RT6257A/B
Functional Block Diagram
BOOT
V
IN
Reg
Minoff
VIBIAS
V
REF
OC
UV
V
CC
Ripple
Gen.
GND LX
EN
+
+
-
V
IN
On-Time
LX
Control
Driver
LGATE
GND
EN
V
CC
UGATE
LX
VIN
V
CC
LX
Comparator
FB
Operation
The RT6257A/B is a high-efficiency, monolithic
synchronous step-down DC-DC converter that can deliver
up to 6A output current from a 4.5V to 18V input supply.
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.
Current Limit
The RT6257A/B current limit is a cycle-by-cycle
“valley”
type, measuring the inductor current through the
synchronous rectifier during the off-time while the inductor
current ramps down. The current is determined by
measuring the voltage between Source and Drain of the
synchronous rectifier, adding temperature compensation
for greater accuracy. If the current exceeds the current
limit, the on-time one-shot is inhibited until it drops below
the current limit level. If the output current exceeds the
available inductor current (controlled by the current limit
mechanism), the output voltage will drop. If it drops below
the output under-voltage protection level (see next section)
the IC will stop switching to avoid excessive heat.
Output Under-Voltage Protection and Hiccup Mode
The RT6257A/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 60% 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 RT6257A/B will enter output under-voltage
protection with hiccup mode. It is shown as Figure 1. During
hiccup mode, the IC will shut down for t
HICCUP_OFF
(5ms),
and then attempt to recover automatically for t
HICCUP_ON
(2.5ms). 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.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6257A/B-03 October 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
