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AS2887
3A Low Dropout Voltage Regulator Adjustable & Fixed Output, Fast Response
FEATURES
* Adjustable Output Down To 1.2V * Fixed Output Voltages 1.5, 2.5, 3.3, 5.0V * Output Current Of 3A * Low Dropout Voltage 1.1V Typ. * Extremely Tight Load And Line Regulation * Current & Thermal Limiting * Standard 3-Terminal Low Cost TO-220, TO-263 & TO-252 * Similar To Industry Standard LT1085/LT1585
APPLICATIONS
* Powering VGA & Sound Card * Power PCSupplies * SMPS Post-Regulator * High Efficiency "Green" Computer Systems * High Efficiency Linear Power Supplies * Portable Instrumentation * Constant Current Regulators * Adjustable Power Supplies * Battery charger
PRODUCT DESCRIPTION
The AS2887 are low power 3A adjustable and fixed voltage regulators that are very easy to use. It requires only 2 external resistors to set the output voltage for adjustable version. The AS2887 is designed for low voltage applications that offer lower dropout voltage and faster transient response. This device is an excellent choice for use in powering low voltage microprocessor that require a lower dropout, faster transient response to regulate from +2.5V to 3.8V supplies and as a post regulator for switching supplies applications. The AS2887 features low dropout of a maximum of 1.5 volts. The AS2887 offers full protection against over-current faults, reversed input polarity, reversed load insertion, and positive and negative transient voltage. On-Chip trimming adjusts the reference voltage to 1%. The IQ of this device flows into load which increases efficiency. The AS2887 are offered in a 3-pin TO-220, TO-263 & TO-252 packages compatible with other 3 terminal regulators. For a 8A low dropout regulator refer to the AS2880 data sheet.
ORDERING INFORMATION
TO-220 3-PIN
AS2887YU AS2887YU-X
DD PLASTIC 3-PIN
AS2887YT AS2887YT-X
TO-252 DPAK
AS2887YR AS2887YR-X
TO-263-3 (T)
PIN CONNECTIONS
TO-220-3 (U)
TO-252 (R)
X = Output Voltage (i.e. 1.5 for 1.5V, 2.5 for 2.5V etc.) Y = Output Tolerance, A for 1% Blank for 2% Consult with factory for other fixed output voltages
AS2887
1 2 3
AS2887
AS2887
1 2 3
1
2
3
ADJ/GND VOUT VIN
ADJ/GND VOUT VIN
Front View
ADJ/GND VOUT VIN
Top View
Front View
Rev. 10/11/00
AS2887
ABSOLUTE MAXIMUM RATINGS
Lead Temp. (Soldering, 10 Seconds) .............................. 300C Storage Temperature Range ............................ -65 to +150C Operating Junction Temperature Range ...................... AS2887 Control Section ............................ -45C +125C AS2887 Power Transistor...........................-45C +150C Input Voltage........................................................ 30V Input to Output Voltage Differential Max ............ 30V
ELECTRICAL CHARACTERISTICS (NOTE 1) at IOUT = 10mA, TA=25C, unless otherwise specified. AS2887A PARAMETER CONDITIONS Typ
Min Max
1.515 1.530 2.525 2.550 3.330 3.360 5.05 5.10
AS2887 Min Max
1.470 1.455 1.530 1.545
Units
V
1.5V Version Output Voltage (Note 2) 2.5V Version Output Voltage (Note 2) 3.3V Version Output Voltage (Note 2) 5.0V Version Output Voltage (Note 2) Adjustable Version Reference Voltage (VREF) All Voltage Options Min. Load Current (Note 3) Line Regulation (VREF(VIN))
AS2887-1.5V, 0 IOUT3A, 2.75VVIN 29V
1.5 1.5 2.5 2.5 3.3 3.3 5.0 5.0
1.485 1.470 2.475 2.450 3.270 3.240 4.95 4.90
AS2887-2.5V,0 IOUT3A, 3.5VVIN 29V
2.450 2.425
3.230 3.201 4.900 4.850
2.550 2.575
3.370 3.399 5.100 5.150
V
AS2887-3.3V, 0 IOUT3A, 4.75VVIN 29V
V
AS2887-5.0V, 0 IOUT3A, 5.5VVIN 29V
V
VIN 7V, P PMAX 1.5V (VIN -VOUT)(VIN-VOUT)MAX, 10mAIOUT3A 1.5V (VIN -VOUT) (VIN-VOUT)MAX 2.75VVIN 29V, IOUT=10mA, TJ=25C (Note 3) VIN 29V, IOUT=0mA, TJ=25C (Note 2)
1.250
1.238
1.263
1.225
1.270
V
5 0.005 0.005 0.05
10 0.2 0.2 0.3
10 0.2 0.2 0.3
mA % % %
Load Regulation(VREF(IOUT))
10mAIOUT 3A, (VIN-VOUT)=3V, TJ=25C (Note 3) 0IOUT 3A, VIN=7V, TJ=25C (Note 2)
0.05 1.1
0.3 1.5
0.3 1.5
% V A
Dropout Voltage Current Limit IOUT(MAX) Long Term Stability Thermal Regulation (VOUT(Pwr)) Temperature Stability (VOUT(T)) Output Noise, RMS Thermal Resistance
VREF=1%
IOUT =3A (Note 3) IOUT 3A (Note 2)
VIN=7V 1.4V (VIN- VOUT) (Note3) TA=125C, 1000 Hrs. TA=25C, 20 ms pulse
4.0 0.3 (Note 2) 0.01 0.25
3.2 1 0.020
3.2 1 0.020 % %/W % % VO 3.0 60 3.0 60 3.0 60 3.0 60 C/W C/W
10Hz to 10kHz TO-220 DD Package
TA=25C Junction to Tab Junction to Ambient Junction to Tab Junction to Ambient
0.003 3.0 60 3.0 60
The Bold specifications apply to the full operating temperature range. Note 1: Changes in output voltage due to heating effects are covered under the specification for thermal regulation. Note 2: Fixed Version Only Note 3: Adjustable Version Only
Rev. 10/11/00
AS2887
APPLICATION HINTS
The AS2887 incorporates protection against over-current faults, reversed load insertion, over temperature operation, and positive and negative transient voltage. However, the use of an output capacitor is required in order to insure the stability and the performances. Stability The output capacitor is part of the regulator's frequency compensation system. Either a 220F aluminum electrolytic capacitor or a 47F solid tantalum capacitor between the output terminal and ground guarantees stable operation for all operating conditions. However, in order to minimize overshoot and undershoot, and therefore optimize the design, please refer to the section `Ripple Rejection'. Ripple Rejection Ripple rejection can be improved by adding a capacitor between the ADJ pin and ground. When ADJ pin bypassing is used, the value of the output capacitor required increases to its maximum (220F for an aluminum electrolytic capacitor, or 47F for a solid tantalum capacitor). If the ADJ pin is not bypass, the value of the output capacitor can be lowered to 100F for an electrolytic aluminum capacitor or 15F for a solid tantalum capacitor. However the value of the ADJ-bypass capacitor should be chosen with respect to the following equation: C = 1 / ( 6.28 * FR * R1 ) Where C = value of the capacitor in Farads (select an equal or larger standard value), FR = ripple frequency in Hz, R1 = value of resistor R1 in Ohms. If an ADJ-bypass capacitor is use, the amplitude of the output ripple will be independent of the output voltage. If an ADJbypass capacitor is not used, the output ripple will be proportional to the ratio of the output voltage to the reference voltage: M = VOUT / VREF Where M = multiplier for the ripple seen when the ADJ pin is optimally bypassed. VREF = Reference Voltage Reducing parasitic resistance and inductance One solution to minimize parasitic resistance and inductance is to connect in parallel capacitors. This arrangement will improve the transient response of the power supply if your system requires rapidly changing current load condition. Thermal Consideration Although the AS2887 offers some limiting circuitry for overload conditions, it is necessary not to exceed the maximum junction temperature, and therefore to be careful about thermal resistance. The heat flow will follow the lowest resistance path, which is the Junction-to-case thermal resistance. In order to insure the best thermal flow of the component, a proper mounting is required. Note that the case of the device is electrically connected to the output. In case the case has to be electrically isolated, a thermally conductive spacer can be used. However do not forget to consider its contribution to thermal resistance. Assuming: VIN = 10V, VOUT = 5V, IOUT = 1.5A, TA = 50C Heatsink = 6C/W, Case-Heatsink = 0.5C/W, JC = 3C/W Power dissipation under this condition PD = (VIN - VOUT) * IOUT = 7.5W Junction Temperature TJ = TA + PD * (Case - HS + HS + JC) For the Control Sections TJ = 50 + 7.5*(0.5 +6=3) = 121.25C 121.25C < TJ(max) for the control section. In both case reliable operation is insured by adequate junction temperature.
Rev. 10/11/00
AS2887
Basic Adjustable Regulator
VIN
AS2887
VREF IADJ 50A R2 VOUT = VREF * ( 1 + R /R1) + IADJ * R2 2 R1
VOUT
Fig.2 Basic Adjustable Regulator
Output Voltage Consider Figure 2. The resistance R1 generates a constant current flow, normally the specified load current of 10mA. This current will go through the resistance R2 to set the overall output voltage. The current IADJ is very small and constant. Therefore its contribution to the overall output voltage is very small and can generally be ignored Load Regulation Parasitic line resistance can degrade load regulation. In order not to affect the behavior of the regulator, it is best to connect directly the R1 resistance from the resistor divider to the case, and not to the load. For the same reason, it is best to connect the resistor R2 to the Negative side of the load.
VIN
R P Parasitic Line Resistance
AS2887
Connect R 1 to Case of Regulator
R1
RL R2
Connect R 2 to Load
Fig.3 Basic Adjustable Regulator
Rev. 10/11/00
AS2887
TYPICAL APPLICATIONS
VIN C1
IN
AS2887
ADJ
OUT R1
VIN C1
IN
AS2887
ADJ
OUT C2 R1
VOUT
LOAD
VOUT = VREF (1 + R2 ) + IADJ R2 R1
R2
Fig. 4 3A Current output Regulator
Fig. 5 Typical Adjustable Regulator
(Note A) VIN + 10F
IN
AS2887
ADJ
5V
OUT
VOUT
R1 121 1% 150F
VIN (Note A)
IN
AS2887
ADJ
OUT
5V 121 1% + 100F
+
10F 1k 2N3904 1k 365 1%
*C 1 improves ripple rejection. Xc should be ~ R 1 at ripple frequency.
R2 365 1%
TTL Input
+
C1 10F*
Note A: V IN(MIN) = (Intended V OUT) + (VDROPOUT (MAX)) Note A: VIN(MIN)= (Intended V ) + (VDROPOUT (MAX)) OUT
Fig. 6
Improving Ripple Rejection
Fig.7 5V Regulator with Shutdown
Rev. 10/11/00
AS2887
TYPICAL CHARACTERISTICS
Rev. 10/11/00

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