MAX4004 Просмотр технического описания (PDF) - Maxim Integrated

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MAX4004

High-Accuracy, High-Side Current Monitors in SOT23

Maxim Integrated 

MAX4004/MAX4006

High-Accuracy, High-Side Current Monitors

in SOT23

In many fiber applications, OUT is connected directly to

an ADC that operates from a supply voltage that is less

than the voltage at BIAS. Connecting the MAX4004/

MAX4006’s clamping diode output, CLAMP, to the ADC

power supply helps avoid damage to the ADC. Without

this protection, voltages can develop at OUT that can

destroy the ADC. This protection is less critical when

OUT 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

conditions, 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. Determining the

required filtering components is straightforward, as the

MAX4004 exhibits a very high output impedance (> 5MΩ),

while the MAX4006 exhibits an output resistance of 10kΩ.

In some applications where pilot tones are used to identify

specific fiber channels, higher bandwidths are desired

at OUT 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 monitor

bandwidth could result, while too high of a current could

result in excessive monitor noise across the desired

bandwidth.

Bypassing and External Components

In applications where power-supply noise can interfere

with DC diode measurements, additional filtering is sug-

gested. Such noise is commonly seen when switching

power supplies are used to generate the photodiode bias

voltage. As shown in the typical operating circuit, a pi filter

(two 0.22μF capacitors and one 2.2μH inductor) greatly

suppress power-supply switching noise. If such a filter is

already present in the bias-generating circuit, only a sim-

ple bypass capacitor at the BIAS pin is suggested. The

output lowpass filter, a 10kΩ resistor and a 10nF capaci-

tor, further reduce permeating power-supply noise as well

as other wideband noise that might otherwise restrict

measurements at low signal levels. Again, reducing the

bandwidth of the OUT signal can affect performance of

pilot-tone systems.

To restrict high-frequency photodiode signals from affect-

ing the current monitor and BIAS power supply, an RF

choke and 10nF capacitor can be added. The capaci-

tance presented to REF should not exceed 10nF; larger

values increase startup time and could cause the thermal

shutdown circuit to activate during startup.

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