RT6222A/B
Operation
The RT6222A/B is a synchronous step-down converter
with advanced constant on-time control mode. Using the
ACOT control mode can reduce the output capacitance
and perform fast transient response. It can minimize the
component size without additional external compensation
network.
Soft-Start (SS)
The soft-start function is used to prevent large inrush
currents while the converter is being powered up. The
RT6222A/B provides an internal soft-start feature for inrush
control. During the start-up sequence, the internal capacitor
is charged by an internal current source I
SS
to generate a
soft-start ramp voltage as a reference voltage to the PWM
comparator. The device will initiate switching and the
output voltage will smoothly ramp up to its targeted
regulation voltage only after this ramp voltage is greater
than the feedback voltage V
FB
to ensure the converters
have a smooth start-up. The typical soft-start time is
0.8ms.
Output Under-Voltage Protection and Hiccup Mode
The RT6222A/B includes output under-voltage protection
(UVP) against over-load or short-circuited condition by
constantly monitoring the feedback voltage V
FB
. If V
FB
drops below the under-voltage protection trip threshold
(typically 75% of the internal feedback reference voltage),
the UV comparator will go high to turn off both the internal
high-side and low-side MOSFET switches.
If the output under-voltage condition continues for a period
of time, the RT6222A/B will enter output under-voltage
protection with hiccup mode. During hiccup mode, the IC
will shut down for t
HICCUP_OFF
(4.5ms), and then attempt
to recover automatically for t
HICCUP_ON
(1.5ms). Upon
completion of the soft-start sequence, if the fault condition
is removed, the converter will resume normal operation;
otherwise, such cycle for auto-recovery will be repeated
until the fault condition is cleared. Hiccup mode allows
the circuit to operate safely with low input current and
power dissipation, and then resume normal operation as
soon as the over-load or short-circuit condition is removed.
Abnormal case
detected (UV)
t
HICCUP_ON
= 1.5ms
EN
VOUT
t
HICCUP_OFF
= 4.5ms
t
FB
0.8ms
SW
t
UVLO Protection
To protect the chip from operating at insufficient supply
voltage, the UVLO is needed. When the input voltage of
VIN is lower than the UVLO falling threshold voltage, the
device will be lockout.
Thermal Shutdown
When the junction temperature exceeds the OTP
threshold value, the IC will shut down the switching
operation. Once the junction temperature cools down and
is lower than the OTP lower threshold, the converter will
autocratically resume switching.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6222A/B-01 August 2018
www.richtek.com
3
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
