Datasheet

LTC6655
12
6655fd
For more information www.linear.com/LTC6655
3.5V
3V
V
IN
V
OUT
50mV/DIV
C
OUT
= 3.3µF
6655 F04
400µs/DIV
applicaTions inForMaTion
The choice of output capacitor also affects the bandwidth
of the reference circuitry and resultant noise peaking. As
shown in Figure 1, the bandwidth is inversely proportional
to the value of the output capacitor.
Noise peaking is related to the phase margin of the output
buffer. Higher peaking generally indicates lower phase mar
-
gin. Other factors affecting noise peaking are temperature,
input voltage, and output load current.
Start-Up and Load Transient Response
Results for the transient response plots (Figures 3 to 8)
were produced with the test circuit shown in Figure 2
unless otherwise indicated.
The turn-on time is slew limited and determined by the
short-circuit current, the output capacitor, and output
voltage as shown in the equation:
tV
C
I
ON OUT
OUT
SC
= •
For example, the LTC6655-2.5V, with a 3.3µF output
capacitor and a typical short-circuit current of 20mA, the
start-up time would be approximately:
25
33 10
002
412
6
.•
.•
.
–
V
F
A
µs
=
The resulting turn-on time is shown in Figure 3. Here
the output capacitor is 3.3µF and the input capacitor is
0.1µF.
Figure 4 shows the output response to a 500mV step on
V
IN
. The output response to a current step sourcing and
sinking is shown in Figures 5 and 6, respectively.
Figure 7 shows the output response as the current goes
from sourcing to sinking.
Shutdown Mode
The LTC6655 family of references can be shut down by
tying the SHDN pin to ground. There is an internal pull-up
resistor tied to this pin. If left unconnected this pin rises to
V
IN
and the part is enabled. Due to the low internal pull-up
current, it is recommended that the SHDN pin be pulled
high externally for normal operation to prevent accidental
LTC6655-2.5
100Ω
V
OUT
1,2
7
6
3,4,5,8
C
IN
0.1µF
C
OUT
3.3µF
V
GEN
6655 F02
0.5V
V
IN
3V
Figure 2. Transient Load Test Circuit
Figure 3. Start-Up Response
Figure 4. Output Response with a 500mV Step On V
IN
Figure 1. Output Voltage Noise Spectrum
V
IN
2V/DIV
V
OUT
1V/DIV
C
OUT
= 3.3µF
6655 F03
200µs/DIV
C
OUT
= 100µF
FREQUENCY (kHz)
60
NOISE VOLTAGE (nV√Hz)
100
0.01 10 100 1000
0
20
0.1 1
120
80
40
6655 F01
C
OUT
= 2.7µF
C
OUT
= 10µF