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LT3085

3µA IQ, 20mA, 45V Low Dropout Fault Tolerant Linear Regulators

Linear Technology 

LT3007 Series

APPLICATIONS INFORMATION

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

can lead to significant shifts in output voltage. 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 LT3007 series does not need these

special considerations.

Output Capacitance and Transient Response

The LT3007 is stable with a wide range of output capaci-

tors. 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 LT3007 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

piezoelectric response. A piezoelectric device generates

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)

3007 F03

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

3007 F04

Figure 3. Ceramic Capacitor DC Bias Characteristics

Figure 4. Ceramic Capacitor Temperature Characteristics

For more information www.linear.com/LT3007

3007f

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