Datasheet

LT3050 Series
18
3050fa
Table 1 shows 1% resistor divider values for some common
output voltages with a resistor divider current of 5µA.
Table 1. Output Voltage Resistor Divider Valves
V
OUT
(V) R1 (kΩ) R2 (kΩ)
1.2 118 118
1.5 121 182
1.8 124 249
2.5 115 365
3124499
3.3 124 562
5115845
Bypass Capacitance, Output Voltage Noise and
Transient Response
The LT3050 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
(C
REF/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 C
REF/BYP,
the output voltage noise decreases to as low as 30µV
RMS
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
C
REF/BYP
by itself.
To lower the output voltage noise for higher output voltages,
include a feedforward capacitor (C
FF
) from V
OUT
to the
ADJ pin. A good quality, low leakage capacitor is recom-
mended. This capacitor will bypass the error amplifi er of
the regulator, providing a low frequency noise pole. With
the use of 10nF for both C
FF
and C
REF/BYP
, output voltage
noise decreases to 30µV
RMS
when the output voltage is
set to 5V by a 5µA feedback resistor divider. If the current
in the feedback resistor divider is doubled, C
FF
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 test-
ing. Crosstalk from nearby traces induces unwanted noise
onto the LT3050’s output. Power supply ripple rejection
must also be considered. The LT3050 regulator does not
have unlimited power supply rejection and passes a small
portion of the input noise through to the output.
APPLICATIONS INFORMATION
Using a feedforward capacitor (C
FF
) from V
OUT
to the ADJ
pin has the added benefi t 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 C
FF
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).
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 by-
pass capacitor is actively pulled low during shutdown to
reset the internal reference.
3050 F02
IN
SHDN
OUT
ADJ
GND REF/BYP
LT3050-5
V
IN
V
OUT
C
REF/BYP
C
FF
C
OUT
C
FF
10nF
10µA
• I
FB DIVIDER
()
I
FB DIVIDER
=
V
OUT
R1+R2
10µA FOR FIXED V
OUT
()
100µs/DIV
V
OUT
= 5V
C
OUT
= 10µF
I
FB-DIVIDER
= 10µA
0
1nF
10nF
LOAD CURRENT
100mA/DIV
FEEDFORWARD
CAPACITOR, C
FF
100pF
3050 F03
V
OUT
50mV/DIV
Figure 2. Feedforward Capacitor for Fast Transient Response
Figure 3. Transient Response vs Feedforward Capacitor