RT8251GSP Richtek USA Inc, RT8251GSP Datasheet - Page 9

• Manufacturer Part Number: RT8251GSP
• Description: IC DCDC CTRLR STP-DN 8SOP
• Manufacturer: Richtek USA Inc
• Type: Step-Down (Buck)r

Specifications of RT8251GSP

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 V ~ 15 V
• Current - Output: 5A
• Frequency - Switching: 570kHz
• Voltage - Input: 4.75 V ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width) Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application Information
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.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 3.
The output voltage is set by an external resistive divider
according to the following equation :
Where V
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.
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.
The external 5V can be a 5V fixed input from system or a
5V output of the RT8251.
DS8251-01 March 2011
V
OUT
= V
FB
FB
Figure 4. External Bootstrap Diode
is the feedback reference voltage (0.8V typ.).
Figure 3. Output Voltage Setting
1
R2
R1
RT8251
RT8251
BOOT
SW
GND
FB
5V
V
OUT
100nF
R1
R2
Soft-Start
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 10 A charge current for the
external capacitor. If 10nF capacitor is used to set the
soft-start time, its period will be 1ms (typ.).
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 25 A.
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
a100k or greater resistor from the VIN pin (see Figure 5).
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current I
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 I
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 :
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.
L =
I =
L
f
V
V
f L
OUT
OUT
I
L(MAX)
1
V
1
V
OUT
IN
V
IN
IN(MAX)
V
. To guarantee that the ripple
OUT
L
increases with higher V
RT8251
www.richtek.com
L
= 0.24(I
MAX
IN
9
)