AME5252 Datasheet, PDF (11/18 Pages) Analog Microelectronics – Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Converter

AME
AME5252
Dual Synchronous, 600mA, 1.5MHz
Step-Down DC/DC Converter
Inductor Core Selection
Once the value for L is known, the type of inductor
Several capacitors may also be paralleled to meet size
must be selected. High efficiency converters generally
or height requirements in the design. The selection of
cannot afford the core loss found in low cost powdered
COUT is determined by the effective series resistance
iron cores, forcing the use of more expensive ferrite or
(ESR) that is required to minimize voltage ripple and load
mollypermalloy cores. Actual core loss is independent of
step transients, as well as the amount of bulk capaci-
core size for a fixed inductor value but it is very depen-
tance that is necessary to ensure that the control loop is
dent on the inductance selected. As the inductance in-
stable. Loop stability can be checked by viewing the load
creases, core losses decrease. Unfortunately, increased
transient response as described in a later section.
inductance requires more turns of wire and therefore cop-
per losses will increase. Ferrite designs have very low
core losses and are preferred at high switching frequen-
The output ripple, V_OUT, is determined by :
cies, so design goals can concentrate on copper loss
and preventing saturation. Ferrite core material saturates
"hard", which means that inductance collapses abruptly
when the peak design current is exceeded. This result in
∆ VOUT
≤
∆
I_L

^ESR
+

1
8f ⋅C_OUT


an abrupt increase in inductor ripple current and conse-
quent output voltage ripple. Do not allow the core to satu-
rate! Different core materials and shapes will change the
size/current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don't radiate energy but generally cost
more than powdered iron core inductors with similar char-
acteristics. The choice of which style inductor to use
mainly depends on the price vs. size requirements and
any radiated field/EMI requirements.
The output ripple is highest at maximum input voltage
since IL increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and
RMS current handling requirements. Dry tantalum, spe-
cial polymer, aluminum electrolytic and ceramic capaci-
tors are all available in surface mount packages. Special
polymer capacitors offer very low ESR but have lower
capacitance density than other types. Tantalum capaci-
tors have the highest capacitance density but it is impor-
tant to only use types that have been surge tested for
CIN and COUT Selection
use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR but can be used
The input capacitance, CIN, is needed to filter the trap- C _{in cost-sensitive applications provided that consideration}
ezoidal current at the source of the top MOSFET. To pre- _{is given to ripple current ratings and long term reliability.}
vent large ripple voltage, a low ESR input capacitor sized _{Ceramic capacitors have excellent low ESR characteris-}
for the maximum RMS current should be used.RMS cur-
tics but can have a high voltage coefficient and audible
rent is given by :
piezoelectric effects. The high Q of ceramic capacitors
I_RMS
=
I_OUT
( max ) ⋅ ^VOUT
V_IN
⋅
V_{IN − 1}
V_OUT
with trace inductance can also lead to significant ringing
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
not offer much relief. Note that ripple current ratings from output. When a ceramic capacitor is used at the input
capacitor manufacturers are often based on only 2000 and the power is supplied by a wall adapter through long
hours of life which makes it advisable to further derate the wires, a load step at the output can induce ringing at the
capacitor, or choose a capacitor rated at a higher tem-
perature than required.
input, V_IN. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at V_IN large enough to damage the
part.
Rev.C.01
11