Datasheet

LT6656
12
6656fc
For more information www.linear.com/LT6656
Output Settling
The output of the LT6656 is primarily designed to source
current into a load, but is capable of sinking current to
aid in output transient recovery. The output stage uses a
class B architecture to minimize quiescent current and
has a crossover dead band as the output transitions from
sourcing to sinking current.
APPLICATIONS INFORMATION
Figure 5. Output Response to 0.5V
P-P
Step on V
IN
, C
L
= 1µF, I
L
= 0
The settling time is typically less than 8ms for output loads
up to 5mA, however the time required to settle when the
load is turned off or in response to an input transient can be
significantly longer due to the dead band (shown in Figure
7). During this interval the output stage is neither sourcing
nor sinking current so the settling time is dominated by
the ability of the application circuit to discharge the output
capacitor to the voltage at which the sourcing circuitry
in the output stage reactivates. Larger load currents will
decrease the settling time and higher output capacitance
will increase the settling time.
In application circuits where the LT6656 is experiencing
a load step greater than 5µA, such as an ADC reference
and supply implementation, the settling time will typically
remain less than 8ms, regardless of the output settling
from a previous load step.
The settling time can be estimated by the following
equation:
Settling time≈
2(Deadband)(C
L
)
I
L
+ (V
OUT
)(0.8ms/V)
The deadband is ≈7mV for the 2.5V option, is proportional
to the voltage option (i.e., ≈14mV for the 5V option) and
can double due to variations in processing.
The graph in Figure 6 shows the settling time versus load
step with no load and with a constant 2µA load applied.
Note the settling time can be longer with load steps that
are not large enough to activate the sinking side of the
output stage.
2.7V
2.5V
V
OUT
2.3V
3.25V
V
IN
2.75V
5ms/DIV
6656 F05
Figure 6. Output Settling Time to 0.05% vs Load Step
LOAD STEP (mA)
OUTPUT SETTLING TIME (ms)
6656 F06
0.001 1
10
0.10.01
2.5V OPTION
V
IN
= 3V
C
L
= 1µF
∆I
L
= LOAD
STEP TO 2µA
∆I
L
= LOAD
STEP TO ZERO
∆I
L
= ZERO TO
LOAD STEP
30
25
20
15
10
5
0
2.52V
2.50V
V
OUT
2.48V
1mA
I
OUT
2mA
5ms/DIV
6656 F04
2.52V
2.50V
V
OUT
2.48V
0µA
I
OUT
100µA
5ms/DIV
6656 F03
Figure 3. Transient Response, 0µA to 100µA Load Step
(R2 = 24.9k, R1 = Open)
Figure 4. Transient Response, 1mA to 2mA Load Step
(R1 = R2 = 2.49k)