RT6211A/B
change causing a switching frequency variation with
load current. Also, at light loads if the inductor current
goes negative, the switch dead-time between the
synchronous rectifier turn-off and the high-side switch
turn-on allows the switching node to rise to the input
voltage. This increases the effective on-time and
causes the switching frequency to drop noticeably.
frequency mode to minimize interference and noise
problems. The RT6211A uses variable-frequency
discontinuous switching at light loads to improve
efficiency. During discontinuous switching, the on-time
is immediately increased to add “hysteresis” to
discourage the IC from switching back to continuous
switching unless the load increases substantially.
One way to reduce these effects is to measure the
actual switching frequency and compare it to the
desired range. This has the added benefit eliminating
the need to sense the actual output voltage, potentially
saving one pin connection. ACOTTM uses this method,
measuring the actual switching frequency (at LX) and
modifying the on-time with a feedback loop to keep the
average switching frequency in the desired range.
The IC returns to continuous switching as soon as an
on-time is generated before the inductor current
reaches zero. The on-time is reduced back to the
length needed for 500kHz switching and encouraging
the circuit to remain in continuous conduction,
preventing repetitive mode transitions between
continuous switching and discontinuous switching.
Current Limit
To achieve good stability with low-ESR ceramic
capacitors, ACOTTM uses a virtual inductor current
ramp generated inside the IC. This internal ramp signal
replaces the ESR ramp normally provided by the output
capacitor's ESR. The ramp signal and other internal
compensations are optimized for low-ESR ceramic
output capacitors.
The RT6211A/B current limit is a cycle-by-cycle “valley”
type, measuring the inductor current through the
synchronous rectifier during the off-time while the
inductor current ramps down. The current is
determined by measuring the voltage between Source
and Drain of the synchronous rectifier, adding
temperature compensation for greater accuracy. If the
current exceeds the current limit, the on-time one-shot
is inhibited until it drops below the current limit level. If
the output current exceeds the available inductor
current (controlled by the current limit mechanism), the
output voltage will drop. If it drops below the output
under-voltage protection level (see next section) the IC
will stop switching to avoid excessive heat.
ACOTTM One-Shot Operation
The RT6211A/B control algorithm is simple to
understand. The feedback voltage, with the virtual
inductor current ramp added, is compared to the
reference voltage. When the combined signal is less
than the reference the on-time one-shot is triggered, as
long as the minimum off-time one-shot is clear and the
measured inductor current (through the synchronous
rectifier) is below the current limit. The on-time
one-shot turns on the high-side switch and the inductor
current ramps up linearly. After the on-time, the
high-side switch is turned off and the synchronous
rectifier is turned on and the inductor current ramps
down linearly. At the same time, the minimum off-time
one-shot is triggered to prevent another immediate
on-time during the noisy switching time and allow the
feedback voltage and current sense signals to settle.
The minimum off-time is kept short (240ns typical) so
that rapidly-repeated on-times can raise the inductor
current quickly when needed.
The RT6211B also includes a negative current limit to
protect the IC against sinking excessive current and
possibly damaging the IC. If the voltage across the
synchronous rectifier indicates the negative current is
too high, the synchronous rectifier turns off until after
the next high-side on-time. The RT6211A does not sink
current and therefore does not need a negative current
limit.
Input Under-Voltage Lockout
To protect the chip from operating at insufficient supply
voltage, the UVLO is needed. When the input voltage
of VIN is lower than the UVLO falling threshold voltage,
the device will be lockout.
Discontinuous Operating Mode (RT6211A Only)
After soft-start, the RT6211B operates in fixed
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6211A/B-07 September 2018
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