5
Date: 3/14/06 SPX2945 400mA Low Dropout Voltage Regulator with Shutdown ©Copyright 2006 Sipex Corporation
Error Detection Comparator Output
The Comparator produces a logic low output
whenever the SPX2945 output falls out of regu-
lation by more than around 5%. This is around
60mV offset divided by the 1.235 reference
voltage. This trip level remains 5% below
normal regardless of the programmed output
voltage of the regulator. Figure 1 shows the
timing diagram depicting the ERROR signal
and the regulator output voltage as the SPX2945
input is ramped up and down. The ERROR
signal becomes low at around 1.3V input, and
goes high around 5V input (input voltage at
which V
OUT
= 4.75). Since the SPX2945’s
dropout voltage is load dependent, the input
voltage trip point (around 5V) will vary with the
load current. The output voltage trip point
(approx. 4.75V) does not vary with load.
The error comparator has an open-collector out-
put, which requires an external pull-up resistor.
Depending on the system requirements the re-
sistor may be returned to 5V output or other
supply voltage. In determining the value of this
resistor, note that the output is rated to sink
400µA; this value adds to battery drain in a low
battery condition. Suggested values range from
100K to 1MΩ. If the output is unused this
resistor is not required.
Programming the Output Voltage of
SPX2945
The SPX2945 may be pin-strapped for 5V or
3.3V using its internal voltage divider by tying
Pin 1 (output) to Pin 2 (sense) and Pin 7 (feed-
back) to Pin 6 (5V/3.3V Tap).
Also, it may be programmed for any output
voltage between its 1.235V reference and its
20V maximum rating. As seen in Figure 2, an
external pair of resistors is required.
Refer to the below equation for the program-
ming of the output voltage::
V
OUT
= V
REF
x ( 1 + R
1
/ R
2
)+ I
FB
R
1
The V
REF
is 1.235 and I
FB
is the feedback bias
current, nominally -20nA. The minimum rec-
ommended load current of 1 µA forces an upper
limit of 1.2 MΩ on value of R
2
. If no load is
presented the I
FB
produces an error of typically
2% in V
OUT
, which may be eliminated at room
temperature by trimming R
1
. To improve the
accuracy choose the value of R
2
= 100k; this
reduces the error by 0.17% and increases the
resistor program current by 12µA. Since the
SPX2945 typically draws 60 µA at no load with
Pin 2 open-circuited this is a small price to pay.
Reducing Output Noise
It may be an advantage to reduce the AC noise
present at the output. One way is to reduce the
regulator bandwidth by increasing the size of
the output capacitor. This is the only way that
noise can be reduced on the 3 lead SPX2945 but
is relatively inefficient, as increasing the ca-
pacitor from 1µF to 220µF only decreases the
noise from 430µV to 160µV Vrms for a 100kHz
bandwidth at 5V output. Noise could also be
reduced fourfold by a bypass capacitor across
R
1
, since it reduces the high frequency gain from
4 to unity. Pick
C
BYPASS
≅1 / 2πR
1
x 200 Hz
or choose 0.01µF. When doing this, the output
capacitor must be increased to 3.3µF to main-
tain stability. These changes reduce the output
noise from 430µV to 100µV Vrms for a 100kHz
bandwidth at 5V output. With the bypass ca-
pacitor added, noise no longer scales with out-
put voltage so that improvements are more dra-
matic at higher output voltages.
+
+
+
+
4.75V
OUTPUT
VOLTAGE
ERROR*
_______
INPUT
VOLTAGE 1.3V
5.0V
* See Application Info.
Figure 1. ERROR Output Timing
