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LT3065 Series

3065fc

For more information www.linear.com/LT3065

applicaTions inForMaTion

During start-up, the internal reference soft-starts when

a REF/BYP capacitor is used. Regulator start-up time is

directly proportional to the size of the bypass capacitor

(see Start-Up Time vs REF/BYP Capacitor in the Typical

Performance Characteristics section). The reference

bypass capacitor is actively pulled low during shutdown

to reset the internal reference.

Using a feedforward capacitor also affects start-up time.

Start-up time is directly proportional to the size of the

feedforward capacitor and the output voltage, and is

inversely proportional to the feedback resistor divider cur-

rent, slowing to 15ms with a 10nF feedforward capacitor

and a 10μF output capacitor for an output voltage set to

5V by a 10μA feedback resistor divider.

Output Capacitance and Transient Response

The LT3065 regulator is stable with a wide range of output

capacitors.TheESRoftheoutputcapacitoraffectsstability,

mostnotablywithsmallcapacitors.Useaminimumoutput

capacitor of 3.3μF with an ESR of 1Ω or less to prevent

oscillations. For VOUT less than 1.2V, use a minimum COUT

of 4.7µF. If a feedforward capacitor is used with output

voltages set for greater than 24V, use a minimum output

capacitor of 10μF. The LT3065 is a micropower device

and output load transient response is a function of output

capacitance.Largervaluesofoutputcapacitancedecrease

the peak deviations and provide improved transient re-

sponseforlargerloadcurrentchanges.Bypasscapacitors,

used to decouple individual components powered by the

LT3065, increase the effective output capacitor value. For

applications with large load current transients, a low ESR

ceramiccapacitorinparallelwithabulktantalumcapacitor

often provides an optimally damped response.

Give extra consideration to the use of ceramic capacitors.

Manufacturers make ceramic capacitors with a variety of

dielectrics, each with different behavior across tempera-

ture and applied voltage. The most common dielectrics

are specified with EIA temperature characteristic codes

of 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, as

shown in Figures 6 and 7. When used with a 5V regulator,

a 16V 10μF Y5V capacitor can exhibit an effective value

Figure 6. Ceramic Capacitor DC Bias Characteristics

Figure 7. Ceramic Capacitor Temperature Characteristics

DC BIAS VOLTAGE (V)

3065 F06

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

3065 F07

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

as low as 1μF to 2μF for the DC bias voltage applied, and

over the operating temperature range. The X5R and X7R

dielectrics yield much more stable characteristics and are

more suitable for use as the output capacitor.

The X7R type works over a wider temperature range and

has better temperature stability, 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

toimproveascomponentcasesizeincreases,butexpected

capacitance at operating voltage should be verified.

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