LT3029

13

3029fa

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

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 15ms.

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

improved 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)

3029 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

3029 F03

–25

0

50

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

Figure 2. Ceramic Capacitor DC Bias Characteristics

Figure 3. Ceramic Capacitor Temperature Characteristics