RT7274/79/80/81
high, the synchronous rectifier turns off until after the next
high-side on-time. RT7274/80 does not sink current and
therefore does not need a negative current limit.
Input Under-voltage Lock-out
In addition to the enable function, the RT7274/79/80/81
feature an under-voltage lock-out (UVLO) function that
monitors the internal linear regulator output (PVCC). To
prevent operation without fully-enhanced internal MOSFET
switches, this function inhibits switching when PVCC
drops below the UVLO-falling threshold. The IC resumes
switching when PVCC exceeds the UVLO-rising threshold.
Output Over-voltage Protection and Under-voltage
Protection
The RT7279/80 include output over-voltage protection
(OVP). If the output voltage rises above the regulation
level, the high-side switch naturally remains off and the
synchronous rectifier turns on. If the output voltage remains
high the synchronous rectifier remains on until the inductor
current reaches the negative current limit (RT7279) or until
it reaches zero (RT7280). If the output voltage remains
high, the IC's switches remain off. If the output voltage
exceeds the OVP trip threshold for longer than 5μs
(typical), the IC's OVP is triggered.
Soft-Start (SS)
The RT7274/79/80/81 soft-start uses an external pin (SS)
to clamp the output voltage and allow it to slowly rise.
After VEN is high and PVCC exceeds its UVLO threshold,
the IC begins to source 2μAfrom the SS pin. An external
capacitor at SS is used to adjust the soft-start timing.
The available capacitance range is from 2.7nF to 220nF.
Do not leave SS unconnected.
The RT7279/80 include output under-voltage protection
(UVP). If the output voltage drops below the UVP trip
threshold for longer than 250μs (typical) the IC's UVP is
triggered.
During start-up, while the SS capacitor charges, the
RT7274/79/80/81 operate in discontinuous switching mode
with very small pulses. This prevents negative inductor
currents and keeps the circuit from sinking current.
Therefore, the output voltage may be pre-biased to some
positive level before start-up. Once the VSS ramp charges
enough to raise the internal reference above the feedback
voltage, switching will begin and the output voltage will
smoothly rise from the pre-biased level to its regulated
level. After VSS rises above about 2.2V output over-and
under-voltage protections are enabled and the RT7279/81
begins continuous-switching operation.
There are two different behaviors for OVP and UVP events
for the TSSOP-14 (Exposed Pad) packages.
` Latch-Off Mode (TSSOP-14 (Exposed Pad) Only)
` The RT7280GCP/RT7279GCP, 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.
Internal Regulator (PVCC)
Shut-down, Start-up and Enable (EN)
An internal linear regulator (PVCC) produces a 5.1V supply
from VIN that powers the internal gate drivers, PWM logic,
reference, analog circuitry, and other blocks. If VIN is 6V
or greater, PVCC is guaranteed to provide significant power
for external loads.
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 1.6V the IC is fully operational.
Between these 2 levels there are 2 thresholds (1.2V typical
and 1.4V typical). When VEN exceeds the lower threshold
the internal bias regulators begin to function and supply
current increases above the shutdown current level.
Switching operation begins when VEN exceeds the upper
threshold. Unlike many competing devices, EN is a high
voltage input that can be safely connected to VIN (up to
18V) for automatic start-up.
PGOOD Comparator
PGOODis an open drain output controlled by a comparator
connected to the feedback signal. If FB exceeds 90% of
the internal reference voltage, PGOOD will be high
impedance. Otherwise, the PGOOD output is connected
to PGND.
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7274/79/80/81-01 February 2013
www.richtek.com
7
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