Datasheet

LT3055
17
3055f
For more information www.linear.com/LT3055
Using a feedforward capacitor (C
FF
) from V
OUT
to the
ADJ pin has the added benefit of improving transient
response for output voltages greater than 0.6V. With no
feedforward capacitor, the settling time increases as the
output voltage increases above 0.6V. Use the equation in
Figure5 to determine the minimum value of C
FF
to achieve
a transient response that is similar to the 0.6V output
voltage performance regardless of the chosen output volt-
age (See Figure 6 and Transient Response in the Typical
Performance Characteristics section).
During start-up, the internal reference soft-starts when
a bypass capacitor is present. 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 by-
pass capacitor is actively pulled low during shutdown to
reset the internal reference.
Start-up time is also affected by the presence of a feed-
forward capacitor. Start-up time is directly proportional to
the size of the feedforward capacitor and the output volt-
age, and is inversely proportional to the feedback resistor
divider current, 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 LT3055 regulator 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 3.3μF with an ESR of 1Ω or less to prevent
oscillations. If a feedforward capacitor is used with output
voltages set for greater than 24V, use a minimum output
capacitor of 10μF. The LT3055 is a micropower device and
output load transient response is a function of output ca-
pacitance. Larger values of output capacitance decrease the
peak deviations and provide improved transient response
for larger load current changes. Bypass capacitors, used to
decouple individual components powered by the LT3055,
increase the effective output capacitor value. For applica-
tions with large load current transients, a low ESR ceramic
capacitor in parallel with a bulk tantalum capacitor 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 7 and 8. 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 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
APPLICATIONS INFORMATION
3055 F05
IN
SHDN
OUT
ADJ
GND REF/BYP
LT3055
V
IN
V
OUT
C
REF/BYP
C
FF
R2
R1
C
OUT
+
C
FF
≥
10nF
10µA
• I
FB _ DIVIDER
( )
I
FB _ DIVIDER
=
V
OUT
R1
+
R2
Figure 5. Feedforward Capacitor for Fast Transient Response
Figure 6. Transient Response vs Feedforward Capacitor
50µs/DIV
V
OUT
= 5V
C
OUT
= 10µF
I
FB-DIVIDER
= 10µA
0
1nF
10nF
LOAD CURRENT
500mA/DIV
FEEDFORWARD
CAPACITOR, C
FF
100pF
3055 F06
V
OUT
50mV/DIV