LT1175
APPLICATIONS INFORMATION
GND
LT1175
+
3.8V
R1
A1
AC
FEEDFORWARD
PATH
C
F
20pF
R
C
0.5Ω
NEGATIVE DC
FEEDBACK
AT LIGHT
LOADS
Q3
R
N
0.3Ω. This condition can occur with ceramic or film
capacitors which often have an ESR under 0.1Ω. With
previous designs, 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 elimi-
nate this problem. C
F
is a conventional feedforward ca-
pacitor 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
unavoidable 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 con-
nections 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.
8
U
–
+
W
U
U
C
OUT
LOAD
ESR
R2
SENSE
Q2
OUT
OUTPUT
Q1
PARASITIC
COLLECTOR
RESISTANCE
POWER
TRANSISTOR
R
LIM
CURRENT LIMIT
SENSE RESISTOR
–V
IN
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 select-
ing 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 capacitors do not fail
1175fd
LINER [ LINEAR TECHNOLOGY ]
