LT1175I Linear Technology, LT1175I Datasheet - Page 8

• Manufacturer Part Number: LT1175I
• Description: 500mA Negative Low Dropout Micropower Regulator
• Manufacturer: Linear Technology

LT1175
0.3 . This condition can occur with ceramic or film
capacitors which often have an ESR under 0.1 . 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
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
unavoidable parasitic collector resistance of the power
transistor. Access to the node at the bottom of R
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
output capacitor and good loop stability is achieved even
with extremely low ESR in the output capacitor.
8
APPLICATIONS
FEEDFORWARD
LT1175
U
NEGATIVE DC
F
FEEDBACK
AT LIGHT
is a conventional feedforward ca-
LOADS
INFORMATION
PATH
U
AC
C
Q3
were in series with the
R
W
N
A1
+
–
C
20pF
F
+
U
3.8V
C
is the
Q2
C
Figure 3
is
–V
Q1
IN
R
R
0.5
R1
R2
LIM
C
The end result of all this attention to loop stability is that
the output capacitor used with the LT1175 can range in
value from 0.1 F to hundreds of microfarads, with an ESR
from 0 to 10 . 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
SENSE
GND
OUT
PARASITIC
COLLECTOR
RESISTANCE
POWER
TRANSISTOR
CURRENT LIMIT
SENSE RESISTOR
ESR
C
OUT
LOAD
OUTPUT
1175 F03