
PWR when the MAX15031 shuts down. Connect SHDN
to IN for always-on operation.
80V, 300mW Boost Converter and Current
Monitor for APD Bias Applications
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