RT2855A/B
Soft-Start (SS)
Over Temperature Protection
The RT2855A/B soft-start uses an external pin (SS) to
clamp the output voltage and allow it to slowly rise. After
VEN is high and VREG5 exceeds its UVLO threshold, the
IC begins to source 6μA from 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 RT2855A/B includes an over temperature protection
(OTP) circuitry to prevent overheating due to excessive
power dissipation. The OTP will shut down switching
operation when the junction temperature exceeds 150°C.
Once the junction temperature cools down by
approximately 20°C the IC will resume normal operation
with a complete soft-start. For continuous operation,
provide adequate cooling so that the junction temperature
does not exceed 150°C.
During start-up, while the SS capacitor charges, the
RT2855A/B operate in discontinuous 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 2.2Voutput over-and under-voltage protections
are enabled and the RT2855B begins continuous-switching
operation.
Output Discharge Control
When EN pin is low, the RT2855A/B will discharge the
output with an internal 50Ω MOSFET connected between
VOUT toGND pin.
OVP/UVP Protection
The RT2855A/B 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.
An internal linear regulator (VREG5) 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, VREG5 is guaranteed to provide significant
power for external loads.
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.
External Bootstrap Capacitor
Connect a 0.1μF low ESR ceramic capacitor between
BOOT and SW. This bootstrap capacitor provides the gate
driver supply voltage for the high sideN-channel MOSFET
switch.
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS2855A/B-01 September 2015
www.richtek.com
7
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
