150mA SOT-23 Ultra Low Noise CMOS RF-LDO™ Regulator
However, being aware of these current flows, there is also
another potential source of induced voltage noise from the
resistance inherent in the PCB trace. Figure 6 shows where
the additive resistance of the PCB can manifest itself. Again
these resistances may be very small, but a summation of
several currents can develop detectable voltage ripple and
will be amplified by the LDO. Particularly the accumulation
of current flows in the ground plane can develop significant
voltages unless care is taken. With a degree of care, the
ILC7082 will yield outstanding performance.
Printed Circuit Board Layout Guidelines
As was mentioned in the previous section, to take full
advantage of any high performance LDO regulator requires
paying careful attention to grounding and printed circuit
board (PCB) layout.
V
OUT
I
OUT
ESR
R
PCB
R
PCB
Figure 7: Effects of poor circuit layout
I
1
C
OUT
ESR
C
NOISE
5
SOT-23-5
4
ILC7082
Figure 8 shows an optimum schematic. In this schematic,
high output surge current has little effect on the ground cur-
rent and noise bypass current return of the LDO regulator.
Note that the key difference here is that C
OUT
and C
NOISE
are
directly connected to the LDO regulator’s ground pin. The
LDO is then separately connected to the main ground plane
and returned to a single point system ground.
The layout of the LDO and its external components are also
based on some simple rules to minimize EMI and output
voltage ripple.
V
IN
C
IN
R
PCB
1
2
R
PCB
3
ON
OFF
Figure 6: Inherent PCB resistance
Figure 7 shows the effects of poor grounding and PCB lay-
out magnified by the ESR and PCB resistances and the
accumulation of current flows.
Note thatparticularly during high output load current, the
LDO regulator’s ground pin and the ground return for C
OUT
and C
NOISE
are not at the same potential as the system
ground. This is due to high frequency impedance caused by
PCB’s trace inductance and DC resistance. The current
loop between C
OUT
, C
NOISE
and the LDO regulator’s ground
pin will degrade performance of the LDO.
Impala Linear Corporation
ILC7082 1.3
(408) 574-3939
www.impalalinear.com
April 1999
8
IMPALA [ Impala Linear Corporation ]
