RT6236A/B
“hysteresis” to discourage the IC from switching back to
continuous switching unless the load increases
substantially.
clock. If the output voltage exceeds the OVP trip threshold
(1.25 times regulation level) for longer than 10μs (typical),
then IC's output Over-Voltage Protection (OVP) is
triggered. The RT6236BL chip enters latch mode.
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
650kHz switching and encouraging the circuit to remain
in continuous conduction, preventing repetitive mode
transitions between continuous switching and
discontinuous switching.
For the RT6236A If the output voltage VOUT rises above
the regulation level and lower 1.2 times regulation level,
the high-side switch naturally remains off and the
synchronous rectifier turns on until the inductor current
reaches zero current. If the output voltage remains high,
then IC's switches remain off. If the output voltage exceeds
the OVP trip threshold (1.25 times regulation level) for
longer than 10μs (typical), the IC's OVP is triggered. The
RT6236AL chip enters latch mode.
Current Limit
The RT6236A/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. If the inductor current exceeds the
current limit, the on-time one-shot is inhibited (Mask high
side signal) until the inductor current ramps down below
the current limit. Thus, only when the inductor current is
well below the current limit is another on time permitted.
This arrangement prevents the average output current from
greatly exceeding the guaranteed current limit value, as
typically occurs with other valley-type current limits. 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, the IC will stop switching (see next
section).
The RT6236A/B include output Under-Voltage Protection
(UVP). If the output voltage drops below the UVP trip
threshold for longer than 250μs (typical) then IC's UVP is
triggered. Chip into latch or hiccup mode. (see next
section).
Hiccup Mode
The RT6236AH/BH, use hiccup mode for UVP. When the
protection function is triggered, the IC will shut down for a
period of time and then attempt to recover automatically.
Hiccup mode allows the circuit to operate safely with low
input current and power dissipation, and then resume
normal operation as soon as UVP is removed. During
hiccup mode, the shutdown time is determined by the
capacitor at SS. A 2μA current source discharges VSS
from its starting voltage (normally VPVCC). The IC remains
shut down until VSS reaches 0.2V, about 10ms for a 3.9nF
capacitor. At that point the IC begins to charge the SS
capacitor at 6μA, and a normal start-up occurs. If the fault
remains, UVP protection will be enabled when VSS reaches
2.2V (typical). The IC will then shut down and discharge
the SS capacitor from the 2.2V level, taking about 4ms for
a 3.9nF SS capacitor.
Output Over-Voltage Protection and Under-Voltage
Protection
If the output voltage VOUT rises above the regulation level
and lower 1.2 times regulation level, the high-side switch
naturally remains off and the synchronous rectifier turns
on. For the RT6236B, if the output voltage remains high
the synchronous rectifier remains on until the inductor
current reaches the low side current limit. If the output
voltage still remains high, then IC's switches remain that
the synchronous rectifier turns on and high-side MOS
keeps off to operate at typical 500kHz switching protection,
again if inductor current reaches low side current, the
synchronous rectifier will turn off until next protection
Latch-Off Mode
The RT6236AL/BL, use latch-off mode OVP and UVP.
When the protection function is triggered the IC will shut
down in Latch-Off Mode. The IC stops switching, leaving
both switches open, and is latched off. To restart operation,
toggle EN or power the IC off and then on again.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6236A/B-03 January 2018
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
