RT9277A/B
Application Information
Preliminary
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 :
I
PEAK
=
I
IN(MAX)
+
1 I
RIPPLE
=
1.2
×
I
IN(MAX)
2
⎡
I
OUT(MAX)
×
V
OUT
⎤
=
1.2
× ⎢
⎥
⎣
η
×
V
IN(MIN)
⎦
Input Capacitor Selection
For better input bypassing, low-ESR ceramic capacitors
are recommended for performance. A 10μF 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.
V
RIPPLE
=
V
RIPPLE_ESR
+
V
RIPPLE_C
−
V
IN
⎞
I
⎛
V
≅
I
PEAK
×
R
ESR
+
PEAK
⎜
OUT
⎟
C
OUT
⎝
V
OUT
×
f
OSC
⎠
Output Voltage
The regulated output voltage is calculated by :
V
OUT
=
V
REF
× ⎛
1
+
R1
⎞
⎜
⎟
⎝
R2
⎠
Where V
REF
= 1.24V typical.
For most applications, R2 is a suggested a value up to
100kΩ Place the resistor-divider as close to the IC as
possible to reduce the noise sensitivity.
Loop Compensation
The minimum inductance value is derived from the following
equation :
The
RT9277A/B
voltage feedback loop can be compensated
with an external compensation network consisted of R3,
C3 and C4 (As shown in Figure 1). Choose R3 to set the
high-frequency integrator gain for fast transient response
without over or under compensation. Once R3 is
determined, C3 is selected to set the integrator zero to
maintain loop stability. The purpose of C4 is to cancel the
zero caused by output capacitor and the capacitor ESR. If
the ceramic capacitor is selected to be the output capacitor,
C4 can be taken off because of the small ESR. C2 is the
output capacitor as shown in Figure 1. The following
equations give approximate calculations of each
component :
L
=
η
×
V
IN(MIN)
×
[
V
OUT
-
V
IN(MIN)
]
2
2
0.4
×
I
OUT(MAX)
×
V
OUT
×
f
OSC
Depending on the application, the recommended inductor
value is between 2.2μH to 10μH.
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.
www.richtek.com
10
DS9277A/B-02 March 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
