The AS1521 is a low power voltage regulator. This device is an excellent choice for use in battery-powered applications such as
cordless telephones, radio control systems, and portable computers. The AS1521 features very low quiescent current and very low
dropout voltage. This includes a tight initial tolerance of
±
1% max, and very low output temperature coefficient, making the AS1521
useful as a low-power voltage reference.
The error flag output feature is used as power-on reset for warning of a low output voltage, due to falling voltage input of batteries.
The AS1521 is offered in a SO-8, SOT-252, TO-223, TO-220 & TO-263 packages.
ORDERING INFORMATION
SO-8
AS1521S-X
TO-252
AS1521R-X
SOT-223
3-PIN
AS1521M3-X
TO-220
3-PIN
AS1521U-X
TO-263
3-PIN
AS1521T-X
Oper. Temp.
Range
-40°C to +125°C
X= Output Voltage (2.5V, 3.3V, 5.0V, or Blank for Adj.)
PIN CONNECTIONS
8-Pin Surface Mount (S)
V
OUT
SENSE/ ADJ
GND
ERROR
1
2
3
4
8
7
V
IN
GND
1
6
5
GND
SHDN
V
IN
GND V
OUT
SOT-223 (M3)
TO-263-3 (T)
TO-220-3 (U)
TO-252 (R)
3
2
1
V
IN
GND
V
OUT
AS1521
2
3
1
AS1521
2
3
AS1521
AS1521
1
2
3
Front View
V
OUT
GND V
IN
Top View
Top View
SHDN
V
OUT
Top View
V
OUT
GND V
IN
<0.25V
>2.8V
NC
OFF
ON
ON
Front View
AS1521
Rev. 10/11/00
AS1521
ABSOLUTE MAXIMUM RATINGS
Power Dissipation……………………...… Internally Limited
Lead Temp. (Soldering, 5 Seconds)…………….……. 260°C
Storage Temperature Range……………… -65°C to + 150°C
Operating Junction Temperature Range
AS1521……………………………..… -40°C to + 125°C
Maximum Input Supply Voltage...................................... +20V
Shutdown Input Voltage ..................................... -0.6V to 6.5V
ELECTRICAL CHARACTERISTICS
at Vs = ±15V, Ta = 25°C, unless otherwise specified.
Boldface
apply over the full
operating temperature range.
PARAMETER
2.5V Version
Output Voltage
Reverse Output Current
3.3V Version
Output Voltage
Reverse Output Current
5V Version
Output Voltage
Reverse Output Current
All Voltage Options
Output Voltage
Temperature Coefficient
Line Regulation ( Note 3)
Load Regulation ( Note 3 )
Dropout Voltage
( Note 5 )
20
(Note 1)
6V
≤
V
IN
≤20V
(Note 4)
I
L
= 1 to 300mA
I
L
= 0.1 to 1mA
I
L
= 1mA
I
L
= 300mA
Ground Current
I
L
= 1mA
I
L
= 10mA
I
L
= 50mA
I
L
= 100mA
I
L
= 300mA
V
OUT
= 0
V
IN
– V
OUT
1V (Avg), V
RIPPLE
= 0.5Vp-p
F
RIPPLE
= 120Hz, I
LOAD
= 0.3A, T
J
= 25°C
V
IN
= -20V, V
OUT
= 0V
1.5
0.04
0.13
0.42
100
350
1.5
4.0
7
200
58
50
1.0
100
10
0.20
0.30
0.16
0.25
0.55
0.70
150
500
2.5
7.0
14
500
ppm/°C
%max
%max
MV
1mA
≤I
L
≤
300mA
V
OUT
= 2.5V, V
IN
= 0V
CONDITIONS
(Note 2)
Typ.
AS1521
Min
Max
2.525
2.550
25
3.333
3.382
25
5.050
5.120
25
2.475
2.450
UNITS
2.5
2.5
16
3.3
3.3
16
5.0
5.0
16
V
µA
V
µA
V
µA
1mA
≤I
L
≤
300mA
V
OUT
= 3.3V, V
IN
= 0V
3.267
3.217
1mA
≤I
L
≤
300mA
V
OUT
= 5.0V, V
IN
= 0V
4.950
4.880
µA
mA
mA
dB
mA
Current Limit
Ripple Rejection
Input Reverse Leakage Current
Rev. 10/11/00
AS1521
Note 1:
Output or reference voltage temperature coefficients defined as the worst case voltage change divided by the total temperature range.
Note 2:
Unless otherwise specified all limits are guaranteed for T
j
= 25°C, V
IN
= 6V, I
L
= 100µA and C
L
= 1µF. Additional conditions for the 8-pin versions are
feedback tied to 5V tap and output tied to output sense (V
OUT
= 5V) and V
SHUTDOWN
≤
0.8V.
Note 3:
Regulation is measured at constant junction temperature, using pulse testing with a low duty cycle. Changes in output voltage due to heating effects are
covered under the specification for thermal regulation.
Note 4:
Line regulation for the AS1521 is tested at 150°C for I
L
= 1 mA. For I
L
= 100µA and T
J
= 125°C, line regulation is guaranteed by design to 0.2%. See typical
performance characteristics for line regulation versus temperature and load current.
Note 5:
Dropout voltage is defined as the input to output differential at which the output voltage drops 100 mV below its nominal value measured at 1V differential at
very low values of programmed output voltage, the minimum input supply voltage of 2V ( 2.3V over temperature) must be taken into account.
Note 6:
Comparator thresholds are expressed in terms of a voltage differential at the feedback terminal below the nominal reference voltage measured at 6V input. To
express these thresholds in terms of output voltage change, multiply by the error amplifier gain = V
OUT
/V
REF
= (R1 + R2)/R2. For example, at a programmed output
voltage of 5V, the Error output is guaranteed to go low when the output drops by 95 mV x 5V/1.235 = 384 mV. Thresholds remain constant as a percent of V
OUT
as
V
OUT
is varied, with the dropout warning occurring at typically 5% below nominal, 7.5% guaranteed.
APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the AS1521 requires a 2.2µF or greater capacitor
between output and ground. Oscillation could occur without this
capacitor. Most types of tantalum or aluminum electrolytic
works fine here. For operations of below -25°C solid tantalum is
recommended since the many aluminum types have electrolytes
the freeze at about -30°C. The ESR of about 5Ω or less and
resonant frequency above 500kHz are the most important
parameters in the value of the capacitor. The capacitor value can
be increased without limit.
At lower values of output current, less output capacitance is
required for stability. For the currents below 10mA the value of
the capacitor can be reduced to 0.5µF and 0.15µF for 1mA. More
output capacitance needed for the 8-pin version at voltages below
5V since it runs the error amplifier at lower gain. At worst case
5µF or greater must be used for the condition of 250mA load at
1.23V output.
The AS1521, unlike other low dropout regulators will remain
stable and in regulation with no load in addition to the internal
voltage divider.
This feature is especially important in
application like CMOS RAM keep-alive. When setting the output
voltage of the AS1521, a minimum load of 10mA is
recommended.
If there is more than 10 inches of wire between the input and the
AC filter capacitor or if a battery is used as the input then a 0.1µF
tantalum or aluminum electrolytic capacitor should be placed
from the input to the ground.
Instability can occur if there is stray capacitance to the AS1521
feedback terminal (pin 7). This could cause more problems when
using a higher value of external resistors to set the output voltage.
4 .7 5 V
O UTPUT
VOLTAGE
_______
ERROR*
This problem can be fixed by adding a 100pF capacitor between
output and feedback and increasing the output capacitor to at least
3.3µF.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the AS1521
output falls out of regulation 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 AS1521
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 = 4.75). Since the AS1521’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 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 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 AS1521
The AS1521 may be pin-strapped for 5V using its internal voltage
divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feedback)
to Pin 6 (5V Tap).
ERROR
SHUTDOWN
V
IN
8 4
V
OUT
5 1
AS1521
+
5 . 0 V
IN P U T
VOLTAGE
+
1 . 3 V
+
+
GND
3, 6, 7
2
ADJ
* S e e A p p lic a t io n In f o .
_______
F ig u r e 1 . E R R O R O u tp u t T im in g
Fig.2 Adjustable Voltage Regulator
Rev. 10/11/00
AS1521
Also, it may be programmed for any output voltage between its
1.235V reference and its 30V maximum rating. As seen in
Figure 2, an external pair of resistors is required.
Refer to the below equation for the programming of the output
voltage::
V
OUT
= V
REF
×
( 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 recommended 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 R2 = 100k this reduces the error by
0.17% and increases the resistor program current by 12µA. Since
the AS1521 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 AS1521 but is relatively inefficient, as increasing the
capacitor 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
×
200 Hz
or choose 0.01µF. When doing this, the output capacitor must be
increased to 3.3µF to maintain stability. These changes reduce the
output noise from 430µV to 100µV Vrms for a 100kHz bandwidth at
5V output. With the bypass capacitor added, noise no longer scales
with output voltage so that improvements are more dramatic at higher
output voltages.
HEAT SINK REQUIREMENTS
Depending on the maximum ambient temperature and maximum
power dissipation a heat sink may be required with the AS1521.
The junction temperature range has to be within the range
specified under Absolute Maximum Ratings under all possible
operating conditions. To find out if a heat sink is required, the
maximum power dissipation of the device needs to be calculated.
This is the maximum specific AC voltage that must be taken into
consideration at input. Figure 3 shows the condition and power
dissipation which should be calculated with the following
formula:
P
TOTAL
= (V
IN
- 5) I
L
+ (V
IN
)I
G
Next step is to calculate the temperature rise T
R
(max). T
J
(max)
maximum allowable junction temperature, T
A
(max) maximum
ambient temperature :
T
R
(max) = T
J
(max) - T
A
(max)
Junction to ambient thermal resistance
θ
(j-A)
can be calculated
after determining of P
TOTAL &
T
R
(max):
θ
(J-A)
= T
R
(max)/P
(max)
If the
θ
(J-A)
is 60°C/W or higher, the device could be operated
without a heat sink. If the value is below 60°C/W then the heat
sink is required and the thermal resistance of the heat sink can be
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