__________Applications Information

Powering Circuitry with STAT

STAT’s function is to indicate the comparator’s operat-

ing mode. When STAT is low, the comparator is in high-

speed mode and will meet the guaranteed propagation

delay. When STAT is high, the comparator is in auto-

standby mode, in low-power mode, or in transition to

high-speed mode. An additional feature of this pin is

that it can source 3mA of current. When STAT is high,

additional circuitry can be powered. This circuitry can

be automatically powered up or powered down,

depending on the input signal or lack of input signal

received by the MAX975/MAX977.

STO_ Considerations

The charge currents for the capacitor connected to

STO_ are on the order of 100nA. This necessitates cau-

tion in capacitor type selection and board layout.

Capacitor leakage currents must be less than 1nA to

prevent timing errors. Ceramic capacitors are available

in values up to 1µF, and are an excellent choice for this

application. If a larger capacitance value is needed,

use parallel ceramic capacitors to get the required

capacitance. Aluminum and tantalum electrolytic

capacitors are not recommended due to their higher

leakage currents.

Board layout can create timing errors due to parasitic

effects. Make the STO_ traces as short as possible to

reduce capacitance and coupling effects. When driving

STO_ to disable auto-standby mode, use standard

CMOS logic isolated with a low-leakage (<1nA) diode,

such as National’s FJT1100 (Figure 3). 15nA leakage

typically results in 10% error.

The MAX977 has separate timing inputs (STOA and

STOB). These pins must have separate capacitors. The

timing circuits will not operate correctly if a single

capacitor is used with STOA and STOB connected

together.

The relationship between the timeout period and the

is in pF. This equation is for larger capacitance values,

and does not take into account variations due to board

capacitance and board leakage. If less than 1ms is

desired, subtract the ~3pF STO_ parasitic capacitance

from the calculated value.

Circuit Layout and Bypassing

The MAX975/MAX977’s high gain bandwidth requires

design precautions to realize the comparator’s full high-

speed capability. The following precautions are recom-

mended:

1) Use a printed circuit board with an unbroken, low-

inductance ground plane.

2) Place a decoupling capacitor (a 0.1µF ceramic

sible.

3) Keep lead lengths short on the inputs and outputs, to

avoid unwanted parasitic feedback around the com-

parators.

4) Solder the devices directly to the printed circuit

board instead of using a socket.

5) Minimize input impedance.

6) For slowly varying inputs, use a small capacitor

(~1000pF) across the inputs to improve stability.

IR Receiver

Figure 4 shows an application using the MAX975 as an

infrared receiver. The infrared photodiode creates a

current relative to the amount of infrared light present.

voltage level crosses the voltage applied by the voltage

divider to the inverting input, the output transitions. If

the photodiode is not receiving enough signal to cause

transitions on the MAX975’s output, STAT is used as a

loss-of-signal indicator. R3 adds additional hysteresis

for noise immunity.

Single/Dual, +3V/+5V Dual-Speed

Comparators with Auto-Standby

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13

STO_

CMOS

LOGIC

Figure 3. Driving STO

GND

STAT

R1

R2

R3

OUT

LOSS OF SIGNAL

MAX975

Figure 4. IR Receiver