SPX2920_06 Datasheet, PDF(5/10 Page) Sipex Corporation – 400mA Low Drop Out Voltage Regulator with Shutdown

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SPX2920_06 Datasheet, PDF (5/10 Pages) Sipex Corporation – 400mA Low Drop Out Voltage Regulator with Shutdown

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Error Detection Comparator Output

The Comparator produces a logic low output

whenever the SPX2920 output falls out of

regulation by more than about 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 ER-

ROR signal and the regulator output voltage

as the SPX2920 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 VOUT = 4.75V).

Since the SPX2920âs dropout voltage is

load dependent, the input voltage trip point

(around 5V) will vary with the load current.

The output voltage trip point (approximately

4.75V) does not vary with load.

The error comparator has an open-col-

lector output, which requires an external

pull-up resistor. Depending on the system

requirements the resistor 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 400ÂµA; this

value adds to battery drain in a low battery

condition. Suggested values range from

100K to 1Mâ¦. If the output is unused this

resistor is not required.

Programming the Output Voltage of

SPX2920

The SPX2920 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 (feedback) 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 on the

front page, an external pair of resistors is

required.

recommended load current of 1ÂµA forces an

upper limit of 1.2Mâ¦ on the value of R2. If no

load is presented, the IFB produces an error

of typically 2% in VOUT, which may be elimi-

nated at room temperature by trimming R1.

To improve the accuracy choose the value of

R2 = 100kâ¦; this reduces the error by 0.17%

and increases the resistor program current

by 12ÂµA. Since the SPX2920 typically draws

60ÂµA 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 increas-

ing the size of the output capacitor. This

is the only way that noise can be reduced

on the 3 lead SPX2920 but it is relatively

inefï¬cient; as increasing the capacitor from

1ÂµF to 220ÂµF only decreases the noise

from 430ÂµV to 160ÂµV Vrms for a 100kHz

bandwidth at 5V output.

Noise could also be reduced fourfold by

a bypass capacitor across R1, since it re-

duces the high frequency gain from 4 to

unity. Pick

CBYPASS â1 / 2ÏR1 x 200 Hz

or choose 0.01ÂµF. When doing this, the

output capacitor must be increased to 3.3ÂµF

to maintain stability. These changes reduce

the output noise from 430ÂµV to 100ÂµV Vrms

for a 100kHz bandwidth at 5V output. With

the bypass capacitor added, noise no longer

scales with output voltage so that improve-

ments are more dramatic at higher output

4.75V

OUTPUT

VOLTAGE

_______

ERROR*

Refer to the equation below for the program-

ming of the output voltage:

VOUT = VREF x ( 1 + R1/ R2 )+ IFBR1

The VREF is 1.235 and IFB is the feedback

bias current, nominally -20nA. The minimum

INPUT

VOLTAGE

+5.0V

+1.3V

* See Application Info.

Figure 1. ERROR Output Timing

Date: 03/02/06

SPX2920 400mA Low Drop Out Voltage Regulator with Shutdown

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