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Detail Description
The RS6517 is a synchronous high voltage buck converter that can support the input voltage range from 4.75V to 20V 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.
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 VFB is the feedback reference voltage (1.23V 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.
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.
V
OUT
⎡
V
OUT
⎤ ⎡
Δ
I
L
= ⎢
× ⎢
1
−
⎥
V
IN
⎣
f
×
L
⎦ ⎣
⎤
⎥
⎦
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
= 0.2375(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：
V
OUT
V
OUT
⎤
⎡
⎤ ⎡
L
=⎢
× ⎢
1
−
f
× Δ
I
L
(
MAX
)
⎥ ⎣
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.
DS‐RS6517‐05
February, 2010
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
