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RT9270 Datasheet PDF : 13 Pages
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RT9270
Preliminary
Component Selection Table
VIN VOUT fOSC
(V) (V) (Hz)
L1
(uH)
3.3 12 670k 10(TDK SLF6028)
3.3 12 670k 4.7(TDK SLF6028)
3.3 12 1.3M 10(TDK SLF6028)
3.3 12 1.3M 4.7(TDK SLF6028)
3.3 7.2 670k 4.7(TDK SLF6028)
3.3 7.2 1.3M 4.7(TDK SLF6028)
C2
(uF)
33 tantalum
33 tantalum
33 tantalum
33 tantalum
47 tantalum
47 tantalum
R3
(k Ω)
36
47
39
51
22
32
C3
(pF)
1200
1000
1000
820
820
1000
C4 Typical IOUT(MAX)
(pF)
(mA)
33
250
22
250
33
210
22
210
56
490
56
450
Application Information
The design procedure of Boost converter can start from
the maximum input current, which is related about
inductor, catch-diode, input/output capacitor selections
and the maximum power which internal switch can stand.
It can be derived from maximum output power, minimum
input voltage and the efficiency of Boost converter. Once
the maximum input current is calculated, the inductor
value can be determined and the other components as
well.
Inductor Selection
For a better efficiency in high switching frequency
converter, the inductor selection has to use a proper core
material such as ferrite core to reduce the core loss and
choose low ESR wire to reduce copper loss. The most
important point is to prevent the core saturated when
handling the maximum peak current. Using a shielded
inductor can minimize radiated noise in sensitive
applications. The maximum peak inductor current is the
maximum input current plus the half of inductor ripple
current. The calculated peak current has to be smaller
than the current limitation in the electrical characteristics.
A typical setting of the inductor ripple current is 20% to
40% of the maximum input current. If the selection is 40%,
the maximum peak inductor current is :
1
IPK = IIN(MAX) + IRIPPLE = 1.2 × IIN(MAX)
2
IOUT(MAX) × VOUT
= 1.2 × [
]
η × VIN(MIN)
The minimum inductance value is derived from the
following equation :
η × VIN(MIN)2 × [VOUT - VIN(MIN)]
L = 0.4 × IOUT(MAX) × VOUT2 × fOSC
Depending on the application, the recommended inductor
value is between 2.2uH and 10uH.
Diode Selection
To achieve high efficiency, Schottky diode is good choice
for low forward drop voltage and fast switching time. The
output diode rating should be able to handle the maximum
output voltage, average power dissipation and the
pulsating diode peak current.
Input Capacitor Selection
For better input bypassing, low-ESR ceramic capacitors
are recommended for performance. A 10uF input capacitor
is sufficient for most applications. For a lower output power
requirement application, this value can be decreased.
Output Capacitor Selection
For lower output voltage ripple, low-ESR ceramic
capacitors are recommended. The tantalum capacitors
can be used as well, but the ESR is bigger than ceramic
capacitor. The output voltage ripple consists of two
components: one is the pulsating output ripple current
flows through the ESR, and the other is the capacitive
ripple caused by charging and discharging.
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DS9270-09 March 2007
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