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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

LIM

LIM

1.23V

R

(k )

x10 2.67(k )

I

(mA)









Ω =

−

Ω





















Clamping the Monitor Output Voltage

(MOUT)

CLAMP provides a means for diode clamping the volt-

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

reference voltage is used as the feedback set point and