RT8298
Soft-Start
Over Temperature Protection
The RT8298 provides soft-start function. The soft-start
function is used to prevent large inrush current while
converter is being powered-up.An internal current source
(6nA) charges an internal capacitor (15pF) to build a soft-
start ramp voltage. The VFB voltage will track the internal
ramp voltage during soft-start interval. The typical soft-
start time is calculated as follows :
The RT8298 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.
(0.808V× 15pF)
t
=
= 2ms
SS
6nA
Under Voltage Protection
Operating Frequency and Synchronization
For the RT8298, 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 UV condition remains for a period, the
RT8298 will retry every 2ms. When the UV condition is
removed, the converter will resume operation. The UVP
is disabled during soft-start period.
The internal oscillator runs at 600kHz (typ.) when the EN/
SYNC pin is at logic-high level (>2V). If the EN pin is
pulled to low-level for 10μs above, the IC will shut down.
The RT8298 can be synchronized with an external clock
ranging from 300kHz to 1.5MHz applied to the EN/SYNC
pin. The external clock duty cycle must be from 10% to
90%.
Hiccup Mode
10µs
3.5ms (Start-up period)
VIN = 12V, IOUT = Short
EN/SYNC
VOUT
(1V/Div)
V
FB
CLK
External CLK
IL
Foldback
600kHz
(5A/Div)
Figure 6. Startup Sequence Using External Sync Clock
Time (2.5ms/Div)
Figure 7. Hiccup Mode Under Voltage Protection
Figure 6 shows the synchronization operation in startup
period. When the EN/SYNC is triggered by an external
clock, the RT8298 enters soft-start phase and the output
voltage starts to rise. When VFB is lower than 0.7V, the
oscillation frequency will be proportional to the feedback
voltage. With higher VFB, the switching frequency is
relatively higher. After startup period about 3.5ms, the IC
operates with the same frequency as the external clock.
Duty Cycle Limitation
The RT8298 has a maximum duty cycle 90%. The
minimum input voltage is determined by the maximum
duty cycle and its minimum operating voltage 4.5V. The
voltage drops of high side MOSFET and low side MOSFET
also must be considered for the minimum input voltage.
The minimum duty cycle can be calculated by the following
equation :
Duty Cycle(min) = fSW x tON(min)
Copyright 2011 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS8298-01 November 2011
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
