RT9511 RICHTEK [Richtek Technology Corporation], RT9511 Datasheet - Page 19

• Manufacturer Part Number: RT9511
• Description: Fully Integrated Battery Charger with Two Step-Down Converters
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Selecting the Input and Output Capacitors
In most applications, the most important is the high
frequency decoupling capacitor on the input of the RT9511.
A 1 F ceramic capacitor, placed in close proximity to input
pin and GND pin is recommended. In some applications
depending on the power supply characteristics and cable
length, it may be necessary to add an additional 10 F
ceramic capacitor to the input. The RT9511 requires a
small output capacitor for loop stability. A 1 F ceramic
capacitor placed between the BATT pin and GND is typically
sufficient.
Step-Down DC-DC Converters
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current I
with higher inductance.
Having a lower ripple current reduces the ESR losses in
the output capacitors and the output voltage ripple. Highest
efficiency operation is achieved at low frequency with small
ripple current. This, however, requires a large inductor.
A reasonable starting point for selecting the ripple current
is I
highest V
below a specified maximum, the inductor value should be
chosen according to the following equation :
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or mollypermalloy
cores. Actual core loss is independent of core size for a
fixed inductor value but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
DS9511-01 April 2011
L =
I =
L
L
= 0.4(I
f
IN
V
V
f L
OUT
. To guarantee that the ripple current stays
OUT
I
MAX
L(MAX)
). The largest ripple current occurs at the
L
increases with higher V
1
V
1
V
OUT
IN
V
IN(MAX)
V
OUT
IN
and decreases
Ferrite designs have very low core losses and are preferred
at high switching frequencies, 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 results in an abrupt increase in
inductor ripple current and consequent output voltage ripple.
Do not allow the core to saturate! 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 materials
are small and don't radiate energy but generally cost more
than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs. size requirements and
any radiated field/EMI requirements.
C
The input capacitance, C
trapezoidal current at the source of the top MOSFET. To
prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used. RMS
current is given by :
This formula has a maximum at V
I
commonly used for design because even significant
deviations do not offer much relief. Choose a capacitor
rated at a higher temperature than required. Several
capacitors may also be paralleled to meet size or height
requirements in the design.
The selection of C
resistance (ESR) that is required to minimize voltage ripple
and load step transients, as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later section.
The output ripple, V
RMS
IN
I
RMS
V
and C
OUT
= I
= I
OUT
OUT(MAX)
OUT
/2. This simple worst-case condition is
I ESR+
L
Selection
OUT
V
OUT
V
is determined by the effective series
OUT
IN
8fC
, is determined by :
1
OUT
V
IN
V
OUT
, is needed to filter the
IN
1
IN
RT9511
= 2V
www.richtek.com
OUT
, where
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