Datasheet

10
LT3020/LT3020-1.2/
LT3020-1.5/LT3020-1.8
3020fc
APPLICATIO S I FOR ATIO
WUUU
an output current change of 1mA to 100mA is typically
0.4mV at V
ADJ
= 200mV. At V
OUT
= 1.5V, load regulation is:
(1.5V/200mV) • (0.4mV) = 3mV
Output Capacitance and Transient Response
The LT3020’s design is stable with a wide range of output
capacitors, but is optimized for low ESR ceramic capaci-
tors. The output capacitor’s ESR affects stability, most
notably with small value capacitors. Use a minimum
output capacitor of 2.2µF with an ESR of 0.3Ω or less to
prevent oscillations. The LT3020 is a low voltage device,
and output load transient response is a function of output
capacitance. Larger values of output capacitance decrease
the peak deviations and provide improved transient re-
sponse for larger load current changes. For output capaci-
tor values greater than 20µF a small feedforward capacitor
with a value of 300pF across the upper divider resistor (R2
in Figure 1) is required.
Give extra consideration to the use of ceramic capacitors.
Manufacturers make ceramic capacitors with a variety of
dielectrics, each with a different behavior across tempera-
ture and applied voltage. The most common dielectrics are
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics
provide high C-V products in a small package at low cost,
but exhibit strong voltage and temperature coefficients.
The X5R and X7R dielectrics yield highly stable
characterisitics and are more suitable for use as the output
capacitor at fractionally increased cost. The X5R and X7R
dielectrics both exhibit excellent voltage coefficient char-
acteristics. The X7R type 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, simi-
lar 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 re-
sulting voltages produced can cause appreciable amounts
of noise. A ceramic capacitor produced Figure 4’s trace in
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3020 F02
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
3020 F03
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 2. Ceramic Capacitor DC Bias Characteristics
Figure 3. Ceramic Capacitor Temperature Characteristics
1ms/DIV 3020 F04
1mV/DIV
V
OUT
= 1.3V
C
OUT
= 10µF
I
LOAD
= 0
Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor