RT6210
Soft-Start
requirement still needs to be met, to prevent the device
from operating in an over-current condition.
When start-up, a large inrush may be observed for high
output voltages and output capacitances. To solve this,
the RT6210 provides an external soft-start function to
reduce input inrush current to meet various applications.
In the current-limited condition, the maximum sourcing
current is fixed because the peak inductor current is
limited. When the load is further increasing and is over
the sourcing capability of the high-side switch, the output
voltage will start to drop and eventually be lower than the
under-voltage protection threshold so that the IC will enter
shutdown mode and may restart with hiccup mechanism.
For the RT6210, the soft-start time tSS can be programmed
by selecting the value of the capacitor CSS connected
between the SS pin andGND.During the soft-start period,
an internal pull-up current source ISS (typically, 6μA)
charges the external capacitor CSS to generate a soft-
start voltage ramp, and then output voltage will follow this
voltage ramp to monotonically start up.
UV Threshold
V
FB
The soft-start time can be calculated as :
+ 0.3V
REF
C
SS
V
t
=
SS
I
SS
where ISS = 6μA(typical), VREF is the feedback reference
voltage, and CSS is the external capacitor placed from the
SS pin to GND, where a 10nF to 100nF capacitor is
recommended to set the soft-start time from 1.833ms to
18.333ms.
I
LOAD
Current Limit
I
_inductor
t
ON_MIN
1.1V
I
SS
V
SW
SS
C
SS
Figure 5. Peak-Current Limit
Under-Voltage Protection
V
SS
V
OUT
The feedback voltage is constantly monitored for under-
voltage protection. When the feedback voltage is lower
than under-voltage protection threshold VTH-UVP, the under-
voltage protection is triggered, the high-side MOSFET will
be turned off and the low-side MOSFET is turned on to
discharge the output voltage. The under-voltage protection
is not a latched mechanism; if the under-voltage condition
remains for a period of time, the RT6210 will enter hiccup
mode. During the soft-start time, the under-voltage
protection is masked and a 5μs deglitch time is built in
the UVP circuit to prevent false transitions.
t
SS
Figure 4. External Soft-Start Time Setting
Current Limit
The RT6210 provides peak current limit function to prevent
chip damaging from short-circuited output, the VINvoltage
and SW voltage are sensed when the internal high-side
MOSFET is turned on. During this period, the VIN-SW
voltage is increasing when the inductor current is
increasing. The peak inductor current will be monitored
every switching cycle. If the current sense signal exceeds
the internal current limit, clamped by the maximum COMP
voltage, the high-side MOSFET will be turned off
immediately, while the minimum on-time tON_MIN
Hiccup Mode
If the under-voltage protection condition continues for a
period of time, the RT6210 will enter hiccup mode, in which
the soft-start process will be initialized without VIN being
Copyright 2019 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6210-01 February 2019
www.richtek.com
13
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