LT3060 Просмотр технического описания (PDF) - Linear Technology
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LT3060 Datasheet PDF : 20 Pages
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LT3060
APPLICATIONS INFORMATION
Bypass Capacitance, Output Voltage Noise and
Transient Response
The LT3060 regulator provides low output voltage noise
over the 10Hz to 100kHz bandwidth while operating at
full load with the addition of a reference bypass capacitor
(CREF/BYP) from the REF/BYP pin to GND. A good quality,
low leakage capacitor is recommended. This capacitor will
bypass the internal reference of the regulator, providing a
low frequency noise pole. With the use of 10nF for CREF/BYP,
the output voltage noise decreases to as low as 30μVRMS
when the output voltage is set for 0.6V. For higher output
voltages (generated by using a feedback resistor divider),
the output voltage noise gains up accordingly when using
CREF/BYP by itself.
To lower the output voltage noise for higher output voltages,
include a feedforward capacitor (CFF) from VOUT to the ADJ
pin. A good quality, low leakage capacitor is recommended.
This capacitor will bypass the error amplifier of the regulator,
providing a low frequency noise pole. With the use of 10nF
for both CFF and CREF/BYP, output voltage noise decreases
to 30μVRMS when the output voltage is set to 5V by a 5μA
feedback resistor divider. If the current in the feedback
resistor divider is doubled, CFF must also be doubled to
achieve equivalent noise performance.
Higher values of output voltage noise are often measured
if care is not exercised with regard to circuit layout and
testing. Crosstalk from nearby traces induces unwanted
noise onto the LT3060’s output. Power supply ripple
rejection must also be considered. The LT3060 regulator
does not have unlimited power supply rejection and passes
a small portion of the input noise through to the output.
Using a feedforward capacitor (CFF) from VOUT 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 will increase as the output
voltage is raised above 0.6V. Use the equation in Figure 2
to determine the minimum value of CFF to achieve a
transient response that is similar to 0.6V output voltage
performance regardless of the chosen output voltage
(See Figure 3 and Transient Response in the Typical Perf-
ormance Characteristics section).
12
During start-up, the internal reference will soft-start if a
reference bypass capacitor is present. Regulator start-
up time is directly proportional to the size of the bypass
capacitor, slowing to 6ms with a 10nF bypass capacitor
(See Start-up Time vs REF/BYP Capacitor in the Typical
Performance Characteristics section). The reference
bypass capacitor is actively drained during shutdown to
reset the internal reference soft-start.
IN
OUT
VOUT
VIN
LT3060
R2
CFF
COUT
SHDN ADJ
GND REF/BYP R1
CREF/BYP
3060 F02
( ) CFF
≥
4.7nF
5μA
•
IFB−DIVIDER
IFB−DIVIDER
=
VOUT
R1+ R2
Figure 2. Feedforward Capacitor for Fast Transient Response
VOUT = 5V
COUT = 10μF
0
IFB-DIVIDER = 5μA
100pF
1nF
10nF
LOAD CURRENT
100mA/DIV
100μs/DIV
3060 F03
Figure 3. Transient Response vs Feedforward Capacitor
Start-up time is also affected by the presence of a feedforward
capacitor. 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
current, slowing to 15ms with a 4.7nF feedforward capacitor
and a 10μF output capacitor for an output voltage set to
5V by a 5μA feedback resistor divider.
Output Capacitance
The LT3060 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 2.2μF with an ESR of 3Ω or less to prevent
oscillations. If a feedforward capacitor is used with output
3060f
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