LT1763 Datasheet, PDF (13/16 Pages) Linear Technology – 500mA, Low Noise, LDO Micropower Regulators

APPLICATIONS INFORMATION
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
0
X5R
–20
–40
–60
Y5V
–80
–100
0 2 4 6 8 10 12 14 16
DC BIAS VOLTAGE (V)
1763 F04
Figure 4. Ceramic Capacitor DC Bias Characteristics
40
20
0
X5R
–20
–40
Y5V
–60
–80 BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
–100
–50 –25 0 25 50 75
TEMPERATURE (°C)
100 125
1763 F05
Figure 5. Ceramic Capacitor Temperature Characteristics
LT1763-5
C_OUT = 10µF
C_BYP = 0.01µf
I_LOAD = 100mA
V_OUT
500µV/DIV
100ms/DIV
1763 F06
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
LT1763 Series
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics 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.
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 micro-
phone 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, especially when a ceramic capacitor is
used for noise bypassing. 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.
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
power dissipated by the device will be made up of two
components:
1. Output current multiplied by the input/output voltage
differential: (I_OUT)(V_IN – V_OUT), and
2. GND pin current multiplied by the input voltage:
(I_GND)(V_IN).
The GND pin current can be found by examining the GND
Pin Current curves in the Typical Performance Character-
istics. Power dissipation will be equal to the sum of the two
components listed above.
1763fa
13