RT6248B
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 the inductor current ramps down
below the current limit. Thus, only when the inductor
current is well below the current limit, another on-time is
permitted. 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.
Enable and Disable
The RT6248B's ENis used to control converter, the enable
voltage (EN) has a logic-low level of 0.4V. When VEN is
below this level the IC enters shutdown mode When VEN
exceeds its logic-high level of 0.8V the converter is fully
operational.
Soft-Start
The RT6248B provide an internal soft-start function to
prevent large inrush current and output voltage overshoot
when the converter starts up. The soft-start (SS)
automatically begins once the chip is enabled.During soft-
start, it clamps the ramping of internal reference voltage
which is compared with FB signal. The typical soft-start
duration is 0.4ms. A unique PWM duty limit control that
prevents output over-voltage during soft-start period is
designed specifically for FB floating.
Output Over-Voltage Protection and Under-Voltage
Protection
The RT6248B 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 will turn on until the inductor current reaches the
zero or next on-time one-shot is triggered If the output
voltage exceeds the OVP threshold for longer than 20μs
(typical), the IC's OVP is triggered. The RT6248B also
include output under-voltage protection (UVP). If the output
voltage drops below the UVP trip threshold for longer than
20μs (typical) the IC's UVP is triggered. The RT6248B
use auto-recovery mode in OVP and Hiccup Mode in UVP.
When the UVP function is triggered remains for a period,
the RT6248B will retry automatically. When the UVP
condition is removed, the converter will resume operation.
the UVP is disabled during soft-start period. If the OVP
function is triggered, the IC will stops switching. When
the OVP condition is removed, the converter will resume
operation.
Power Good Output (PGOOD)
The power good output is an open drain output that requires
a pull-up resistor. When the output voltage is 15% (typical)
below its set voltage, PGOODwill be pulled low. It is held
low until the output voltage returns to 90% of its set voltage
once more. During soft-start, PGOODis actively held low
and only allowed to be pulled high after soft-start is over
and the output reaches 90% of its set voltage. There is a
10μs delay built into PGOOD circuitry to prevent false
transition.
External Bootstrap Capacitor (CBOOT
)
Connect a 0.1μF low ESR ceramic capacitor between
BOOT pin and LX pin. This bootstrap capacitor provides
the gate driver supply voltage for the high-sideN-channel
MOSFET switch.
Input Under-Voltage Lock-out
In addition to the enable function, the RT6248B provide
an Under-Voltage Lock-out (UVLO) function that monitors
the input voltage. To prevent operation without fully-
enhanced internal MOSFET switches, this function inhibits
switching when input voltage drops below the UVLO-falling
threshold. The IC resumes switching when input voltage
exceeds the UVLO-rising threshold.
The internal power MOSFET switch gate driver is
optimized to turn the switch on fast enough for low power
loss and good efficiency, but also slow enough to reduce
EMI. Switch turn-on is when most EMI occurs since VLX
rises rapidly. In some cases, it is desirable to reduce EMI
further, by the expense of some additional power
dissipation. The switch turn-on can be slowed by placing
a small (10Ω ≤ RBOOT ≤ 30Ω) resistance between BOOT
and the external bootstrap capacitor. This will slow the
high-side switch turn-on and VLX's rise. In order to improve
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6248B-01 May 2018
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