RT6204
MOSFET will be turned off immediately, while the
minimum on-time tON_MIN requirement still needs to be
met, to prevent the device from operating in an
over-current condition.
Soft-Start
When start-up, a large inrush may be observed for high
output voltages and output capacitances. To solve this,
the RT6204 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 RT6204, the soft-start time tSS can be
programmed by selecting the value of the capacitor
CSS connected between the SS pin and GND. During
the soft-start period, an internal pull-up current source
ISS (typically, 6A) 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
VFB
The soft-start time can be calculated as :
+ 0.3V
REF
C
SS
V
t
=
SS
I
SS
ILOAD
where ISS = 6A (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.
Current Limit
I_inductor
tON_MIN
VSW
Figure 5. Peak-Current Limit
Under-Voltage Protection
1.1V
ISS
SS
CSS
VSS
The feedback voltage is constantly monitored for
under-voltage protection. When the feedback voltage is
VOUT
lower than under-voltage protection threshold VTH-UVP
,
tSS
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 RT6204 will enter hiccup mode. During the
soft-start time, the under-voltage protection is masked
and a 5s deglitch time is built in the UVP circuit to
prevent false transitions.
Figure 4. External Soft-Start Time Setting
Current Limit
The RT6204 provides peak current limit function to
prevent chip damaging from short-circuited output, the
VIN voltage 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
Hiccup Mode
If the under-voltage protection condition continues for a
period of time, the RT6204 will enter hiccup mode, in
which the soft-start process will be initialized without
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6204-04 February 2018
www.richtek.com
13
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