Rainbow Electronics MAX15031 - Manual (Page 10)

MAX15031
80V, 300mW Boost Converter and Current
Monitor for APD Bias Applications
10 ______________________________________________________________________________________
Functional Diagram
80V
DMOS
+A
-C
-A
+C
CLK
V
REF
V REF
SHDN BIAS
V REF
LX
ILIM
PWR
PGND
RLIM
MOUT
1x
APD
FB
SGND
CNTRL
CN
CP
IN
SWITCH
CONTROL
LOGIC SOFT-
START
OUTPUT ERROR AND CURRENT
COMPARATOR
REFERENCE
COMPARATOR
CURRENT
MONITOR
OSCILLATOR
400kHz
CHARGE
PUMP
(DOUBLER)
BIAS AND
REFERENCE
THERMAL
SHUTDOWN
SWITCH
CURRENT
SENSE
CURRENT-
LIMIT
ADJUSTMENT
UVLO
CURRENT
LIMIT
MUX
PEAK CURRENT-LIMIT
COMPARATOR
CLAMP
10x
MAX15031
Detailed Description
The MAX15031 constant-frequency, current-mode, PWM
boost converter is intended for low-voltage systems that
require a locally generated high voltage. This device can
generate a low-noise, high output voltage required for
PIN and varactor diode biasing and LCD displays. The
MAX15031 operates either from +2.7V to +5.5V or from
+5.5V to +11V. For 2.7V to 5.5V operation, an internal
charge pump with an external 10nF ceramic capacitor is
used. For 5.5V to 11V operation, connect CP to IN and
leave CN unconnected.
The MAX15031 operates in discontinuous mode in
order to reduce the switching noise caused by reverse-
voltage recovery charge of the rectifier diode. 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 out-
put, diode, and switch to ground during the time need-
ed 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 vertical DMOS device used as the internal
power switch is ideal for boost converters with output
voltages up to 76V. The MAX15031 can also be used in
other topologies where the PWM switch is grounded,
like SEPIC and flyback converters.