RT8265 Datasheet, PDF (9/13 Pages) Richtek Technology Corporation – The RT8265 is an asynchronous high voltage buck converter

Application Information
The RT8265 is an asynchronous high voltage buck converter
that can support the input voltage range from 4.75V to 24V
and the output current can be up to 3A.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
V_OUT
R1
FB
RT8265
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
V_OUT
=
V_FB
⎛⎜⎝₁₊
R1
R2
⎞⎟⎠
Where V_FB is the feedback reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin for efficiency
improvement. The external 5V can be a 5V fixed input
from system or a 5V output of the RT8265. The bootstrap
diode can be a low cost one such as IN4148 or BAT54.
This diode is also recommended for high duty cycle
operation (when
VOUT
VIN
> 65%) applications.
5V
BOOT
RT8265
SW
10nF
Figure 2
DS8265-02 March 2011
RT8265
Soft-Start
The RT8265 contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timming
can be programed by the external capacitor between SS
pin and GND. The chip provides a 15μA charge current for
the external capacitor. If 0.1μF capacitor is used to set
the soft-start and it's period will be 10ms(typ.).
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔI_L increases with higher V_IN
and decreases with higher inductance.
ΔI_L
=
⎡
⎢⎣
V_OUT
f ×L
⎤
⎥⎦
×
⎡⎢⎣_1−
V_OUT
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 achieve
highest efficiency operation. However, it requires a large
inductor to achieve this goal.
For the ripple current selection, the value of ΔI_L = 0.4(I_MAX)
will be a reasonable starting point. The largest ripple current
occurs at the highest V_IN. To guarantee that the ripple
current stays below the specified maximum, the inductor
value should be chosen according to the following
equation :
L
=
⎡
⎢⎣
f
×
V_OUT
ΔI_L(MAX)
⎤
⎥⎦
×
⎡⎢⎣_1−
V_OUT
V_IN(MAX)
⎤
⎥⎦
Inductor Core Selection
The inductor type must be selected once the value for L is
known. Generally speaking, high efficiency converters can
not afford the core loss found in low cost powdered iron
cores. So, the more expensive ferrite or mollypermalloy
cores will be a better choice.
The selected inductance rather than the core size for a
fixed inductor value is the key for actual core loss. As the
inductance increases, core losses decrease. Unfortunately,
increase of the inductance requires more turns of wire and
therefore the copper losses will increase.
Ferrite designs are preferred at high switching frequency
due to the characteristics of very low core losses. So,
design goals can focus on the reduction of copper loss
and the saturation prevention.
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