RT6238A/B
Input Under-Voltage Lockout
PGOOD Comparator
In addition to the enable function, the RT6238A/B feature
an Under-Voltage Lockout (UVLO) function that monitors
the internal linear regulator output (VIN). To prevent
operation without fully-enhanced internal MOSFET
switches, this function inhibits switching when VIN drops
below the UVLO-falling threshold. The IC resumes
switching when VIN exceeds the UVLO-rising threshold
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
toGND.
External Bootstrap Capacitor (CBOOT
)
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.
Soft-Start (SS)
The RT6238A/B soft-start uses an external pin (SS) to
clamp the output voltage and allow it to slowly rise. After
V
EN is high and VIN exceeds its UVLO threshold, the IC
Some of case, such like duty ratio is higher than 65%
application or input voltage is lower than 5.5V which are
recommended to add an external bootstrap diode between
an external 5V and BOOT pin for efficiency improvement
The bootstrap diode can be a low cost one such as IN4148
or BAT54. The external 5V can be a 5V fixed input from
system or a 5V output of the RT6238A/B. Note that the
external boot voltage must be lower than 5.5V
begins to source 6μAfrom the SS pin.An external capacitor
at SS is used to adjust the soft-start timing. Following
below equation to get the minimum capacitance range in
order to avoid UV occur.
C
I
V
0.751.2
OUT
OUT
T =
C
Load Current 0.8
LIM
T6μA
SS
V
REF
Over-Temperature Protection
Do not leave SS unconnected. During start-up, while the
SS capacitor charges, the RT6238A/B operates 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 RT6238A/B begins continuous-switching operation.
The RT6238A/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.
Internal Regulator (PVCC)
An internal linear regulator (PVCC) produces a 5V supply
from VIN. The 5V power supplies the internal control
circuit, such as internal gate drivers, PWM logic, reference,
analog circuitry, and other blocks. 1μF ceramic capacitor
for decoupling and stability is required.
Copyright 2017 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6238A/B-08 July 2017
www.richtek.com
7
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
