14 _____________________________________________________________________________________

76V, 300mW Boost Converter and Current

Monitor for APD Bias Applications

Applications Information

Using APD or PIN Photodiodes

in Fiber Applications

When using the MAX15059 to monitor APD or PIN photo-

diode currents in fiber applications, several issues must

be addressed. In applications where the photodiode

must be fully depleted, keep track of voltages budgeted

for each component with respect to the available supply

voltage(s). The current monitors require as much as 3.5V

between BIAS and APD, which must be considered part

of the overall voltage budget.

Additional voltage margin can be created if a nega-

tive supply is used in place of a ground connection,

as long as the overall voltage drop experienced by

the MAX15059 is less than or equal to 76V. For this

type of application, the MAX15059 is suggested so the

output can be referenced to “true” ground and not the

negative supply. The MAX15059’s output current can

be referenced as desired with either a resistor to ground

or a transimpedance amplifier. Take care to ensure

that output voltage excursions do not interfere with the

required margin between BIAS and MOUT. In many fiber

applications, MOUT is connected directly to an ADC

that operates from a supply voltage that is less than the

voltage at BIAS. Connecting the MAX15059’s clamping

diode output, CLAMP, to the ADC power supply helps

avoid damage to the ADC. Without this protection, volt-

ages can develop at MOUT that might destroy the ADC.

This protection is less critical when MOUT is connected

directly to subsequent transimpedance amplifiers (linear

or logarithmic) that have low-impedance, near-ground-

referenced inputs. If a transimpedance amp is used on

the low side of the photodiode, its voltage drop must

also be considered. Leakage from the clamping diode

is most often insignificant over nominal operating condi-

tions, but grows with temperature.

To maintain low levels of wideband noise, lowpass filter-

ing the output signal is suggested in applications where

only DC measurements are required. Connect the filter

capacitor at MOUT. Determining the required filtering

components is straightforward, as the MAX15059 exhib-

its a very high output impedance of 5GI.

In some applications where pilot tones are used to identi-

fy specific fiber channels, higher bandwidths are desired

at MOUT to detect these tones. Consider the minimum

and maximum currents to be detected, then consult the

frequency response and noise typical operating curves.

If the minimum current is too small, insufficient band-

width could result, while too high a current could result in

excessive noise across the desired bandwidth.

Layout Considerations

Careful PCB layout is critical to achieve low switching

losses and clean and stable operation. Protect sensitive

analog grounds by using a star ground configuration.

Connect SGND and PGND together close to the device

at the return terminal of the output bypass capacitor. Do

not connect them together anywhere else. Keep all PCB

traces as short as possible to reduce stray capacitance,

trace resistance, and radiated noise. Ensure that the

feedback connection to FB is short and direct. Route

high-speed switching nodes away from the sensitive

analog areas. Use an internal PCB layer for SGND as an

EMI shield to keep radiated noise away from the device,

feedback dividers, and analog bypass capacitors. Refer

to the MAX15059 Evaluation Kit data sheet for a layout

example.

Chip Information

PROCESS: BiCMOS

Package Information

For the latest package outline information and land pat-

terns, go to www.maxim-ic.com/packages. Note that

a “+”, “#”, or “-” in the package code indicates RoHS

status only. Package drawings may show a different suf-

fix character, but the drawing pertains to the package

regardless of RoHS status.

PACKAGE TYPE

PACKAGE CODE

DOCUMENT NO.

16 TQFN-EP

T1633-4

21-0136