The AS2920 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 AS2920 features very low quiescent current (140µA Typ.)
and very low dropout voltage. This includes a tight initial tolerance of 1% max and very low output temperature coefficient, making
the AS2920 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 a falling voltage input of batteries.
Another feature is the logic-compatible shutdown input which enables the regulator to be switched on and off. The AS2920 is offered
in 3-pin and 5-pin TO-220 package, SO-8 (same pin out as AS2951), SOT-223, and surface mount TO-263 packages.
The regulator output voltage (of the 8-pin SO-8 and 5-pin TO-220 & TO-263) may be pin-strapped for a 2.5V, 3.3V and 5.0V or
programmed from 1.24V to 26V with an external pair of resistors..
ORDERING INFORMATION
TO-220
3-PIN
AS2920U3-X
TO-220
5-PIN
AS2920U51-X
TO-263
3-PIN
AS2920T3-X
TO-263
5-PIN
AS2920T51-X
SOT-223
3-PIN
AS2920M3-X
SOIC-8
AS2920S-X
OPER JUNCTION
TEMP RANGE
-40°C to 125°C
X= Output Voltage (3.3V, 2.5V, 5.0V, ADJ)
PIN CONNECTIONS
TO-220-5 Package
TO-263 Package
TO-220 Package
TO-263-5 Package
Five Lead Package Pin Functions:
SOT-223 Package
V
OUT
8-Pin Surface Mount
1
2
8
7
V
IN
FEEDBACK
5V/ 3.3V TAP
ERROR
AS2920
1 2 3 4 5
AS2920U51
AS2920T51
1)
ERROR
2)
V
IN
3)
GND
4)
V
OUT
5)
SHUTDOWN
AS2920U52
AS2920T52
ADJ
SHUTDOWN
GND
V
IN
V
OUT
AS2920
1 2 3 4 5
1
AS2920
2
3
AS2920
1 2 3
AS2920
1
2
3
SENSE
SHUTDOWN
AS2920
3
4
6
5
GND
V GND V
IN
OUT
Top View
V
IN
GND V
OUT
Top View
Top View
Front View
V
IN
GND V
OUT
Top View
Front View
Rev. 10/11/00
AS2920
ABSOLUTE MAXIMUM RATINGS
Power Dissipation..........................................Internally Limited
Lead Temp. (Soldering, 5 Seconds) ................................ 260°C
Storage Temperature Range .............................. -65° to +150°C
Operating Junction Temperature Range (Note 9)
AS2920 .................................................. -40C° to +125°C
ESD Rating ................................................................ 2KV Min
Input Supply Voltage .....................................+3.0V to +60V
Feedback Input Voltage ..................................-1.5V to +30V
Shutdown Input Voltage..................................-0.3V to +30V
Error Comparator Output ................................-0.3V to +30V
ESD Rating ............................................................ 2KV Min
ELECTRICAL CHARACTERISTICS
at V
IN
=V
OUT
±15V,T
A
=25°C, unless otherwise specified.
Boldface
applies over the
full operating temperature range.
PARAMETER
2.5V Version
Output Voltage
1mA
≤I
L
≤
400mA
3.3V Version
Output Voltage
1mA
≤I
L
≤
400mA
5V Version
Output Voltage
All Voltage Options
Output Voltage
Temperature Coefficient
Line Regulation ( Note 3)
Load Regulation ( Note 3 )
Dropout Voltage
( Note 5 )
Ground Current
1mA
≤I
L
≤
400mA
CONDITIONS
Typ.
(Note 2)
2.5
2.5
3.3
3.3
5.0
5.0
20
(Note 1)
6V
≤
V
IN
≤30V
(Note 4)
I
L
= 1 to 400mA
I
L
= 0.1 to 1mA
I
L
= 1mA
I
L
= 400mA
I
L
= 1mA
I
L
= 100mA
I
L
= 250mA
I
L
= 400mA
Current Limit
Thermal Regulation
Output Noise, 10Hz to 100kHz
I
L
= 100mA
V
OUT
= 0
0.05
400
260
Typ
1.235
0.03
0.04
60
375
140
1.3
5
13
AS2920
Min
Max
2.475
2.450
3.267
3.234
4.950
4.9
2.525
2.550
3.333
3.366
5.050
5.10
100
0.1
0.40
0.20
0.30
100
150
400
500
200
300
2
2.5
9
12
15
25
1000
1200
0.2
UNITS
V
V
V
ppm/°C
%max
%max
mV
µA
mA
mA
mA
mA
%/w
µV
Vrms
C
L
= 10µF
C
L
= 100µF
Adjustable Versions only
Reference Voltage
Reference Voltage
Feedback Pin Bias Current
Reference Voltage Temperature
Coefficient
Feedback Pin Bias Current
Temperature Coefficient
Over Temperature (Note 6)
AS2920
1.223
1.210
1.185
1.247
1.260
1.285
40
60
V
V
nA
ppm/°C
nA/°C
20
(Note 7)
50
0.1
Rev. 10/11/00
AS2920
(Continued)
PARAMETER
Error Comparator
Output Leakage Current
Output Low Voltage
Upper Threshold Voltage
Lower Threshold Voltage
Hysteresis
V
0H
= 30V
V
IN
= 4.5V
I
0L
= 400µA
(Note 8)
(Note 8)
(Note 8)
0.01
150
60
75
15
1.3
Low (Regulator ON)
High (Regulator OFF)
V
S
= 2.4V
V
S
= 26V
Regulator Output Current in Shutdown
0.7
2.0
30
450
3
10
20
50
100
600
750
µA
40
25
95
140
1.00
2.00
250
400
µA
mV
mV
mV
mV
V
CONDITIONS
(Note 2)
Typ.
Min
AS2920
Max
UNITS
Shutdown Input
Input logic Voltage
Shut down Pin Input Current
µA
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 AS2920 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:
V
REF
≤V
OUT
≤
(Vin - 1V), 2.3
≤Vin≤30V,
100µA≤I
L
≤
250 mA, T
J
≤
T
JMAX
.
Note 7:
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.
Note 8:
V
SHUTDOWN
≥
2V, V
IN
≤
30V, V
OUT
=0, Feedback pin tied to 5V Tap.
Note 9:
The junction -to-ambient thermal resistance of the TO-92 package is 180°C/ W with 0.4” leads and 160°C/ W with 0.25” leads to a PC board.
The thermal resistance of the 8-Pin DIP package is 105°C/W junction-to-ambient when soldered directly to a PC board. Junction-to-ambient thermal resistance for the
SOIC (S) package is 160°C/W.
UNREGULATED DC
5V @ 500mA
MAX
+
7
8
1
2
+
_
FROM
TTL
180k
Ω
..
ERROR
AMPLIFIER
6
330k
Ω
..
60k
Ω
..
3
+
60 mV
+ 1.23V
5
+
1 uF
TO CMOS OR
TTL
+
_
ERROR DETECTION
COMPARATOR
REFERENCE
4
AS2920 Block Diagram
Rev. 10/11/00
AS2920
APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the AS2920 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 AS2920, 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 AS2920, 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 AS2920
feedback terminal (pin 7). This could cause more problems when
using a higher value of external resistors to set the output voltage.
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 AS2920
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 AS2920
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 AS2920’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 AS2920
The AS2920 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).
4 .7 5 V
O U T PU T
V OL TA GE
_______
ERRO R*
+V
IN
100k
Shutdown
Input
OFF
ON
8
+V
IN
V
OUT
1
1.2V TO 26V
+
0.01uF
10uF
ERROR
AS2920
FB
3
SD
GND
4
ERROR
7
R
1
5
V
REF
R
2
OUTPUT
+
5 .0 V
IN P U T
V OL TA GE
+
1 .3 V
+
+
* S e e A p p lica tio n In fo .
_______
F ig u r e 1 . E R R O R O u tp u t T im in g
Rev. 10/11/00
AS2920
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 AS2920 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 AS2920 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 AS2920.
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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