LT6657
applicaTions inForMaTion
LT6657 Sinking Current Dropout Description
The LT6657 output stage can source and sink current
of equal magnitude. When sourcing current it performs
as a conventional low dropout regulating device. When
sinking current, it can maintain a regulated output with
an input voltage equal to, more positive than, or also
slightly less than the output voltage. The specification
for dropout voltage while sinking is expressed in negative
2
3
IN
I
OUT
6
LT6657
OUT
+V
–V
OUT
SHDN
C
OUT
1µF
GND
4
I
R
SHUNT
SHUNT
I
OUT
–V
DD
OUT
6657 F13
Figure 13. Negative Shunt Mode Operation
voltage values for V – V . A typical unit will maintain
IN
OUT
The I
current has to be operated above 2.5mA to
obtain the same performance as the series mode opera-
tion. In shunt mode operation I is less than or
a regulated output voltage while sinking current with an
inputvoltage250mV(50mVguaranteed)belowtheoutput
voltage. Lower input voltage will cause the output to drop
out of regulation. This allows shunt reference applications
where the output and input can be tied together and sink
current from the output to ground.
SHUNT
OUT_MAX
equal to 8.5mA. A C
output for stability.
of 1µF or more is required on the
OUT
Shutdown Mode
Positive or Negative Shunt Mode Operation
When the SHDN pin is pulled below 0.8V with respect to
ground, the LT6657 enters a low power state and turns the
output off. Quiescent current is typically 2µA. If SHDN is
set less than 0.4V the quiescent current drops to 0.01µA
typical. The SHDN pin turn-on threshold is 1.26V and it has
approximately 150mV hysteresis for the turn-off threshold.
The turn-on logic high voltage is 1.6V. Drive the SHDN
with either logic or an open-collector/drain with a pull-up
resistor. The resistor supplies the pull-up current to the
open-collector/drainlogic,normallyseveralmicroamperes,
plus the SHDN pin current, typically less than 5µA at 6V. If
In addition to the series mode operation, the LT6657 can
be operated in shunt mode. In this mode the reference is
wired as a two terminal circuit which can be used both
as a positive or a negative voltage reference, as shown in
Figures 12 and 13.
R
is chosen using the following formula:
VDD – VOUT
SHUNT
RSHUNT
=
ISHUNT _MAX
unused, connect the SHDN input pin to V .
where: I
= 2.5mA + I
IN
SHUNT_MAX
OUT_MAX
Power Dissipation
PowerdissipationforLT6657dependsonV ,loadcurrent
IN
R
SHUNT
2
V
and the package type. The MSOP-8 package has a thermal
IN
DD
I
OUT
6
LT6657
OUT
+V
OUT
resistance of θ = 273°C/W.
3
JA
SHDN
C
OUT
1µF
Althoughthemaximumjunctiontemperatureis150°C,for
best performance it is recommended to limit the change
in junction temperature as much as possible. The plot in
GND
4
6657 F12
Figure 12. Positive Shunt Mode Operation
6657fb
12
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Linear Systems [ Linear Systems ]
