RT2101B
Application Information
The basic RT2101B application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by CIN and COUT. The switching frequency range
from 700kHz to 2MHz. It is adjusted by using a resistor
to ground on the RT/SYNC pin.
Input and Output Capacitors Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the top MOSFET. A
low ESR input capacitor with larger ripple current rating
should be used for the maximum RMS current. RMS
current is given by :
V
V
V
IN
V
OUT
OUT
I
= I
1
RMS
OUT(MAX)
IN
Output Voltage Setting
This formula has a maximum at VIN = 2VOUT, where IRMS
=
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
IOUT / 2. This simple worst case condition is commonly
used for design because even significant deviations do
not offer much relief. Note that ripple current ratings from
capacitor manufacturers are often based on only 2000
hours of life, which makes it advisable to either further
derate the capacitor or choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be placed in parallel to meet size or height requirements
in the design. The selection of COUT is determined by the
effective series resistance (ESR) that is required to
minimize voltage ripple, load step transients, and the
amount of bulk capacitance that is necessary to ensure
that the control loop is stable. Loop stability can be
examined by viewing the load transient response as
described in a later section. The output ripple, ΔVOUT, is
determined by :
V
OUT
R1
FB
RT2101B
GND
R2
Figure 1. Setting the Output Voltage
The output voltage setting range is 0.827V to 3.6V and
the set by an external resistive divider is according to the
following equation :
R1
R2
VOUT = VFB 1
where VFB is the feedback reference voltage 0.827V (typ.).
Inductor Selection
1
VOUT I ESR
L
8fOSCCOUT
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance :
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
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
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, VIN. At best, this ringing can couple with 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 VIN large enough to damage the
part.
VOUT
fOSC L
VOUT
V
IN
1
IL =
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 ΔIL = 0.4 (IMAX). The largest ripple current occurs at
the highest VIN. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation :
V
I
V
OUT
OUT
L =
1
f
V
IN(MAX)
OSC
L(MAX)
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10
DS2101B-06 March 2020
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
