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RC5053
PRODUCT SPECIFICATION
12
P
Peak inductor current at 14.2A load:
The ripple current should generally be between 10% and
40% of the output current. The inductor must be able to with-
stand this peak current without saturating, and the copper
resistance in the winding should be kept as low as possible to
minimize resistive power loss. Note that in noncurrent lim-
ited circuits, the current in the inductor may rise above this
maximum under short circuit or fault conditions; the inductor
should be sized accordingly to withstand this additional
current. Inductors with gradual saturation characteristics are
often the best choice.
Input and Output Capacitors
A typical RC5053 design puts significant demands on both
the input and the output capacitors. During constant load
operation, a buck converter like the RC5053 draws square
waves of current from the input supply at the switching
frequency. The peak current value is equal to the output load
current plus 1/2 peak-to-peak ripple current, and the mini-
mum value is zero. Most of this current is supplied by the
input bypass capacitor. The resulting RMS current flow in the
input capacitor will heat it up, causing premature capacitor
failure in extreme cases. Maximum RMS current occurs with
50% PWM duty cycle, giving an RMS current value equal to
I
OUT
/2. A low ESR input capacitor with an adequate ripple
current rating must be used to ensure reliable operation.
Note that capacitor manufacturers’ ripple current ratings are
often based on only 2000 hours (three months) lifetime at
rated temperature. Further derating of the input capacitor
ripple current beyond the manufacturer’s specification is
recommended to extend the useful life of the circuit.
Lower operating temperature will have the largest effect
on capacitor longevity.
The output capacitor in a buck converter sees much less rip-
ple current under steady-state conditions than the input
capacitor. Peak-to-peak current is equal to that in the induc-
tor, usually 10% to 40% of the total load current. Output
capacitor duty places a premium not on power dissipation but
on ESR. During an output load transient, the output capacitor
must supply all of the additional load current demanded by
the load until the RC5053 can adjust the inductor current to
the new value. Output capacitor ESR results in a step in the
output voltage equal to the ESR value multiplied by the
change in load current. An 11A load step with a 0.05
ESR
output capacitor will result in a 550mV output voltage
shift; this is 27.5% of the output voltage for a 2.0V supply!
Because of the strong relationship between output capacitor
ESR and output load transient response, the output capacitor
is usually chosen for ESR, not for capacitance value;
a capacitor with suitable ESR will usually have a larger
capacitance value than is needed for energy storage.
14.2A
2
+
15.2A
=
Electrolytic capacitors rated for use in switching power
supplies with specified ripple current ratings and ESR can be
used effectively in RC5053 applications. OS-CON electro-
lytic capacitors from SANYO and other manufacturers give
excellent performance and have a very high performance/size
ratio for electrolytic capacitors. Surface mount applications
can use either electrolytic or dry tantalum capacitors.
Tantalum capacitors must be surge tested and specified for
use in switching power supplies. Low cost, generic tantalums
are known to have very short lives followed by explosive
deaths in switching power supply applications. AVX TPS
series surface mount devices are popular surge tested tanta-
lum capacitors that work well in RC5053 applications.
A common way to lower ESR and raise ripple current is to
parallel several capacitors. A typical RC5053 application
might exhibit 5A input ripple current. SANYO OS-CON part
number 10SA220M (220
μ
F/10V) capacitors feature 2.3A
allowable ripple current at 85
°
C; three in parallel at the input
(to withstand the input ripple current) will meet the above
requirements. Similarly, AVX TPSE337M006R0100
(330
μ
F/6V) have a rated maximum ESR of 0.1
; seven in
parallel will lower the net output capacitor ESR to 0.014
.
For low cost application, SANYO MV-GX series of capaci-
tors can be used with acceptable performance.
Feedback Loop Compensation
The RC5053 voltage feedback loop is compensated at the
COMP pin, attached to the output node of the internal gm
error amplifier. The feedback loop can generally be compen-
sated properly with an RC + C network from COMP to GND
as shown in Figure 8a.
Figure 8a. Compensation Pin Hook-Up
Loop stability is affected by the values of the inductor, output
capacitor, output capacitor ESR, error amplifier transconduc-
tance and error amplifier compensation network. The induc-
tor and the output capacitor creates a double pole at the
frequency:
DAC
RC5053
SENSE
COMP
10
R
C
C
C
C1
–
+
ERR
6
f
LC
)
C
OUT
2
π
L
O
(
(
)
--------------------1
=