Copyright © 2005 ADD Microtech Corp.
8
AMC2596_A (LF) May 2005
AMC2596
APPLICATION INFORMATION (continued)
Inductor Selection
The AMC2596 can be used for either continuous or discontinuous modes of operation. Each mode has distinctively
different operating characteristics, which can affect the regulator performance and requirements.
With relatively heavy load currents, the circuit operates in the continuous mode (inductor current always flowing), but
under light load conditions, the circuit will be forced to the discontinuous mode (inductor current falls to zero for a
period of time). For light loads (less than approximately 300 mA) it may be desirable to operate the regulator in the
discontinuous mode, primarily because of the lower inductor values required for the discontinuous mode.
Inductors are available in different styles such as pot core, toroid, E-frame, bobbin core, et., as well as different core
materials, such as ferrites and powdered iron. The least expensive, the bobbin core type, consists of wire wrapped on a
ferrite rod core. This type of construction makes for an inexpensive inductor, but since the magnetic flux is not
completely contained within the core, it generates more electromagnetic interference (EMI). This EMI can cause
problems in sensitive circuits, or can give incorrect scope readings because of induced voltages in the scope probe.
An inductor should not be operated beyond its maximum rated current because it may saturate. When an inductor
begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the DC resistance
of the winding). This will cause the switch current to rise very rapidly. Different inductor types have different
saturation characteristics, and this should be well considered when selecting as inductor.
Catch Diode
This diode is required to provide a return path for the inductor current when the switch is off. It should be located
close to the AMC2596 using short leads and short printed circuit traces as possible. To satisfy the need of fast
switching speed and low forward voltage drop, Schottky diodes are widely used to provide the best efficiency,
especially in low output voltage switching regulators (less than 5V).
Besides, fast-Recovery, high-efficiency, or
ultra-fast recovery diodes are also suitable. But some types with an abrupt turn-off characteristic may cause instability
and EMI problems. A fast-recovery diode with soft recovery characteristics is a better choice.
Feedback Connection
For fixed output voltage version, the FB (feedback) pin must be connected to VOUT. For the adjustable version, it is
important to place the output voltage ratio resistors near AMC2596 as possible in order to minimize the noise
introduction.
ENABLE
It is required that the ENABLE must not be left open. For normal operation, connect this pin to a “LOW” voltage
(typically, below 1.6V). On the other hand, for standby mode, connect this pin with a “HIGH” voltage. This pin can
be safely pulled up to +VIN without a resistor in series with it.
Grounding
To maintain output voltage stability, the power ground connections must be low-impedance. For the 5-lead TO-220
and TO-263 style package, both the tab and pin 3 are ground and either connection may be used.
Heat Sink and Thermal Consideration
Although the AMC2596 requires only a small heat sink for most cases, the following thermal consideration is
important for all operation. With the package thermal resistances θJA and θJC, total power dissipation can be estimated
as follows:
PD = (VIN × IQ)+(VOUT / VIN)(ILOAD × VSAT);
When no heat sink is used, the junction temperature rise can be determined by the following:
∆TJ = PD × θJA;
With the ambient temperature, the actual junction temperature will be:
TJ = ∆TJ +TA ;
If the actual operating junction temperature is out of the safe operating junction temperature (typically 125°C), then a
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