RT5779A/B
Functional Block
Diagram
EN
VOUT
UVLO
OTP
Shutdown
Control
Comparator
+
-
AGND
TON
LX
VIN
FB
Error Amplifier
+
-
+
Logic
Control
Current
Limit
Detector
AZC
Driver
LX
V
REF
PGOOD
-
+
Ramp
Generator
LX
LX
PGND
FB
Operation
The RT5779A/B is a low-voltage, high-efficiency,
synchronous step-down DC-DC converter that can deliver
up to 5A output current from a 2.5V to 6V input supply.
The RT5779A/B adopts ACOT
TM
control mode, which can
reduce the output capacitance and provide ultrafast
transient responses, and allow minimal component sizes
without any additional external compensation network. The
device includes a built-in ramp voltage generator, which
takes up the virtual inductor current as an input. With the
internal ramp signal, the device can be compensated to
achieve good stability even with low-ESR ceramic
capacitors, since the need for the output capacitor's ESR
to generate an ESR ramp voltage can be eliminated.
Low V
IN
ACOT
TM
One-Shot Operation
For a low V
IN
ACOT converter, a built-in error amplifier is
used to keep track of the feedback voltage, as shown in
the Functional Block Diagram. In steady state, the error
amplifier compares the feedback voltage V
FB
and an
internal reference voltage. If the virtual inductor current
ramp voltage is lower than the output of the error amplifier,
a new pre-determined fixed on-time will be triggered by
the on-time one-shot generator, provided that minimum-
off-time one-shot is cleared and the measured inductor
current through the synchronous rectifier (low-side switch)
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
is below the current limit I
LIM_L
. During the on-time, the
high-side switch is turned on and the inductor current
ramps up linearly. After the on-time, the high-side switch
is turned off and the synchronous rectifier is turned on
and the inductor current ramps down linearly. If the output
voltage has not reached its nominal level, another on-time,
however, can only be generated after a short blanking time,
or called minimum off-time, which is triggered by the
minimum-off-time one-shot generator. It is to prevent
another immediate on-time being triggered during the noisy
switching time and allow the feedback voltage and current
sense signals to settle. The minimum off-time t
OFF_MIN
is
kept short so that the inductor current can be raised up
quickly by rapidly repeated on-times when needed. Such
feature makes reaction speed of the ACOT-based
converters to load transients extremely fast.
Enable Control
The RT5779A/B provides an EN pin, as an external chip
enable control, to enable or disable the device. If V
EN
is
held below a logic-low threshold voltage (V
ENL
) of the
enable input (EN), the converter will enter into shutdown
mode, 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
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS5779A/B-00 October 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
