Datasheet
LT3022/LT3022-1.2
LT3022-1.5/LT3022-1.8
13
3022fb
APPLICATIONS INFORMATION
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3022 F02
20
0
–20
–40
–60
–80
–100
0
4
8
10
2 6
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
3022 F03
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 3. Ceramic Capacitor Temperature Characteristics
Figure 2. Ceramic Capacitor DC Bias Characteristics
and are more suitable for use as the output capacitor at
fractionally increased cost. X5R and X7R dielectrics both
exhibit excellent voltage coefficient characteristics. X7R
works over a larger temperature range and exhibits better
temperature stability whereas X5R is less expensive and
is available in higher values. Figures 2 and 3 show voltage
coefficient and temperature coefficient comparisons
between Y5V and X5R material.
Voltage and temperature coefficients are not the only
sources of problems. 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. A ceramic capacitor produced Figure 4’s
trace in response to light tapping from a pencil. Similar
vibration induced behavior can masquerade as increased
output voltage noise.
1mV/DIV
1ms/DIV
3022 F04
V
OUT
= 1.3V
C
OUT
= 10µF
I
LOAD
= 0
Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor
No-Load/Light-Load Recovery
A possible transient load step that occurs is where the
output current changes from its maximum level to zero
current or a very small load current. The output voltage
responds by overshooting until the regulator lowers the
amount of current it delivers to the new level. The regulator
loop response time and the amount of output capacitance
control the amount of overshoot. Once the regulator has
decreased its output current, the current provided by
the resistor divider (which sets V
OUT
) is the only current
remaining to discharge the output capacitor from the level
to which it overshot. The amount of time it takes for the
output voltage to recover easily extends to milliseconds
with minimum divider current and many microfarads of
output capacitance.