RT6252A/B
17V Input, 2A, ACOT
TM
Buck Converter with Both FETs OC
Protection
General Description
The RT6252A/B is a simple, easy-to-use, 2A
synchronous step-down DC-DC converter with an input
supply voltage range of 4.5V to 17V. The device
possesses an accurate reference voltage and
integrates low R
DS(ON)
power MOSFETs to achieve
high efficiency.
The RT6252A/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 RT6252A
operates in automatic PSM that maintains high
efficiency during light load operation. The RT6252B
operates in Forced PWM that helps meet tight voltage
regulation accuracy requirements.
The RT6252A/B senses both FETs current for a robust
over-current protection (OCP). The device features
cycle-by-cycle current limit protection to prevent the
device from the catastrophic damage in output short
circuit, over-current or inductor saturation conditions. A
built-in soft-start function prevents inrush current during
start-up. The device also includes input under-voltage
lockout, output under-voltage protection, and
over-temperature protection (OTP) to provide safe and
smooth operation in all operating conditions.
Features
2A Converter Integrated 140m and 84m FETs
Input Supply Voltage Range : 4.5V to 17V
Maximum Output Voltage Range : 7V
Advanced Constant On-Time (ACOT
TM
) Control
Ultrafast Transient Response
Optimized
for Low-ESR Ceramic Output
Capacitors
High Accuracy Feedback Reference Voltage : Typ.
1%
Optional for Operation Modes :
Automatically
(RT6252A)
Power
Saving
Mode
(PSM)
Forced PWM Mode (RT6252B)
Fixed Switching Frequency : 580kHz
Enable Control and Internally Fixed Soft-Start
Safe Start-Up from Pre-biased Output
Input Under-Voltage Lockout (UVLO)
Output Under-Voltage Protection (UVP) with
Hiccup Mode
High- /Low-side MOSFET OCP and OTP Function
RoHS Compliant and Halogen Free
Simplified Application Circuit
RT6252A/B
V
IN
C
IN
SW
Enable
EN
GND
1
FB
R
FB2
R
FB1
C
FF
VIN
BOOT
C
BOOT
L
V
OUT
C
OUT
R
BOOT
(Optional)
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6252A/B-00
July 2019
www.richtek.com
1
RT6252A/B
Ordering Information
RT6252A/B
Package Type
J6F : TSOT-23-6 (FC)
H6F : SOT-563 (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
UVP Option
H : Hiccup
PWM Operation Mode
A : Automatic PSM
B : Forced PWM
Note :
2
3
Applications
Set-Top Boxes
LCD TVs
Home Networking Devices
Surveillance
General Purpose
Pin Configuration
(TOP VIEW)
BOOT EN
6
5
FB
4
Richtek products are :
GND SW VIN
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
TSOT-23-6 (FC)
FB
EN
BOOT
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
6
5
4
RT6252AHGJ6F
34=DNN
RT6252BHGJ6F
34= : Product Code
DNN : Date Code
1
2
3
VIN
SW
GND
33=DNN
33= : Product Code
DNN : Date Code
SOT-563 (FC)
RT6252AHGH6F
01W
RT6252BHGH6F
01 : Product Code
W : Date Code
00W
00 : Product Code
W : Date Code
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS6252A/B-00
July 2019
RT6252A/B
Functional Pin Description
Pin No.
TSOT23-6(FC) SOT-563 (FC)
1
2
3
2
Pin Name
GND
SW
Power ground.
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 17V. Connect
input bypass capacitors directly to this pin and GND pins. The
MLCC with capacitance higher than 20F is recommended.
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 reference voltage, typically 0.765V.
Enable control input. Connect this pin to logic high enables the
device and connect this pin to GND disables the device.
Bootstrap capacitor connection node to supply the high-side gate
driver. Connect a 0.1F ceramic capacitor between this pin and
the SW pin.
Pin Function
3
1
VIN
4
6
FB
5
5
EN
6
4
BOOT
Functional Block Diagram
VIN
SW
EN
VEN_TH
R
EN_DN
+
-
UVLO
On-
Time
OC
Internal
Regulator
VCC
PVCC
BOOT
UV
Protection
65%
Control
Soft-Start
Ramp
Gen.
+
+
-
Gate Driver &
Dead-Time
Control
SW
FB
Comparator
MIN OFF
GND
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6252A/B-00
July 2019
www.richtek.com
3
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
is a registered trademark of Richtek Technology Corporation.
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4
DS6252A/B-00
July 2019
RT6252A/B
prevented. After the device is powered up, if the input
voltage VIN goes below the UVLO falling threshold
voltage (V
UVLO
V
UVLO
), this switching will be
inhibited; if VIN rises above the UVLO rising threshold
(V
UVLO
), the device will resume normal operation with a
complete soft-start.
Output Under-Voltage Protection and Hiccup Mode
The RT6252A/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 65% 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 RT6252A/B will enter output
under-voltage protection with hiccup mode. During
hiccup mode, the IC will shut down for t
HICCUP_OFF
(15ms), and then attempt to recover automatically for
t
HICCUP_ON
(1.8ms). 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. The hiccup mode allows the
circuit to operate safely with low input current and
power dissipation, and then the converter resumes
normal operation as soon as the over-load or
short-circuit condition is removed.
Output short
MOSFETs and prevents the device from the
catastrophic
damage
in
output
short-circuit,
over-current or inductor saturation conditions.
The high-side MOSFET over-current protection is
achieved by an internal current comparator that
monitors the current in the high-side MOSFET during
each on-time. The switch current is compared with the
high-side switch peak-current limit (I
LIM_H
) after a
certain amount of delay when the high-side switch
being turned on each cycle. If an over-current condition
occurs, the converter will immediately turns off the
high-side switch and turns on the low-side switch to
prevent the inductor current exceeding the high-side
current limit.
The low-side MOSFET over-current protection is
achieved by measuring the inductor current through the
synchronous rectifier (low-side switch) during the
low-side on-time. Once the current rises above the
low-side switch valley current limit (I
LIM_L
), the on-time
one-shot will be inhibited until the inductor current
ramps down to the current limit level (I
LIM_L
), that is,
another on-time can only be triggered when the
inductor current goes below the low-side current limit. If
the output load current exceeds the available inductor
current (clamped by the low-side 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.
Negative Over-Current Limit
V
OUT
, 2V/Div
Fault condition removed
Resume normal operation
The RT6252B is the part which is forced to PWM and
allows negative current operation.
In case of PWM operation, high negative current may
be generated as an external power source is tied to
output terminal unexpectedly. As the risk described
above, the internal circuit monitors negative current in
each on-time interval of low-side MOSFET and
compares it with NOC threshold.
Once the negative current exceeds the NOC threshold,
the low-side MOSFET is turned off immediately, and
then the high-side MOSFET will be turned on to
discharge the energy of output inductor. This behavior
can keep the valley of negative current at NOC
threshold to protect low-side MOSFET. However, the
is a registered trademark of Richtek Technology Corporation.
I
SW
, 4A/Div
V
SW
, 10/Div
10ms/Div
The Over-Current Protection
The RT6252A/B features cycle-by-cycle current-limit
protection on both the high-side and low-side
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
DS6252A/B-00
July 2019
www.richtek.com
5
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