LMV7271
,
LMV7272
,
LMV7275
SNOSA56I –FEBRUARY 2003–REVISED SEPTEMBER 2015
www.ti.com
9 Power Supply Recommendations
To minimize supply noise, power supplies should be decoupled by a 0.01-μF ceramic capacitor in parallel with a
10-μF capacitor.
Due to the nanosecond edges on the output transition, peak supply currents will be drawn during the time the
output is transitioning. Peak current depends on the capacitive loading on the output. The output transition can
cause transients on poorly bypassed power supplies. These transients can cause a poorly bypassed power
supply to ring due to trace inductance and low self-resonance frequency of high ESR bypass capacitors.
Treat the LMV727x as a high-speed device. Keep the ground paths short and place small (low-ESR ceramic)
bypass capacitors directly between the V+ and V– pins.
Output capacitive loading and output toggle rate will cause the average supply current to rise over the quiescent
current.
10 Layout
10.1 Layout Guidelines
10.1.1 Circuit Techniques for Avoiding Oscillations in Comparator Applications
Feedback to almost any pin of a comparator can result in oscillation. In addition, when the input signal is a slow
voltage ramp or sine wave, the comparator may also burst into oscillation near the crossing point. To avoid
oscillation or instability, PCB layout should be engineered thoughtfully. Several precautions are recommended:
1. Power supply bypassing is critical, and will improve stability and transient response. Resistance and
inductance from power supply wires and board traces increase power supply line impedance. When supply
current changes, the power supply line will move due to its impedance. Large enough supply line shift will
cause the comparator to mis-operate. To avoid problems, a small bypass capacitor, such as 0.1-µF ceramic,
should be placed immediately adjacent to the supply pins. An additional 6.8 μF or greater tantalum capacitor
should be placed at the point where the power supply for the comparator is introduced onto the board. These
capacitors act as an energy reservoir and keep the supply impedance low. In a dual-supply application, a
0.1-μF capacitor is recommended to be placed across V+and V−pins.
2. Keep all leads short to reduce stray capacitance and lead inductance. It will also minimize any unwanted
coupling from any high-level signals (such as the output). The comparators can easily oscillate if the output
lead is inadvertently allowed to capacitively couple to the inputs through stray capacitance. This shows up
only during the output voltage transition intervals as the comparator changes states. Try to avoid a long loop
which could act as an inductor (coil).
3. It is a good practice to use an unbroken ground plane on a printed-circuit-board to provide all components
with a low inductive ground connection. Make sure ground paths are low-impedance where heavier currents
are flowing to avoid ground level shift. Preferably there should be a ground plane under the component.
4. The output trace should be routed away from inputs. The ground plane should extend between the output
and inputs to act as a guard. This can be achieved by running a topside ground plane between the output
and inputs. A typical PCB layout is shown in Figure 43.
5. When the signal source is applied through a resistive network to one input of the comparator, it is usually
advantageous to connect the other input with a resistor with the same value, for both DC and AC
consideration. Input traces should be laid out symmetrically if possible.
6. All pins of any unused comparators should be tied to the negative supply.
10.1.2 DSBGA Light Sensitivity
Exposing the DSBGA device to direct sunlight will cause mis-operation of the device. Light sources such as
Halogen lamps can also affect electrical performance if brought near to the device. The wavelengths, which have
the most detrimental effect, are reds and infrareds. Be aware of internal light sources, such as keyboard or
display backlights, that may pass through a PCB. A copper plane should be placed on a lower layer under the
DSBGA to block light. Be careful using vias under the device, as they may pass light.
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