RT6264A/B
Operation
The RT6264A/B is a high-efficiency, synchronous
step-down DC-DC converter that can deliver up to 4A
output current from a 4.5V to 18V input supply.
Advanced Constant On-Time Control and PWM
Operation
The RT6264A/B adopts ACOT
®
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 (RT6264A Only)
The RT6264A 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
VOUT
EN
0.3ms
1ms
VCC
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 RT6264A/B provides an internal soft-start feature
for inrush control. 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
converters have a smooth start-up from pre-biased
output. The output voltage starts to rise in 0.3ms from
EN rising, and the soft-start ramp-up time (V
FB
from 0V
to 0.765V) is 1ms.
VIN = 12V
VIN
VCC = 5V
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 RT6264A/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.
Input Under-Voltage Lockout
In addition to the EN pin, the RT6264A/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
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6264A/B-01
December 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
