LT6656 Linear Technology Corporation, LT6656 Datasheet - Page 8

• Manufacturer Part Number: LT6656
• Description: 1?a Precision Series Voltage Reference
• Manufacturer: Linear Technology Corporation

LT6656
applicaTions inForMaTion
Output Settling
The output of the LT6656 is primarily designed to source
current, but is capable of sinking current to aid in output
transient recovery. The output stage uses a class B archi-
tecture to minimize quiescent current, and has a typical
crossover dead band of 6mV as the output transitions
from sourcing to sinking current, and twice the deadband
as the output transitions from sinking back to sourcing
current.
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. Settling time is dominated by the
ability of the application circuit to discharge the output
capacitor. Larger load currents decrease settling time.
The settling time can be estimated by the following
equation:
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.
The photo in Figure 7 shows the output response to a 0.5V
input step in both a no-load and 5µA load condition. In
the no-load condition only the bias current of the internal
bandgap reference (about 400nA) is available to discharge
the output capacitor.

Figure 5. Output Response to 0.5V
V
Settling time
OUT
V
IN
3.25V
2.75V
2.7V
2.5V
2.3V
≈
2(Deadband)(C )
5ms/DIV
I
L
P-P
Step on V
L
+ 2ms
IN
, C
6656 F05
L
= 1µF , I
L
= 0
10mV/DIV
Output Noise
In general, output noise in the LT6656 is proportional to the
bandwidth of the output stage and therefore increases with
higher load current and lower output capacitance. However,
peaking in the noise response may be the dominant factor
in determining the output noise level. Noise peaking can
be reduced by increasing the size of the output capacitor
when driving heavier loads, or conversely, reducing the
size of the output capacitor when driving lighter loads.
Noise plots may be found in the Typical Performance
Characteristics section.
Internal Protection
The LT6656 incorporates several internal protection
features that make it ideal for use in battery powered
systems. Reverse input protection limits the input cur-
rent to typically less than 40µA when either the LT6656
V
OUT
Figure 7. Detailed Output Response to a 0.5V Input Step,
C
V
Figure 6. Output Settling Time to 0.05% vs Load Step
IN
IN
I
L
3.25V
2.75V
= 5µA
I
= C
L
= 0
L
30
25
20
15
10
0.001
= 1µF
5
0
V
C
IN
L
= 1µF
= 3V
0.01
LOAD STEP (mA)
5ms/DIV
∆I
STEP TO ZERO
0.1
∆I
STEP TO 2µA
L
= LOAD
L
= LOAD
∆I
LOAD STEP
L
= ZERO TO
1
6656 F06
10
6656 F07
6656f