G771 datasheet(7/15 Pages) GMT

7 / 15 page

Ver 0.2 Preliminary

Dec 11, 2001

TEL: 886-3-5788833

http://www.gmt.com.tw

G771

Global Mixed-mode Technology Inc.

Keep in mind that copper can't be used as an EMI

shield, and only ferrous materials such as steelwork

will. Placing a copper ground plane between the DXP-

DXN traces and traces carrying high-frequency noise

signals do not help reduce EMI.

PC Board Layout Checklist

Place the G771 close to a remote diode.

Keep traces away from high voltages (+12V bus).

Keep traces away from fast data buses and CRTs.

Use recommended trace widths and spacing.

Place a ground plane under the traces

Use guard traces flanking DXP and DXN and con-

necting to GND.

Route two DXPx-DXN pairs independently

Connect the common DXN as close as possible to

the DXN pin on IC.

Place the noise filter and the 0.1µF Vcc bypass

capacitors close to the G771.

Fig 2(a) Connect the common DXN as close as

possible to the DXN pin on IC.

Fig 2 (b) Recommended DXP/DXN PC

Twisted Pair and Shielded Cables

For remote-sensor distances longer than 8 in., or in

particularly noisy environments, a twisted pair is rec-

ommended. Its practical length is 6 feet to 12feet (typi-

cal) before noise becomes a problem, as tested in a

noisy electronics laboratory. For longer distances, the

best solution is a shielded twisted pair like that used

for audio microphones. Connect the twisted pair to

DXP and DXN and the shield to GND, and leave the

shield's remote end unterminated.

Excess capacitance at DX_limits practical remote

sensor distances (see Typical Operating Characteris-

tics), For very long cable runs, the cable's parasitic

capacitance often provides noise filtering, so the

2200pF capacitor can often be removed or reduced in

value. Cable resistance also affects remote-sensor

accuracy; 1

Ω series resistance introduces about + 1°C

error.

Low-Power Standby Mode

Standby mode disables the ADC and reduces the sup-

ply-current drain to less than 10µA. Enter standby

mode via the RUN/STOP bit in the configuration byte

ory, and the SMB interface is alive and listening for

reads and writes. This is valid for temperature sensor

only.

Standby mode is not a shutdown mode. With activity

on the SMBus, extra supply current is drawn (see

Typical

Operating

Characteristics).

software

standby mode, the G771 can be forced to perform

temperature measurement via the one-shot command,

despite the RUN/STOP bit being high.

Supply-current drain during the 125ms conversion

period is always about 500µA. Slowing down the con-

version rate reduces the average supply current (see

Typical Operating Characteristics). In between con-

versions, the instantaneous supply current is about

200µA due to the current consumed by the system

resetting circuit.

Reset Immunity Negative-Going VCC Transients

In addition to issuing a reset to the microprocessor (µP)

during power-up, power-down, and brownout condi-

tions, the G771 is relatively immune to short duration

negative-going VCC transients (glitches).

Typically, for the G771, a VCC transient that goes

100mV below the reset threshold and lasts 20µs or

less will not cause a reset pulse. A 0.1µF bypass ca-

pacitor mounted as close as possible to the VCC pin

provides additional transient immunity.

Ensuring a Valid Reset Output Down to VCC = 0V

When VCC falls below 1V, the G771 RESET output

no longer sinks current-it becomes an open circuit.

Therefore, high-impedance CMOS logic inputs con-

nected to RESET can drift to undetermined voltages.

This presents no problem in most applications, since

most µP and other circuitry is inoperative with VCC be-

low 1V. However, in applications where RESET

must be valid down to 0V, adding a pull-down resistor

to RESET causes any stray leakage currents to flow

to ground, holding RESET low (Figure 3). R1's value

is not critical; 100k

Ω is large enough not to load

RESET and small enough to pull RESET to ground.

DXP1

DXN

DXP2

DXP1

DXN

G771

DXP2

GND

Chip Boundary

GND

DXP1

DXN

DXP2

DXP1

DXN

G771

DXP2

GND

Chip Boundary

GND

DXP

DXN

GND

10 MILS

MINIMUM

10 MILS

GND

DXP

DXN

GND

10 MILS

MINIMUM

10 MILS

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