RT8251
Application Information
Soft-Start
The RT8251 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 5A.
The RT8251 contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timming
can be set by the external capacitor between SS pin and
GND. The chip provides a 10μA charge current for the
external capacitor. If 10nF capacitor is used to set the
soft-start time, its period will be 1ms (typ.).
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 3.
V
OUT
Chip Enable Operation
The EN pin is the chip enable input. Pull the EN pin low
(<0.4V) will shutdown the device.During shutdown mode,
the RT8251 quiescent current drops to lower than 25μA.
Drive the EN pin to high ( >1.4V, < 5.5V) will turn on the
device again. If the EN pin is open, it will be pulled to high
by internal circuit. For external timing control (e.g.RC),
the EN pin can also be externally pulled to High by adding
a100kΩ or greater resistor from the VIN pin (see Figure 5).
R1
FB
RT8251
GND
R2
Figure 3. Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
R1
R2
⎛
⎝
⎞
⎟
⎠
VOUT = VFB 1+
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.
Where VFB is the feedback reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 100nF low ESR ceramic capacitor between
the BOOT pin and SW pin. This capacitor provides the
gate driver voltage for the high side MOSFET.
V
VOUT
⎡
OUT ⎤ ⎡
× 1−
⎥ ⎢
⎤
ΔIL =
⎢
⎣
⎥
⎦
f ×L
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.
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
cycle is higher than 65%. The bootstrap diode can be a
low cost one such as 1N4148 or BAT54.
For the ripple current selection, the value of ΔIL= 0.24(IMAX
)
The external 5V can be a 5V fixed input from system or a
5V output of the RT8251.
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 :
5V
⎡
⎤ ⎡
⎤
V
f × ΔI
V
OUT
V
IN(MAX)
OUT
L =
× 1−
BOOT
⎢
⎥ ⎢
⎥
L(MAX)
⎣
⎦ ⎣
⎦
100nF
RT8251
SW
The inductor 's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit. Please
see Table 2 for the inductor selection reference.
Figure 4. External Bootstrap Diode
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS8251-04 February 2013
www.richtek.com
9
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