RT7237A
Under Voltage Protection
Hiccup Mode
Clamp Mode
For the RT7237AN, it provides inductor current clamp
mode. In shutdown condition, the RT7237ANcan be reset
by removing short condition.
For the RT7237AH, 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
RT7237AH will retry automatically. When the UVP
condition is removed, the converter will resume operation.
The UVP is disabled during soft-start period.
Clamp Mode
VOUT
(2V/Div)
Hiccup Mode
ISW
VOUT
(1A/Div)
(2V/Div)
Time (5ms/Div)
Figure 7. Clamp Mode
Over Temperature Protection
ISW
(1A/Div)
The RT7237A 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.
IOUT = Short
Time (25ms/Div)
Figure 5. Hiccup Mode Under Voltage Protection
Latch-Off Mode
For the RT7237AL, 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 RT7237ALwill shutdown in Latch-
Off Mode. In shutdown condition, the RT7237AL can be
reset by EN pin or power input VIN.
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.
Latch-Off Mode
V
f ×L
VOUT
V
IN
⎡
OUT ⎤ ⎡
× 1−
⎥ ⎢
⎤
ΔIL =
⎢
⎣
⎥
⎦
⎦ ⎣
VOUT
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.
(2V/Div)
ISW
(1A/Div)
For the ripple current selection, the value of ΔIL= 0.24(IMAX
)
IOUT = Short
will be a reasonable starting point. The largest ripple
current occurs at the highest VIN. To guarantee that the
Time (25μs/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.
DS7237A-01 September 2012
www.richtek.com
11
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
