RT6252A/B
Operation
The RT6252A/B is a high-efficiency, synchronous
step-down DC-DC converter that can deliver up to 2A
output current from a 4.5V to 17V input supply.
Advanced Constant On-Time Control and PWM
Operation
The RT6252A/B adopts ACOT
TM
control for its ultrafast
transient response, low external component counts and
stable with low ESR MLCC output capacitors. When
the feedback voltage falls below the feedback
reference voltage, the minimum off-time one-shot
(200ns, typ.) has timed out and the inductor current is
below the current limit threshold, then the internal
on-time one-shot circuitry is triggered and the high-side
switch is turn-on. Since the minimum off-time is short,
the device exhibits ultrafast transient response and
enables the use of smaller output capacitance.
The on-time is inversely proportional to input voltage
and directly proportional to output voltage to achieve
pseudo-fixed frequency over the input voltage range.
After the on-time one-shot timer expired, the high-side
switch is turned off and the low-side switch is turned on
until the on-time one-shot is triggered again. To
achieve stable operation with low-ESR ceramic output
capacitors, an internal ramp signal is added to the
feedback reference voltage to simulate the output
voltage ripple.
Power Saving Mode (RT6252A Only)
The RT6252A automatically enters power saving mode
(PSM) at light load to maintain high efficiency. As the
load current decreases, the inductor current ripple
valley eventually touches the zero current, which is the
boundary between continuous conduction and
discontinuous conduction modes. The low-side switch
is turned off when the zero inductor current is detected.
In this case, the output capacitor is only discharged by
load current so that the switching frequency decreases.
As the result, the light-load efficiency can be enhanced
due to lower switching loss.
Enable Control
The RT6252A/B provides an EN pin, as an external
chip enable control, to enable or disable the device. If
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
V
EN
is held below a logic-low threshold voltage (V
EN_L
)
of the enable input (EN), the converter will disable
output voltage, that is, the converter is disabled and
switching is inhibited even if the VIN voltage is above
VIN under-voltage lockout threshold (V
UVLO
). During
shutdown mode, the supply current can be reduced to
I
SHDN
(10A or below). If the EN voltage rises above
the logic-high threshold voltage (V
EN_H
) while the VIN
voltage is higher than UVLO threshold, the device will
be turned on, that is, switching being enabled and
soft-start sequence being initiated. An internal resistor
R
EN_DN
from EN to GND allows EN float to shutdown
the chip.
Soft-Start (SS)
The RT6252A/B provides an internal soft-start feature
for inrush control, and the output voltage starts to rise
in 0.3ms from EN rising edge. At power up, the internal
capacitor is charged by an internal current source 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 converter has a smooth start-up from
pre-biased output.
V
IN
= 12V
VIN
V
CC
= 5V
VCC
EN
0.3ms
t
SS
VOUT
Input Under-Voltage Lockout
In addition to the EN pin, the RT6252A/B also provides
enable control through the VIN pin. It features an
under-voltage lockout (UVLO) function that monitors
the internal linear regulator (VCC). If V
EN
rises above
V
EN_H
first, switching will still be inhibited until the VIN
voltage rises above V
UVLO
. It is to ensure that the
internal regulator is ready so that operation with
not-fully-enhanced internal MOSFET switches can be
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www.richtek.com
4
DS6252A/B-00
July 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
