RT6207A/B
“hysteresis”
to discourage the IC from switching back to
continuous switching unless the load increases
substantially.
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.
Current Limit
The RT6207A/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).
Output Over-Voltage Protection and Under-Voltage
Protection
If the output voltage V
OUT
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 RT6207B, 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
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
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. RT6207BL chip enters latch mode.
For RT6207A If the output voltage Vo 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. RT6207AL chip
enters latch mode.
The RT6207A/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 RT6207AH/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 V
PVCC
). The IC remains
shut down until V
SS
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 V
SS
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.
Latch-Off Mode
The RT6207AL/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.
is a registered trademark of Richtek Technology Corporation.
DS6207A/B-02
December 2015
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
