RT7247A
11
DS7247A-03 October 2016
www.richtek.com
©
Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Under Voltage Protection
Hiccup Mode
For the RT7247AH, it provides Hiccup Mode Under Voltage
Protection (UVP). When the V
FB
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
RT7247AH will retry automatically. When the UVP
condition is removed, the converter will resume operation.
The UVP is disabled during soft-start period.
Figure 5. Hiccup Mode Under Voltage Protection
Latch-Off Mode
For the RT7247AL, it provides Latch-Off Mode Under
Voltage Protection (UVP). When the FB voltage drops
below half of the feedback reference voltage, V
FB
, UVP
will be triggered and the RT7247AL will shut down in Latch-
Off Mode. In shutdown condition, the RT7247AL can be
reset by EN pin or power input VIN.
Figure 6. Latch-Off Mode Under Voltage Protection
OUT
OUT
L
IN
V
V
I =
1
f L
V
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 val
ue of
Δ
I
L
= 0.24(I
MAX
)
will be a reasonable starting point. The large
st ripple
current occurs at the highest V
IN
. To guarantee that the
Over Temperature Protection
The RT7247A 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
Δ
I
L
increases with higher V
IN
and decreases with higher inductance.
Clamp Mode
For the RT7247AN, it provides inductor current clamp
mode.
Figure 7. Clamp Mode
Time (1ms/Div)
Clamp Mode
V
OUT
(2V/Div)
I
LX
(1A/Div)
I
OUT
= Short
Time (25ms/Div)
Hiccup Mode
V
OUT
(2V/Div)
I
LX
(1A/Div)
I
OUT
= Short
Time (25
μ
s/Div)
Latch-Off Mode
V
OUT
(2V/Div)
I
LX
(2A/Div)
I
OUT
= Short
