lt3015edd Linear Technology Corporation, lt3015edd Datasheet - Page 13

• Manufacturer Part Number: lt3015edd
• Description: 1.5a, Low Noise, Negative Linear Regulator With Precision Current Limit
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
can exhibit an effective value as low as 1μF to 2μF for the
DC bias voltage applied and 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 tem-
perature, while the X5R is less expensive and is available in
higher values. Care still must be exercised when using X5R
and X7R capacitors; the X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
Z5U capacitors, but can still be significant enough to drop
capacitor values below appropriate levels. Capacitor DC bias
characteristics tend to improve as component case size
increases, but expected capacitance at operating voltage
should be verified in situ for all applications.
Figure 4. Ceramic Capacitor Temperature Characteristics
Figure 3. Ceramic Capacitor DC Bias Characteristics
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
40
20
20
0
0
–50
0
2
–25
4
DC BIAS VOLTAGE (V)
0
TEMPERATURE (°C)
BOTH CAPACITORS ARE 16V,
BOTH CAPACITORS ARE 16V,
6
25
8
1210 CASE SIZE, 10μF
1210 CASE SIZE, 10μF
Y5V
X5R
50
10
Y5V
75
12
X5R
100
14
3015 F04
3015 F03
125
16
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 microphone works. For
a ceramic capacitor, the stress can be induced by vibra-
tions in the system or thermal transients. The resulting
voltages produced can cause appreciable amounts of
noise. A ceramic capacitor produced the trace in Figure 5
in response to light tapping from a pencil. Similar vibration
induced behavior can masquerade as increased output
voltage noise.
Overload Recovery
Like many IC power regulators, the LT3015 has safe oper-
ating area protection. The safe operating area protection
activates at IN-to-OUT differential voltages greater than
8V. The safe area protection decreases current limit as
the IN-to-OUT differential voltage increases and keeps
the power transistor inside a safe operating region for
all values of forward input-to-output voltage up to the
LT3015’s Absolute Maximum Ratings.
When power is first applied and input voltage rises, the
output follows the input and keeps the IN-to-OUT differential
voltage small, allowing the regulator to supply large output
currents and start-up into high current loads. With a high
input voltage, a problem can occur wherein removal of
an output short does not allow the output voltage to fully
recover. Other LTC negative linear regulators such as the
LT1175 and LT1964 also exhibit this phenomenon, so it
is not unique to the LT3015.
Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor
1mV/DIV
V
OUT
V
C
I
L
OUT
OUT
= 10μA
= –1.3V
= 10μF
1ms/DIV
LT3015
3015 F05
13
3015f