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參數(shù)資料
| 型號(hào)： | LMC7225 |
| 文件頁數(shù)： | 8/14頁 |
| 文件大?。?/td> | 277K |
| 代理商： | LMC7225 |
Application Information
(Continued)
3.0 Shoot-Through Current
The shoot-through current is defined as the current surge,
above the quiescent supply current, between the positive
and negative supplies of a device. The current surge occurs
when the output of the device switches states. The shoot-
through current results in glitches in the supply voltages.
Usually, glitches in the supply lines are prevented by bypass
capacitors. When the glitches are minimal, the value of the
bypass capacitors can be reduced.
TL/H/12337–6
FIGURE 2. Circuit for Measurement of the
Shoot-Through Current
TL/H/12337–7
FIGURE 3. Measurement of the Shoot-Through Current
From Figure 3, the shoot-through current for the LMC7211
can be calculated to be 0.2 mA (typical), and the duration is
1
m
s. The values needed for the bypass capacitors can be
calculated as follows:
TL/H/12337–8
Area of
D
e
(/2
(1
m
s
c
200
m
A)
e
100 pC
The capacitor needs to supply 100 picocolumb. To avoid
large shifts in the comparator threshold due to changes in
the voltage level, the voltage drop at the bypass capacitor
should be limited to 100 mV or less.
The charge needed (100 picocolumb) and the allowable
voltage drop (100 mV) will give us the minimum capacitor
value required.
D
Q
e
C (
D
V)
C
e
D
Q/
D
V
e
100 picocolumb/100 mV
C
e
10
-10
/10
-1
e
10
-9
e
1 nF
e
0.001
m
F
10
-9
e
1 nF
e
0.001
m
F
The voltage drop of
E
100 mV will cause a threshold shift in
the comparator. This threshold shift will be reduced by the
power supply rejection ratio, (PSRR). The PSRR which is
applicable here is not the DC value of PSRR (
E
80 dB), but
a transient PSRR which will be usually about 20 dB–40 dB,
depending on the circuit and the speed of the transient. This
will result in an effective threshold shift of about 1 mV to
10 mV.
For precision and level sensing circuits, it is generally a
good goal to reduce the voltage delta on the power supply
to a value equal to or less than the hysteresis of the com-
parator circuit. If the above circuit was to be used with
50 mV of hysteresis, it would be reasonable to increase the
bypass capacitor to 0.01
m
F to reduce the voltage delta to
10 mV. Larger values may be useful for obtaining more ac-
curate and consistent switching.
Note that the switching current of the comparator can
spread to other parts of the board as noise. The bypass
capacitor reduces this noise. For low noise systems this
may be reason to make the capacitor larger.
For non-precision circuits, such as using a comparator to
determine if a push-button switch is on or off, it is often
cheaper and easier to use a larger value of hysteresis and a
small value or bypass capacitance. The low shoot-through
current of the LMC7211 can allow the use of smaller and
less expensive bypass capacitors in non-critical circuits.
4.0 Output Short Circuit Current
The LMC7211 has short circuit protection of 40 mA. Howev-
er, it is not designed to withstand continuous short circuits,
transient voltage or current spikes, or shorts to any voltage
beyond the supplies. A resistor in series with the output
should reduce the effect of shorts. For outputs which send
signals off PC boards additional protection devices, such as
diodes to the supply rails, and varistors may be used.
5.0 Hysteresis
If the input signal is very slow or very noisy, the comparator
output might trip several times as the input signal passes
through the threshold. Using positive feedback to add hys-
teresis to the switching can reduce or eliminate this prob-
lem. The positive feedback can be added by a high value
resistor (R
F
). This will result in two switching thresholds, one
for increasing signals and one for decreasing signals. A ca-
pacitor can be added across R
F
to increase the switching
speed and provide more short term hysteresis. This can re-
sult in greater noise immunity for the circuit.
See Figures 4, 5 and 6.
8
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