Datasheet

LT3029
13
3029fb
For more information www.linear.com/LT3029
applicaTions inForMaTion
Bypass Capacitance and Low Noise Performance
Using a bypass capacitor connected between a channel’s
BYP pin and its corresponding OUT pin significantly low
-
ers LT3029 output voltage noise, but is not required in
all applications. Linear Technology recommends a good
quality low leakage capacitor
. This capacitor bypasses the
regulator’s reference, providing a low frequency noise pole.
A 10nF bypass capacitor introduces a noise pole that de
-
creases output voltage noise to as low as 20µV
RMS
. Using
a bypass capacitor provides the added benefit of improving
transient response. With no bypass capacitor, and a 10µF
output capacitor, a 100mA to 500mA load step settles to
within 1% of its final value in approximately 100µs. With
the addition of a 10nF bypass capacitor and evaluating
the same load step, output voltage excursion stays within
1% (see Transient Response in the Typical Performance
Characteristics section). Using a bypass capacitor makes
regulator start-up time proportional to the value of the
bypass capacitor. For example, a 10nF bypass capacitor
and 10µF output capacitor slow start-up time to 7ms.
Output Capacitance and Transient Response
The LT3029 design is stable with a wide range of output
capacitors. The ESR of the output capacitor affects stabil
-
ity, most notably with small capacitors. Linear Technology
recommends a minimum output capacitor of 3.3µF with
an ESR of 3Ω, or less, to prevent oscillations. The LT3029
is a micropower device, and output transient response is
a function of output capacitance. Larger values of output
capacitance decrease the peak deviations and provide im
-
proved transient response for larger load current changes.
Ceramic capacitors require extra consideration. Manufac-
turers make ceramic capacitors with a variety of dielectrics,
each with different behavior across temperature and
applied voltage. The most common dielectrics specify
the EIA temperature characteristic codes of Z5U, Y5V
,
X5R and X7R. Z5U and Y5V dielectrics provide high C-V
products in a small package at low cost, but exhibit strong
voltage and temperature coefficients, as shown in Figures
2 and 3. When used with a 5V regulator, a 16V 10µF Y5V
capacitor can exhibit an effective value as low as 1µF to
2µF for the applied DC bias voltage and over the operat
-
ing temperature range. 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.
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3029 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
3029 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