RT7258
Soft-Start
Over Temperature Protection
The RT7258 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
charges an internal capacitor to build a soft-start ramp
voltage. The VFB voltage will track the internal ramp voltage
during soft-start interval. The typical softstart time is 2ms.
The RT7258 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.
Operating Frequency and Synchronization
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 RT7258 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%.
Under Voltage Protection
For the RT7258, 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
RT7258 will retry every 2ms. When the UV condition is
removed, the converter will resume operation. The UVP
is disabled during soft-start period.
10µs
3.5ms (Start-up period)
Hiccup Mode
EN/SYNC
V
FB
VOUT
(1V/Div)
CLK
External CLK
Foldback
600kHz
IL
(10A/Div)
Figure 6. Startup Sequence Using External Sync Clock
VIN = 12V, IOUT = Short
Figure 6 shows the synchronization operation in startup
period. When the EN/SYNC is triggered by an external
clock, the RT7258 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.
Time (2.5ms/Div)
Figure 7. Hiccup Mode Under Voltage Protection
Duty Cycle Limitation
The RT7258 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 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS7258-00 January 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
