RT8298
where fsw is the switching frequency, tON (min) is the
minimum switch on time (100ns). This equation shows
that the minimum duty cycle increases when the switching
frequency is increased. Therefore, slower switching
frequency is necessary to achieve high VIN/VOUT ratio
application.
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 :
⎡
⎤ ⎡
× 1−
⎤
V
f × ΔI
V
OUT
V
IN(MAX)
OUT
L =
⎢
⎥ ⎢
⎥
External N-MOSFET Selection
L(MAX)
⎣
⎦ ⎣
⎦
The RT8298 is designed to operate using an external low
side N-MOSFET. Important parameters for the power
MOSFETs are the breakdown voltage (BVDSS), threshold
voltage (VGS_TH), on-resistance (RDS(ON)), total gate charge
(Qg) and maximum current (ID(MAX)). The gate driver voltage
is from internal regulator (5V, VCC). Therefore logic level
N-MOSFET must be used in the RT8298 application. The
total gate charge (Qg) must be less than 50nC, lower Qg
characteristics results in lower power losses.Drain-source
on-resistance (RDS(ON)) should be as small as possible,
less than 30mΩ is desirable. Lower RDS(ON) results in
higher efficiency.
The inductor's current rating (cause 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 3 for the inductor selection reference.
Table 3. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Dimensions
(mm)
Series
10 x 10 x 4
6 x 6 x 3
Zenithtek
ZPWM
WE
74477
10 x 10 x 4
8 x 10 x 4
Table 2. External N-MOSFET Selection
TAIYOYUDEN
NR8040
Part No.
Si7114
Manufacture
Vishay
CIN and COUT Selection
A04474
ALPHA & OMEGA
Fairchild
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 :
FDS6670AS
IRF7821
International Rectifier
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.
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.
V
VOUT
⎡
OUT ⎤ ⎡
× 1−
⎥ ⎢
⎤
ΔIL =
⎢
⎣
⎥
⎦
f ×L
V
IN
⎦ ⎣
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 reduce
voltage. For the highest efficiency operation, however, it
requires a large inductor to achieve this goal.
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.
Copyright 2011 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS8298-01 November 2011
www.richtek.com
13
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