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RT7257C Datasheet(PDF) 10 Page - Richtek Technology Corporation

Part # RT7257C
Description  3A, 18V, 340kHz Synchronous Step-Down Converter
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Manufacturer  RICHTEK [Richtek Technology Corporation]
Direct Link  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT7257C Datasheet(HTML) 10 Page - Richtek Technology Corporation

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RT7257C
10
DS7257C-02
September 2012
www.richtek.com
©
Copyright
2012 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
Under Voltage Protection
Hiccup Mode
For the RT7257CH, it provides Hiccup Mode Under Voltage
Protection (UVP). When the VFB voltage drops below 0.4V,
the UVP function will be triggered to shut down switching
operation. If the UVP condition remains for a period, the
RT7257CH will retry automatically. When the UVP
condition is removed, the converter will resume operation.
The UVP is disabled during soft-start period.
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
the highest efficiency operation. However, it requires a
large inductor to achieve this goal.
For the ripple current selection, the value of
ΔIL= 0.24(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 :
Table 2. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Series
Dimensions
(mm)
TDK
VLF10045
10 x 9.7 x 4.5
TDK
SLF12565
12.5 x 12.5 x 6.5
TAIYO
YUDEN
NR8040
8 x 8 x 4
OUT
OUT
L(MAX)
IN(MAX)
VV
L =
1
fI
V
⎡⎤ ⎡
×−
⎢⎥ ⎢
×Δ
⎣⎦ ⎣
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.
.
Latch-Off Mode
For the RT7257CL, it provides Latch-Off Mode Under
Voltage Protection (UVP). When the FB voltage drops
below half of the feedback reference voltage, VFB, UVP
will be triggered and the RT7257CL will shutdown in Latch-
Off Mode. In shutdown condition, the RT7257CL can be
reset by EN pin or power input VIN.
Figure 5. Hiccup Mode Under Voltage Protection
Figure 6. Latch-Off Mode Under Voltage Protection
Over Temperature Protection
The RT7257C features an Over Temperature Protection
(OTP) circuitry to prevent from overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature exceeds
150
°C. Once the junction temperature cools down by
approximately 20
°C, the converter will resume operation.
To maintain continuous operation, the maximum junction
temperature should be lower than 125
°C.
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
Time (250
μs/Div)
Latch-Off Mode
VOUT
(2V/Div)
ILX
(2A/Div)
IOUT = Short
OUT
OUT
L
IN
VV
I =
1
fL
V
⎡⎤ ⎡
Δ× −
⎢⎥ ⎢
×
⎣⎦ ⎣
Time (50ms/Div)
Hiccup Mode
VOUT
(2V/Div)
ILX
(2A/Div)
IOUT = Short


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