RT8258 RICHTEK [Richtek Technology Corporation], RT8258 Datasheet - Page 7

• Manufacturer Part Number: RT8258
• Description: 1.2A, 24V, 700kHz Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Application Information
The RT8258 is an asynchronous high voltage buck converter
that can support the input voltage range from 4.5V to 24V
and the output current can be up to 1.2A.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
For adjustable voltage mode, the output voltage is set by
an external resistive voltage divider according to the
following equation :
Where V
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and PHASE 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 RT8268.
DS8258-02 March 2011
V
OUT
= V
FB
FB
Figure 2. External Bootstrap Diode
is the feedback reference voltage (0.8V typ.).
Figure 1. Output Voltage Setting
⎛
⎜
⎝
1
+
R2
R1
RT8258
RT8258
⎞
⎟
⎠
PHASE
BOOT
GND
FB
5V
V
OUT
10nF
R1
R2
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔI
and decreases with higher inductance.
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
will be a reasonable starting point. The largest ripple current
occurs at the highest V
current stays below the specified maximum, the inductor
value should be chosen according to the following
equation :
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.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design current
is exceeded. The previous situation results in an abrupt
increase in inductor ripple current and consequent output
voltage ripple.
L =
Δ
I =
L
⎡
⎢
⎣
f
⎡
⎢
⎣
× Δ
V
V
f L
OUT
×
OUT
I
L(MAX)
⎤ ⎡
⎥ ⎢
⎦ ⎣
×
⎤ ⎡
⎥ ⎢
⎦ ⎣
1
×
−
1
V
V
−
OUT
IN
V
IN
IN(MAX)
V
. To guarantee that the ripple
⎤
⎥
⎦
OUT
L
increases with higher V
⎤
⎥
⎦
RT8258
www.richtek.com
L
= 0.34(I
MAX
IN
7
)