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LT3008 Series

3008fc

APPLICATIONS INFORMATION

DC BIAS VOLTAGE (V)

3008 F03

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

3008 F04

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

Figure 3. Ceramic Capacitor DC Bias Characteristics

Figure 4. Ceramic Capacitor Temperature Characteristics

Because the ADJ pin is relatively high impedance (de-

pending on the resistor divider used), stray capacitances

at this pin should be minimized. Special attention should

be given to any stray capacitances that can couple ex-

ternal signals onto the ADJ pin, producing undesirable

output transients or ripple.

Extra care should be taken in assembly when using high

valued resistors. Small amounts of board contamination

canleadtosigniﬁcantshiftsinoutputvoltage.Appropriate

post-assembly board cleaning measures should be

implemented to prevent board contamination. If the

board is to be subjected to humidity cycling or if board

cleaning measures cannot be guaranteed, consideration

should be given to using resistors an order of magnitude

smaller than in Table 1 to prevent contamination from

causing unwanted shifts in the output voltage. A ﬁxed

voltage option in the LT3008 series will not need these

special considerations.

Output Capacitance and Transient Response

The LT3008 is stable with a wide range of output capacitors.

The ESR of the output capacitor affects stability, most

notably with small capacitors. Use a minimum output

capacitor of 2.2μF with an ESR of 3Ω or less to prevent

oscillations. The LT3008 is a micropower device and output

load 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.

Give extra consideration to the use of ceramic capacitors.

Manufacturers make ceramic capacitors with a variety of

dielectrics, each with different behavior across temperature

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 3 and 4. When used with a 5V regulator,

a 16V 10μF Y5V capacitor 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 yield 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.

One must still exercise care 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.

Voltage and temperature coefﬁcients are not the only

sources of problems. Some ceramic capacitors have a

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