MAX15059 Datasheet, PDF (11/15 Pages) Maxim Integrated Products – 76V, 300mW Boost Converter and Current Monitor for APD Bias Applications

76V, 300mW Boost Converter and Current
Monitor for APD Bias Applications
Detailed Description
The MAX15059 constant-frequency, current-mode, PWM
boost converters are intended for low-voltage systems
that require a locally generated high voltage. These
devices are capable of generating a low-noise, high out-
put voltage required for PIN and varactor diode biasing.
The MAX15059 operates from +2.8V to +5.5V.
The MAX15059 operates in discontinuous mode in
order to reduce the switching noise caused by reverse
recovery charge of the rectifier diode and eliminates
the need for external compensation components. Other
continuous-mode boost converters generate large volt-
age spikes at the output when the LX switch turns on
because there is a conduction path between the output,
diode, and switch to ground during the time needed
for the diode to turn off and reverse its bias voltage. To
reduce the output noise even further, the LX switch turns
off by taking 10ns typically to transition from on to off.
As a consequence, the positive slew rate of the LX node
is reduced and the current from the inductor does not
“force” the output voltage as hard as would be the case
if the LX switch were to turn off faster.
The constant-frequency (400kHz) PWM architecture
generates an output voltage ripple that is easy to filter.
An 80V lateral DMOS device used as the internal power
switch is ideal for boost converters with output voltages
up to 76V. The MAX15059 can also be used in other
topologies where the PWM switch is grounded, like
SEPIC and flyback converters.
The MAX15059 includes a versatile current monitor
intended for monitoring the APD, PIN, or varactor
diode DC current in fiber and other applications. The
MAX15059 features more than three decades of dynam-
ic current ranging from 500nA to 4mA and provides an
output current accurately proportional to the APD current
at MOUT. MOUT output accuracy is Q10% from 500nA to
1mA and Q5% from 1mA to 2mA.
The MAX15059 also features a shutdown logic input to
disable the device and reduce its standby current to 2FA
(max).
Fixed-Frequency PWM Controller
The heart of the MAX15059 current-mode PWM control-
ler is a BiCMOS multi-input comparator that simulta-
neously 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 configura-
tion approaches ideal cycle-by-cycle control over the
output voltage since there is no conventional error ampli-
fier in the feedback path.
The devices operate in PWM mode using a fixed-
frequency, current-mode operation. The current-mode
frequency loop regulates the peak inductor current as a
function of the output-voltage error signal.
The current-mode PWM controller is intended for DCM
operation. No internal slope compensation is added to
the current signal.
Current Limit
The current limit of the current monitor is programmable
from 1mA to 4.6mA (typ). Connect RLIM to SGND to get
a default current-limit threshold of 4.6mA or connect a
resistor from RLIM to SGND to program the current-limit
threshold below the default setting of 4.6mA. Calculate
the value of the external resistor, RLIM, for a given cur-
rent limit, ILIM, using the following equation:
R_LIM(kΩ)
=
 _1.23V
_ILIM(mA)



x
10
−

2.67(kΩ)_

Clamping the Monitor Output Voltage
(MOUT)
CLAMP provides a means for diode clamping the volt-
age at MOUT; thus, VMOUT is limited to (VCLAMP + 0.6V).
CLAMP can be connected to either an external supply or
BIAS. Leave CLAMP unconnected if voltage clamping is
not required.
Shutdown
The MAX15059 features an active-low shutdown input
(SHDN). Pull SHDN low or leave it unconnected to enter
shutdown. During shutdown, the supply current drops
to 2FA (max). The output remains connected to the
input through the inductor and output rectifier, holding
the output voltage to one diode drop below IN when
the MAX15059 is in shutdown. Connect SHDN to IN for
always-on operation.
Adjusting the Feedback
Set-Point/Reference Voltage
Apply a voltage to the CNTRL input to set the feedback
set-point reference voltage, VREF (see the Functional
Diagram). For VCNTRL > 1.3V, the internal 1.23V (typ)
reference voltage is used as the feedback set point and
for VCNTRL < 1.2V, the CNTRL voltage is used as the
reference voltage (VFB set equal to VCNTRL).
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