Datasheet

9
LTC1844 Series
1844fa
APPLICATIONS INFORMATION
WUU
U
temperature and applied voltage. The most common di-
electrics used are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but exhibit strong voltage and tem-
perature coefficients as shown in Figures 4 and 5. When
used with a 5V regulator, a 10µF Y5V capacitor can exhibit
an effective value as low as 1µF to 2µF over the operating
temperature range. The X5R and X7R dielectrics result in
more stable characteristics and are more suitable for use
as the output capacitor. The X7R type has better stability
across temperature, while the X5R is less expensive and
is available in higher values.
Additionally, some ceramic capacitors have a piezoelectric
response. A piezoelectric device generates voltage across
its terminals due to mechanical stress, similar to the way
a piezoelectric accelerometer or microphone works. For a
ceramic capacitor the stress can be induced by vibrations
in the system or thermal transients. The resulting voltages
produced can cause appreciable amounts of noise, espe-
cially when a ceramic capacitor is used for noise bypass-
ing. A ceramic capacitor produced Figure 6’s trace in
response to light tapping from a pencil. Similar vibration-
induced behavior can masquerade as increased output
voltage noise.
Figure 4. Ceramic Capacitor DC Bias Characteristics
Figure 5. Ceramic Capacitor Temperature Characteristics
V
OUT
500µV/DIV
100ms/DIV
1844 F06
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
LTC1844-2.8
C
OUT
= 10µF
C
BYP
= 0.01µF
I
LOAD
= 100mA
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
1844 F04
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
1844 F05
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Dropout Recovery and Output Overshoot
If the input supply voltage drops too low for the LTC1844
to maintain regulation, the internal feedback loop goes
into dropout and the internal pass transistor turns fully on.
If the input supply then suddenly rises, the output may
briefly overshoot the intended output voltage while the
LTC1844 transitions back from dropout to normal opera-
tion. This behavior occurs when the input supply slew rate
is greater than 1V/ms and the output bypass capacitor is
small. If the input is expected to slew rapidly, an output
bypass capacitor of 10µF or greater should be used to
minimize output overshoot. Note that overshoot typically
does not occur at start-up since the feedback loop does
not spend a significant amount of time in dropout.