RT7257B
Under Voltage Protection
Over Temperature Protection
The RT7257B 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.
Hiccup Mode
For the RT7257BH, 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
RT7257BH will retry automatically. When the UVP
condition is removed, the converter will resume operation.
The UVP is disabled during soft-start period.
Hiccup Mode
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.
VOUT
(2V/Div)
V
f ×L
VOUT
V
IN
⎡
OUT ⎤ ⎡
× 1−
⎥ ⎢
⎤
ΔIL =
⎢
⎣
⎥
⎦
⎦ ⎣
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.
ILX
(2A/Div)
IOUT = Short
Time (50ms/Div)
Figure 5. Hiccup Mode Under Voltage Protection
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 :
Latch-Off Mode
For the RT7257BL, 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 RT7257BLwill shutdown in Latch-
Off Mode. In shutdown condition, the RT7257BL can be
reset by EN pin or power input VIN.
⎡
⎤ ⎡
⎤
V
OUT
V
OUT
L =
× 1−
⎢
⎥ ⎢
⎥
f × ΔI
V
IN(MAX)
L(MAX)
⎣
⎦ ⎣
⎦
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
VOUT
(2V/Div)
Table 2. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Dimensions
(mm)
Series
ILX
(2A/Div)
TDK
TDK
VLF10045
SLF12565
NR8040
10 x 9.7 x 4.5
12.5 x 12.5 x 6.5
8 x 8 x 4
IOUT = Short
TAIYO YUDEN
Time (250μs/Div)
Figure 6. Latch-Off Mode Under Voltage Protection
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS7257B-03 September 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
