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2945M3

400mA Low Dropout Voltage Regulator with Shutdown

Signal Processing Technologies 

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 = 4.75). Since the SPX2945’s

OUT

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 = V x ( 1 + R / R )+ I R

OUT

REF

12

FB 1

The VREF is 1.235 and IFB 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 R2. If no load is

presented the IFB produces an error of typically

2% in VOUT, which may be eliminated at room

temperature by trimming R1. To improve the

accuracy choose the value of R2 = 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

R1, since it reduces the high frequency gain from

4 to unity. Pick

C

BYPASS

≅1

/

2πR1

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

+5.0V

+1.3V

* See Application Info.

Figure 1. ERROR Output Timing

+

+

Rev. B Date:01/20/04

SPX2945 400mA Low Dropout Voltage Regulator with Shutdown

5

©Copyright 2004 Sipex Corporation

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