RT6218A/B
18V, 2A, ACOT
TM
Step-Down Converter in 8 Pin TSOT-23
General Description
The
RT6218A/B
is
a
simple,
easy-to-use,
2A
synchronous step-down DC-DC converter with an input
supply voltage range of 4.5V to 18V. The device
integrates low R
DS(ON)
power MOSFETs to achieve
high efficiency in a SOT23 package and build-in
accurate reference voltage (0.765V by product
options).
The RT6218A/B adopts Advanced Constant On-Time
(ACOT
TM
) control architecture to provide an ultrafast
transient response with few external components and
to operate in nearly constant switching frequency over
the line, load, and output voltage range. The RT6218A
operates in automatic PSM that maintains high
efficiency during light load operation. RT6218B
operates in Forced PWM that helps meet tight voltage
regulation accuracy requirements.
The RT6218A/B senses both FETs current for a robust
over-current
protection. The
device features
cycle-by-cycle current limit protection and prevent the
device from the catastrophic damage in output short
circuit, over current or inductor saturation. The
RT6218A/B series offers programmable start-up by
connecting a capacitor at external SS pin. The device
also includes input under-voltage lockout, output
under-voltage protection, and over-temperature
protection (thermal shutdown) to provide safe and
smooth operation in all operating conditions. The
RT6218A/B are offered in TSOT-23-8(FC) package.
Features
Input Supply Voltage Range : 4.5V to 18V
2A Converter With Built-in 120m/65m Low
R
DS(
ON)
Power FETs
Advanced Constant On-Time (ACOT
TM
) Control
for Ultrafast Transient Response
Fixed Switching Frequency : 650kHz (SOT23-8)
Optional for Operation Modes :
Automatically
(RT6218A)
Power
Saving
Mode
(PSM)
Forced PWM Mode (RT6218B)
Optimized for Low-ESR Ceramic Output
Capacitors
1.5%
High-Accuracy Feedback Reference
Voltage
Output Voltage Range : up to 6.5V
Externally Adjustable Soft-Start
Monotonic Start-Up for Pre-biased Outputs
Both HS/LS FETs Protection for Robust Over
Current Protection
Input Under-Voltage Lockout (UVLO)
Output Under-Voltage Protection (UVP) with
Hiccup Mode
Over-Temperature Protection (OTP) (Thermal
Shutdown)
Enable Control
Power Good Indication
RoHS Compliant and Halogen Free
Available in TSOT-23-8 Package
Applications
Marking Information
RT6218AHRGJ8F
25=DNN
RT6218BHRGJ8F
26=DNN
25= : Product Code
DNN : Date Code
Set-Top Boxes
LCD TVs
Home Networking Devices
Surveillance
General Purpose
26= : Product Code
DNN : Date Code
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6218A/B-01
October
2018
www.richtek.com
1
RT6218A/B
Ordering Information
RT6218A/B
R
Package Type
J8F : TSOT-23-8 (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Reference Voltage
V
REF
= 0.765V
GND
Pin Configuration
(TOP VIEW)
BOOT
EN
FB
6
3
8
7
2
PWM Operation Mode
A : Automatic PSM
B : Forced PWM
TSOT-23-8 (FC)
Note :
Richtek products are :
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Functional Pin Description
Pin No.
1
2
3
4
Pin Name
GND
LX
VIN
PGOOD
Pin Function
Power ground. This pin, connected to analog ground, must be soldered to a
large PCB copper area for maximum power dissipation.
Switch node between the internal switch and the synchronous rectifier.
Connect this pin to the inductor and bootstrap capacitor.
Power input. The input voltage range is from 4.5V to 18V. Connect a suitable
input capacitor between this pin and GND, with a typical capacitance of 22F.
Open-drain power-good indication output. Once being started-up, PGOOD will
be pulled low to GND if any internal protection is triggered.
Soft-start capacitor connection node. Connect an external capacitor between
this pin and ground to set the soft-start time. Do not leave this pin
unconnected. A capacitor of 8.2nF is suggested.
Feedback voltage input. Connect this pin to the midpoint of the external
feedback resistive divider to set the output voltage of the converter to the
desired regulation level. The device regulates the FB voltage at Feedback
Threshold Voltage, typically 0.765V.
Enable control input. Floating this pin or connecting this pin to GND can disable
the device and connecting this pin to logic high can enable the device.
Bootstrap capacitor connection node to supply the high-side gate driver.
Connect a 0.1F ceramic capacitor between this pin and LX pin.
5
SS
6
FB
7
8
EN
BOOT
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS6218A/B-01
PGOOD
VIN
UVP Option
H : Hiccup
LX
SS
5
4
October
2018
RT6218A/B
Functional Block Diagram
VIN
6.5K
EN
3V
VEN_
REF
LX
-
+
UVLO
On-
Time
OC
Shutdown
Comparator
Internal
Regulator
VCC
PVCC
BOOT
UGATE
Control
Power
Stage &
Dead-time
Control
LX
UV
Protection
VCC
6µA
SS
FB
Ripple
Gen.
+
+
-
-
+
LGATE
MIN OFF
Comparator
VCC
PGOOD
Comparator
VIBIAS
VRef
GND
0.9V
REF
PGOOD
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6218A/B-01
October
2018
www.richtek.com
3
RT6218A/B
Operation
The RT6218A/B is a high-efficiency, synchronous
step-down DC-DC converter that can deliver up to 2A
output current from a 4.5V to 18V input supply. The
RT6218A/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.
Enable Control
The RT6218A/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
ENH
V
EN
) 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 supply current
can be reduced to I
SHDN
(5A or below). If the EN
voltage rises above the logic-high threshold voltage
(V
ENH
) while the VIN voltage is higher than UVLO
threshold (V
UVLO
), the device will be turned on, that is,
switching being enabled and soft-start sequence being
initiated.
Low-Side Current Limit Protection
The RT6218A/B features a cycle-by-cycle valley-type
current limit protection, measuring the inductor current
through the synchronous rectifier (low-side switch). The
inductor current level is determined by measuring the
low-side switch voltage between the LX pin and GND,
which is proportional to the switch current, during the
low-side on-time. For greater accuracy, temperature
compensation is added to the voltage sensing. Once
the current rises above the low-side switch valley
current limit (I
LIM
), the on-time one-shot will be inhibited
until the inductor current ramps down to the current
limit level (I
LIM
), that is, another on-time can only be
triggered when the inductor current goes below the
low-side current limit. This function can prevent the
average output current from greatly exceeding the
guaranteed low-side current limit value.
If the output load current exceeds the available inductor
current (clamped by the above-mentioned low-side
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
current limit), the output capacitor needs to supply the
extra current such that the output voltage will begin to
drop. If it drops below the output under-voltage
protection trip threshold, the IC will stop switching to
avoid excessive heat.
High-Side Current Limit Protection
The RT6218A/B also includes a cycle-by-cycle
peak-type current limit protection against the condition
that the inductor current increasing abnormally, even
over the inductor saturation current rating. To ensure
the low-side current limit protection can still function
properly, the inductor current through the high-side
switch will only be measured after a certain amount of
delay when the high-side switch being turned on. If an
over-current condition occurs, the converter will
immediately turn off the high-side switch and turn on
the low-side switch to prevent the inductor current
exceeding the high-side switch peak-current limit.
Output Under-Voltage Protection and Hiccup Mode
The RT6218A/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 62.5% 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 RT6218A/B will enter output
under-voltage protection with hiccup mode. During
hiccup mode, the IC will shut down for t
HICCUP_OFF
(C
SS
x 6.3 / I
SS
), and then attempt to recover
automatically for t
HICCUP_ON
(C
SS
x 0.85 / 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.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6218A/B-01
October
2018
RT6218A/B
Abnormal case
detected (UV)
VOUT
t
HICCUP_ON
= C
SS
x 0.85 / I
SS
t
HICCUP_OFF
= C
SS
x 6.3 / I
SS
t
EN
SS = 1.9V
SS
V1
FB
LX
t
T1
T2
T3
External Bootstrap Capacitor
PG
Connect a 0.1F low-ESR ceramic capacitor between
the BOOT and LX pins. This bootstrap capacitor
supplies for the gate driver of the high-side N-channel
MOSFET switch.
Over-Temperature Protection (Thermal Shutdown)
The RT6218A/B includes an over-temperature
protection (OTP) circuitry to prevent overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature
exceeds a thermal shutdown threshold T
SD
. Once the
junction temperature cools down by a thermal
shutdown hysteresis (T
SD
), the IC will resume normal
operation with a complete soft-start.
Soft-Start
The RT6218A/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 CSS between
SS and GND. An internal current source I
SS
(6A)
charges an external capacitor to build a soft-start ramp
voltage. The VFB voltage will track the internal ramp
voltage during soft-start interval. The typical soft-start
time is calculated as follows :
Soft-Start time t
SS
= 55s + C
SS
x 0.55 / 11A + C
SS
x
0.86 / I
SS
t
SS
= SS rising to FB settled (T2 + T3)
T1 : EN delay, from EN go high to SS start rising, T1 = 55μs;
T2 : speed up SS, from SS rising to FB start rising, T2 = C
SS
x 0.55 / 11μA;
T3 : normal SS, from FB rising to settled, T3 = C
SS
x 0.86 / I
SS
;
V1 : offset voltage between SS and FB, V1 = 700mV;
PG go high after SSOK (SS = 1.9V)
Power Good Indication
The PGOOD pin is an open-drain output and is
connected to an external pull-up resistor. It is controlled
by a comparator, which the feedback signal V
FB
is fed
to. If V
FB
is above 90% of the internal reference voltage
after soft-start finished, the PGOOD pin will be in high
impedance and V
PGOOD
will be held high. Otherwise,
the PGOOD output will be pulled low.
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6218A/B-01
October
2018
www.richtek.com
5
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