LT3008 Series
13
3008fc
APPLICATIONS INFORMATION
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3008 F03
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
3008 F04
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
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
can lead to signifi cant 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 fi xed
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 3 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
specifi ed 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 coeffi cients 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 signifi cant 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 verifi ed.
Voltage and temperature coeffi cients are not the only
sources of problems. Some ceramic capacitors have a
