AME5286 Datasheet, PDF (10/20 Pages) Analog Microelectronics – Rectified Step-Down Converter

AME
AME5286
3A, 300KHz ~ 2MHz Synchronous
Rectified Step-Down Converter
Efficiecny Considerations
Although all dissipative elements in the circuit produce
losses, one major source usually account for most of the
losses in AME5286 circuits: I₂R losses. The I₂R loss
dominates the efficiency loss at medium to high load
currents.
The I₂R losses are calculated from the resistances of
the internal switches, R_SW, and external inductor R_L. In
continuous mode, the average output current flowing
through inductor L is "chopped" between the main switch
and the synchronous switch. Thus the series resistance
looking into the SW pin is a function of both top and
bottom MOSFET R_DS(ON) and the duty cycle (D) as fol-
lows:
R_SW = (R_DS(ON)TOP)(D) + (R_DS(ON)BOTTOM)(1-D)
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from Electrical Characteristics table. Thus,
to obtained I₂R losses, simply add R_SW to R_L and multiply
the result by the square of the average output current.
Other losses including C_IN and C_OUT ESR dissipative
losses and inductor core losses generally account for
less than 2% total additional loss.
Thermal Considerations
In most application the AME5286 does not dissipate
much heat due to its high efficiency. But, in applications
where the AME5286 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maxi-
mum junction temperature of the part. If the junction
temperature reaches approximately 160^oC, both power
switches will be turned off and the SW switch will be-
come high impedance.
10
Rev. A.02