80V, 300mW Boost Converter and Current

Monitor for APD Bias Applications

10

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Functional Diagram

80V

DMOS

+A

-C

-A

+C

CLK

SHDN

BIAS

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.