LT1175
applicaTions inForMaTion
GND
LT1175
+
3.8V
R1
C
OUT
ESR
R2
Q2
SENSE
LOAD
A1
AC
FEEDFORWARD
PATH
NEGATIVE DC
FEEDBACK
AT LIGHT
LOADS
Q3
R
N
This condition can occur with ceramic or film capacitors
which often have an ESR under 0.1Ω. With previous de-
signs, the user was forced to add a real resistor in series
with the capacitor to guarantee loop stability. The LT1175
uses a unique AC feedforward technique to eliminate
this problem. C
F
is a conventional feedforward capacitor
often used in regulators to cancel the pole formed by the
output capacitor. It would normally be connected from the
regulated output node to the feedback node at the R1/R2
junction or to an internal node on the amplifier as shown.
In this case, however, the capacitor is connected to the
internal structure of the power transistor. R
C
is the unavoid-
able parasitic collector resistance of the power transistor.
Access to the node at the bottom of R
C
is available only
in monolithic structures where Kelvin connections can
be made to the NPN buried collector layer. The loop now
responds as if R
C
were in series with the output capacitor
and good loop stability is achieved even with extremely
low ESR in the output capacitor.
0
–
+
C
F
20pF
V
IN
OUTPUT
OUTPUT
R
C
0.5
PARASITIC
COLLECTOR
RESISTANCE
POWER
TRANSISTOR
Q1
R
LIM
CURRENT LIMIT
SENSE RESISTOR
1175 F03
Figure 3
The end result of all this attention to loop stability is that
the output capacitor used with the LT1175 can range in
value from 0.1µF to hundreds of microfarads, with an ESR
from 0Ω to 10Ω. This range allows the use of ceramic,
solid tantalum, aluminum and film capacitors over a wide
range of values.
The optimum output capacitor type for the LT1175 is
still solid tantalum, but there is considerable leeway in
selecting the exact unit. If large load current transients
are expected, larger capacitors with lower ESR may be
needed to control worst-case output variation during
transients. If transients are not an issue, the capacitor
can be chosen for small physical size, low price, etc.
Concerns about surge currents in tantalum capacitors are
not an issue for the output capacitor because the LT1175
limits inrush current to well below the level which can
cause capacitor damage. Surges caused by shorting the
regulator output are also not a problem because tantalum
1175fe
LINER [ LINEAR TECHNOLOGY ]
