Datasheet

LTC3375
28
3375fc
For more information www.linear.com/3375
applicaTions inForMaTion
V
CC
Shunt Regulator
If load steps seen on V
CC
are of great concern, then the
compensation capacitor should be tied from V
CC
to ground
as shown in Figure 6a. If load steps are not of a concern,
but instead smaller compensation components are desired
then the compensation capacitor should be tied from V
SHNT
to ground as shown in Figure 6b.
For an NPN device the pullup resistor between V
SHNT
and
the supply voltage should be sized such that:
R
PULLUP
<
V
SUPPLY(MINIMUM)
−(V
CC
+ V
BE
)
I
VCC(MAX)
•β
Where V
SUPPLY(MINIMUM)
is the lowest possible collector
voltage, V
BE
and β are specific to the NPN in the applica-
tion, and I
VCC(MAX)
is the maximum desired load current
from V
CC
.
Likewise R
PULLUP
may be sized such to current limit I
VCC
from an NPN device to prevent damage to the circuit
from a short on the V
CC
pin, and to prevent the NPN from
exceeding its safe operating current:
R
PULLUP
>
V
SUPPLY(MAXIMUM)
−(V
CC
+ V
BE
)
I
VCC(LIMIT)
•β
Where V
CC
= 0V in the case of a grounded output.
Alternatively, the current may be limited by adding a re-
sistor between V
CC
and the emitter of the NPN such that:
R
LIM
=
6.1V −(V
CC
+ V
BE
)
I
VCC(LIMIT)
In this case when I
VCC
exceeds I
VCC(LIMIT)
V
CC
will start to
collapse. The NPN should be sized to be able to survive
at least:
I
VCC(MAX)
=
6.1V − V
BE
R
LIM
for the given supply voltage, where 6.1V is the maximum
V
SHNT
voltage (typical).
The user should verify that the circuit is stable over the
specific conditions of the desired application. In general
increasing the value of the compensation capacitor used or
increasing R
PULLUP
can improve stability. The user should
keep in mind that increasing R
PULLUP
also decreases
I
VCC(MAX)
. In general the highest V
SUPPLY
at I
VCC(MAX)
yields the worst stability for the circuit in Figure 6a, while
the highest V
SUPPLY
at no load on V
CC
yields the worst
stability for the circuit in Figure 6b.
+
–
FBV
CC
1.02M
V
CC
V
SHNT
3375 F06a
1.2V
V
CC
REGULATOR
301k
1Ω
22µF
576k
+
–
FBV
CC
1.02M
V
CC
V
SHNT
3375 F06b
1.2V
V
CC
REGULATOR
301k
576k
2.2µF
Figure 6a. V
CC
Regulator Compensated from the V
CC
Pin
Figure 6b. V
CC
Regulator Compensated from the V
SHNT
Pin
The exact components used in the V
CC
shunt regulator are
dependent on the specific conditions used in the applica-
tion. Care should be taken to make sure that the power
dissipation limits of the specific N-type device used are
not exceeded, because damage to the external device can
lead to damage to the L
TC3375.