RT8256 데이터 시트보기 (PDF) - Richtek Technology
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RT8256 Datasheet PDF : 12 Pages
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RT8256
Application Information
The RT8256 is an asynchronous high voltage buck
converter that can support the input voltage range from
4.75V to 22V and the output current can be up to 2A.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
VOUT
R1
FB
RT8256
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
VOUT
=
VFB
⎛⎜⎝1+
R1
R2
⎞⎟⎠
Where VFB is the feedback reference voltage (1.222V 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 the BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65%. The bootstrap diode can be a
low cost one such as 1N4148 or BAT54.
The external 5V can be a 5V fixed input from system or a
5V output of the RT8256.
5V
BOOT
RT8256
SW
10nF
Figure 2. External Bootstrap Diode
Soft-Start
The RT8256 contains an internal soft-start clamp that
gradually raises the output voltage. The soft-start time is
designed by the internal capacitor. The typical soft-start
time is 2ms.
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.
ΔIL
=
⎡
⎢⎣
VOUT
f ×L
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN
⎤
⎥⎦
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 ΔIL = 0.4(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 :
L
=
⎡
⎢⎣
f
×
VOUT
ΔIL(MAX)
⎤
⎥⎦
×
⎡⎢⎣1−
VOUT
VIN(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.
www.richtek.com
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DS8256-01 March 2011
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