RT2862A
Hiccup Mode
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit. Please
see Table 2 for the inductor selection reference.
VOUT
(2V/Div)
Table 2. Suggested Inductors for Typical
Application Circuit
ILX
(2A/Div)
Component
Supplier
Dimensions
(mm)
Series
TDK
TDK
VLF10045
SLF12565
10 x 9.7 x 4.5
IOUT = Short
12.5 x 12.5 x 6.5
Time (50ms/Div)
TAIYO
YUDEN
NR8040
8 x 8 x 4
Figure 5. Hiccup Mode Under-Voltage Protection
Over-Temperature Protection
CIN and COUT Selection
The RT2862A features an Over-Temperature Protection
(OTP) circuitry to prevent overheat due to excessive power
dissipation. The OTP will shut down switching operation
when junction temperature exceeds 150°C. Once the
junction temperature cools down by approximately 20°C,
the converter will resume operation. To maintain continuous
operation, the maximum junction temperature should be
lower than 125°C.
The input capacitance, CIN, is needed to filter the
trapezoidal current at the Source of the high-side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
approximate RMS current equation is given :
V
V
V
IN
V
OUT
OUT
I
= I
−1
RMS
OUT(MAX)
IN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT / 2. This simple worst case condition is
commonly used for design because even significant
deviations do not offer much relief.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ∆IL increases with higher VIN
and decreases with higher inductance.
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.
V
f ×L
VOUT
V
IN
OUT
∆IL =
× 1−
For the input capacitor, two 10µF low ESR ceramic
capacitors are suggested. For the suggested capacitor,
please refer to Table 3 for more details.
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal.
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for COUT selection 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.
For the ripple current selection, the value of ∆IL= 0.24(IMAX
)
will be a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the
ripple current stays below the specified maximum, the
inductor value should be chosen according to the following
equation :
The output ripple, ∆VOUT, is determined by :
1
∆VOUT ≤ ∆IL ESR +
8fCOUT
V
f ×∆I
V
OUT
V
IN(MAX)
OUT
L =
× 1−
L(MAX)
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS2862A-02 January 2018
www.richtek.com
11
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