RT8284N
1
3
2
V
12V
IN
Table 2. Suggested Inductors for Typical
Application Circuit
BOOT
VIN
V
OUT
C
IN
10µF
8V
C
R
100k
RT8284N
BOOT
EN1
L
Component
Supplier
Dimensions
(mm)
7
SW
EN
Series
R1
R2
C
R
EN2
OUT
TDK
TDK
VLF10045
SLF12565
10 x 9.7 x 4.5
5
6
FB
8
SS
12.5 x 12.5 x 6.5
C
C
C
SS
4,
R
C
COMP
9 (Exposed Pad)
TAIYO
YUDEN
GND
C
P
NR8040
8 x 8 x 4
Figure 4. The Resistors can be Selected to Set IC
Lockout Threshold
CIN and COUT Selection
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
RMS current is given by :
Hiccup Mode
For the RT8284N, Hiccup Mode Under Voltage Protection
(UVP) is provided. When the FB voltage, VFB, drops below
0.5V, the UVP function will be triggered and the RT8284N
will shut down for a period of time and then recover
automatically. The Hiccup Mode UVP can reduce input
current in short-circuit conditions.
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 ΔIL 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 ⎤ ⎡
× 1−
⎥ ⎢
⎤
ΔIL =
⎢
⎣
⎥
⎦
For the input capacitor, one 10μF low ESR ceramic
capacitors are recommended. For the recommended
capacitor, please refer to table 3 for more detail.
⎦ ⎣
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
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 ΔIL= 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, ΔVOUT , is determined by :
1
⎡
⎤
ΔVOUT ≤ ΔIL ESR +
⎢
⎣
⎥
⎦
8fCOUT
⎡
⎤ ⎡
⎤
V
f × ΔI
V
OUT
V
IN(MAX)
OUT
L =
× 1−
The output ripple will be highest at the maximum input
⎢
⎥ ⎢
⎥
L(MAX)
⎣
⎦ ⎣
⎦
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement.Dry tantalum,
special polymer, aluminum electrolytic and ceramic
capacitors are all available in surface mount packages.
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.
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS8284N-03 May 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
