RT6233A/B
Operation
The RT6233A/B is a synchronous step-down converter
with advanced constant on-time control mode. Using
the ACOT
TM
control mode can reduce the output
capacitance and provide fast transient response. It can
minimize the component size without additional
external compensation network.
Current Protection
The inductor current is monitored via the internal
switches cycle-by-cycle. Once the output voltage drops
under UV threshold, the RT6233A/B will enter hiccup
mode.
Soft-Start
The RT6233A/B provides adjustable soft-start function.
When the EN pin becomes high, the SS charge current
(I
SS
) begins charging the capacitor which is connected
from the SS pin to GND (C
SS
).The soft-start function is
used to prevent large inrush current while converter is
being powered-up. The soft-start timing can be
programmed by the external capacitor C
SS
between
SS and GND. An internal current source I
SS
(1A)
charges an external capacitor to build a soft-start ramp
voltage. The V
FB
voltage will track the soft-start ramp
voltage during soft-start interval. The typical soft-start
time is calculated as follows :
Soft-Start time t
SS
= T1 + T2 = 50s + C
SS
x 0.9 / I
SS
Time from EN high to PG signal ready = 50s + C
SS
x
0.9 / I
SS
+ C
SS
x 0.65 / I
SS
LX
Output Under-Voltage Protection and Hiccup
Mode
The RT6233A/B includes output
protection (UVP) against over-load or
condition by constantly monitoring
voltage V
FB
. If V
FB
drops below the
under-voltage
short-circuited
the feedback
under-voltage
protection trip threshold (typically 50% 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 RT6233A/B
will enter output under-voltage protection with hiccup
mode. During hiccup mode, the IC will shut down for
tHICCUP_OFF
(C
SS
x 3.6/I
SS
), and then attempt to
recover automatically for
tHICCUP_ON
(C
SS
x 1.3/I
SS
).
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)
VOUT
t
HICCUP_ON
= C
SS
x 1.3/I
SS
t
HICCUP_OFF
= CSS x 3.6/ISS
t
EN
t
SS = 1.55V
SS
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.
FB
T1
T2
Thermal Shutdown
PG
T1 : EN delay, from EN go high to SS start rising, T1 = 50μs;
T2 : normal SS, from FB rising to settled, T2 = C
SS
x 0.9/I
SS
;
PG go high after SSOK (SS = 1.55V)
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
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.
is a registered trademark of Richtek Technology Corporation.
DS6233A/B-02
December
2018
www.richtek.com
3
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
