RT2875A/B
Under-Voltage Protection
Hiccup Mode
ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
The RT2875B provides Hiccup Mode Under-Voltage
Protection (UVP). When the VFB voltage drops below 0.3V,
the UVP function will be triggered to shut down switching
operation. If the UVP condition remains for a period, the
RT2875B will retry automatically. When the UVP condition
is removed, the converter will resume operation. The UVP
is disabled during soft-start period.
V
f L
VOUT
V
IN
OUT
1
IL =
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
)
Latch Mode
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 :
For the RT2875Ait provides Latch-Off Mode Under Voltage
Protection (UVP). When the VFB voltage drops below 0.3V,
UVP will be triggered and the RT2875A will shut down in
Latch-Off Mode. In shutdown condition, the RT2875Acan
be reset by EN pin or power input VIN.
V
OUT
V
OUT
L =
1
Hiccup Mode
f I
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.
VOUT
(2V/Div)
Table 2. Suggested Inductors for Typical
Application Circuit
IL
(2A/Div)
Component
Supplier
Dimensions
(mm)
Series
IOUT = Short
TDK
TDK
VLF10045
SLF12565
10 x 9.7 x 4.5
Time (50ms/Div)
12.5 x 12.5 x 6.5
TAIYO
YUDEN
Figure 5. Hiccup Mode Under Voltage Protection
NR8040
8 x 8 x 4
Over-Temperature Protection
CIN and COUT Selection
The RT2875A/B 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
180°C. Once the junction temperature cools down by
approximately 15°C, the converter will resume operation.
To maintain continuous operation, the maximum junction
temperature should be lower than 150°C.
The input capacitance, CIN, is needed to filter the
trapezoidal current at the Source of the high side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
approximate RMS current equation is given :
V
V
V
IN
V
OUT
OUT
I
= I
1
RMS
OUT(MAX)
IN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT / 2. This simple worst case condition is
commonly used for design because even significant
deviations do not offer much relief.
Inductor Selection
The inductor value and operating frequency determine the
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is a registered trademark of Richtek Technology Corporation.
DS2875A/B-08 November 2018
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13
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