RT2858B
Operation
In normal operation, the high-side N-MOSFET is turned
on when the FB Comparator sets the Switch Controller,
and it is turned off when On-Time Controller resets the
Switch Controller. While the high-side N-MOSFET is turned
off, the low-side N-MOSFET is turned on and waits for the
FB Comparator to set the beginning of next cycle.
The FB Comparator sets the Switch Controller by
comparing the feedback signal (FB) from output voltage
with the internal 0.765V reference. When load transient
induces VOUT drop, the FB voltage will be less than its
threshold voltage. This means that the high-side N-
MOSFET will turn on again immediately after minimum
off-time expired. The switching frequency will vary during
the transient period thus can provide a very fast transient
response. After the load transient finished, the RT2858B
will be back to steady state with a constant switching
frequency.
Enable
Activate internal regulator once EN input level is higher
than the target level. Force IC to enter shutdown mode
when the EN input level is lower than 0.4V
Internal Regulator
Provide internal power for logic control and switch gate
drivers.
On-Time Controller
Control on-time according to VIN and SW to obtain
constant switching frequency.
OVP/UVP Protection
The RT2858B detects over and under voltage conditions
by monitoring the feedback voltage on FB pin. The two
functions are enabled after approximately 1.7 times the
soft-start time. When the feedback voltage becomes
higher than 120% of the target voltage, the OVP
comparator will go high to turn off both internal high side
and low side MOSFETs. When the feedback voltage is
lower than 70% of the target voltage for 250μs, the UVP
comparator will go high to turn off both internal high side
and low side MOSFETs.
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2858B-00
September 2013
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
