AS2730/31/32/33
Adjustable Regulator Design
APPLICATION HINTS
AS2730/31are adjustable regulators and maybe programmed
for any value between 1.25V and 16V using two resistors.
The relation between the resistors is given by:
R1=R2 (VOUT/1.240 –1)
Resistors have a large value up to 1mΩ in order to reduce the
current consumption. This might be interesting in the case of
widely varying load currents.
The AS2730/31/32/33 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.
Thermal Consideration
Although the AS2730/31/32/33 offers 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. 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.
Error Flag
AS2731/33 features an error indicating either an over current
fault or a low input voltage. This flag pulls low when such a
problem occurs and may sink 10mA. It is inoperative during
thermal shutdown.
Enable Input
AS2731/32 features enable input allowing turning ON & OFF
the device. EN has been designed to be compatible with
TTL/CMOS logic. When the regulator is ON, the current
flowing through this pin is approximately 20µA.
Assuming:
VIN = 10V, VOUT = 5V, IOUT = 1.5A, TA = 50°C/W,
θHeatsink Case= 6°C/W, θHeatsink Case = 0.5°C/W, θ JC = 3°C/W
VOUT
VIN
AS2730
Power dissipation under this condition
PD = (VIN – VOUT) * IOUT = 7.5W
Junction Temperature
TJ = TA + PD * (θCase – HS + θ HS θ JC
)
Fig.1 Basic Fixed Output Regulator
For the Control Section
TJ = 50 + 7.5*(0.5 + 6=3) = 121.25°C
121.25°C < TJ (max) for the Control & Power Sections.
In both case reliable operation is insured by adequate junction
temperature.
VOUT
4
1
2
VIN
AS2732/33
3
Capacitor Requirements
R1
5
The output capacitor is needed for stability and to minimize
the output noise. The required value of the capacitor varies
with the load. However, a minimum value of 10µF Aluminum
will guarantee stability over load. A tantalum capacitor is
recommended for a fast load transient response
R2
V
OUT = VREF
(R /R ) ]
1 2
X [ 1 +
For best results, the total series resistance should be
small enough to pass the minimum regulator load current
If the power source has high AC impedance, a 0.1µF capacitor
between input & ground is recommended. This capacitor
should have good characteristics up to 250 kHz.
Fig. 2 Adjustable Output Voltage Regulator
Minimum Load Current
To ensure a proper behavior of the regulator at light load, a
minimum load of 10mA for AS2730/31/32/33 is required.
Rev. 10/17/00
SIPEX [ SIPEX CORPORATION ]
