RT2855A/B
“hysteresis” to discourage the IC from switching back to
continuous switching unless the load increases
substantially.
Hiccup Mode
The RT2855AHGQW/ RT2855BHGQW, use hiccup mode
OVP and 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 the overload or short circuit is removed. During hiccup
mode, the shutdown time is determined by the capacitor
at SS. A 0.5μA current source discharges VSS from its
starting voltage (normally VREG5). The IC remains shut
down until VSS reaches 0.2V, about 38ms for a 3.9nF
capacitor. At that point the IC begins to charge the SS
capacitor by 6μA, and a normal start-up occurs. If the
fault remains, OVP and 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 16ms for a 3.9nF SS capacitor.
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 RT2855A/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 upper
current limit, the on-time one-shot is inhibited until the
inductor current ramps down below the upper current limit
plus a wide hysteresis band of about 1A until it drops
below the lower current limit level. Thus, only when the
inductor current is well below the upper current limit is
another on-time permitted. This arrangement prevents the
average output current from greatly exceeding the
guaranteed upper 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 (see
next section) the IC will stop switching to avoid excessive
heat.
Latch-Off Mode
The RT2855ALGQW/ RT2855BLGQW, use latch-off mode
OVP and UVP. When the protection function is triggered
the IC will shut down. 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.
Input Under-voltage Lock-out
In addition to the enable function, the RT2855A/B feature
an under-voltage lock-out (UVLO) function that monitors
the internal linear regulator output (VREG5). To prevent
operation without fully-enhanced internal MOSFET
switches, this function inhibits switching when VREG5
drops below the UVLO-falling threshold. The IC resumes
switching when VREG5 exceeds the UVLO-rising
threshold.
The RT2855B 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 RT2855A does not sink current
and therefore does not need a negative current limit.
Shut-down, Start-up and Enable (EN)
The enable input (EN) has a logic-low level of 0.4V. When
VEN is below this level the IC enters shutdown mode and
supply current drops to less than 10μA. When VEN exceeds
its logic-high level of 2V the IC is fully operational.
ENis a high voltage input that can be safely connected to
VIN (up to 18V) for automatic start-up.
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
6
DS2855A/B-01 September 2015
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
