®
RT6268
3.7A, 36V, 100kHz Asynchronous Step-Down Converter with
Load Line Compensation
General Description
The RT6268 is a high-efficiency, monolithic asynchronous
step-down DC/DC converter that can deliver up to 3.7A
output current from a 7V to 36V input supply. The RT6268's
current mode architecture with internal compensation is
optimized for 5V car charger application over a wide range
of loads and output capacitors. Cycle-by-cycle current limit
provides protection against shorted outputs and soft-start
eliminates input current surge during start-up. The RT6268
also provides output over voltage protection output under
voltage protection and thermal shutdown protection. The
low current (<3μA) shutdown mode provides output
disconnect, enabling easy power management in battery-
powered systems. The RT6268 is available in a SOP-8
(Exposed Pad) package.
Features
±
1% High Accuracy Feedback Voltage
7V to 36V Input Voltage Range
3.7A Peak Output Current
CC/CV Mode Control
Adjustable Load Line Compensation
Short Circuit Protection
Integrated N-MOSFET Switches
Current Mode Control
Fixed Frequency Operation : 100kHz
Programmable Output Current Limit
Ω
60mΩ Internal Power MOSFET Switch
Low EMI signature
Up to 95% Efficiency
Cycle-by-Cycle Over Current Protection
Input Under Voltage Lockout
Output Under Voltage Protection
Thermal Shutdown Protection
Ordering Information
RT6268
Package Type
SP: SOP-8(Exposed Pad-Option 2)
Note :
½
Applications
Lead Plating System
G : Green (Halogen Free and Pb Free)
Richtek products are :
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
½
USB Power Supplies
Automotive Cigarette Lighter Adapters
Power Supply for Linear Chargers
DC/DC Converters with Current Limited
Suitable for use in SnPb or Pb-free soldering processes.
Pin Configurations
(TOP VIEW)
VIN
BOOT
EN
FB
2
3
4
8
Marking Information
RT6268GSP : Product Number
SW
GND
CSP
CSN
RT6268
GSPYMDNN
YMDNN : Date Code
GND
9
7
6
5
SOP-8 (Exposed Pad)
Simplified Application Circuit
V
IN
C
IN
R
EN
VIN
C
BOOT
L1
D1
R
SENSE
C
OUT
V
OUT
SW
RT6268
GND
BOOT
EN
FB
CSP
CSN
R1
R2
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6268-00
September 2015
www.richtek.com
1
RT6268
Functional Pin Description
Pin No.
1
2
3
Pin Name
VIN
BOOT
EN
Pin Function
Input Supply Voltage, 7V to 36V. Must bypass with a suitably large ceramic
capacitor.
Bootstrap for High-Side Gate Driver. Connect 0.1F or greater ceramic
capacitor from BOOT to SW pins.
Enable Input Pin. A logic high enables the converter; a logic low forces the
RT6268 into shutdown mode reducing the supply current to less than 3A.
Attach this pin to VIN with a 100k pull up resistor for automatic startup.
Feedback Input Pin. This pin is connected to the converter output. It is used to
set the output of the converter to regulate to the desired value via an external
resistive voltage divider. For an adjustable output, an external resistive divider
is connected to this pin.
Current Sense Negative Input. It is used for load current limiting and load line
drop compensation.
Current Sense Positive Input. It is used for load current limiting and load line
drop compensation.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation. (Connect the exposed pad to Pin 7)
Switch Output -- Connect to external L-C filter and Schottky diode.
4
FB
5
6
7,
9 (Exposed Pad)
8
CSN
CSP
GND
SW
Function Block Diagram
EN
VIN
Internal
Regulator
VCC
VCC
VIBIAS
Oscillator
Ramp
Foldback
Control
OC
GND
UV&OV
Current
Control
Comp
UGATE
Driver
LGATE
Current
Limit
SW
Current
Sense
VCC
V
REF
EN
BOOT
+
-
2Meg
80p
1p
SS
Circuit
+
+
EA
-
100mV
+
-
1Meg
10p
1Meg
CSP
CSN
0.8V
Line
Comp
+
EA
-
FB
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS6268-00
September 2015
RT6268
Operation
The RT6268 is a constant frequency, current mode
asynchronous step-down converter with CC and CV
control. In normal operation, the high side N-MOSFET is
turned on when the S-R latch is set by the oscillator and
is turned off when the current comparator resets the S-R
latch. While the N-MOSFET is turned off, the inductor
current conducts through the external diode.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (V
FB
) with the internal 0.8V reference.
When the load current increases, it causes a drop in the
feedback voltage relative to the reference, the error
amplifier's output voltage then rises to allow higher inductor
current to match the load current.
Oscillator
The internal oscillator runs at fixed frequency 100kHz. In
short circuit condition, the frequency is reduced to 20kHz
for low power consumption.
Internal Regulator
The regulator provides low voltage power to supply the
internal control circuits and the bootstrap power for high
side gate driver.
Enable
The converter is turned on when the EN pin is higher than
2.5V and turned off when the EN pin is lower than 0.4V.
Attach this pin to VIN with a 100kΩ pull up resistor for
automatic startup.
Soft-Start (SS)
An internal current source charges an internal capacitor
to build a soft-start ramp voltage. The FB voltage will track
the internal ramp voltage during soft-start interval. The
typical soft-start time is 3.5ms.
Output Line Drop Compensation
If the trace from RT6268 output terminator to the load is
too long, there will be a voltage drop on the long trace
which is variable with load current. RT6268 is capable of
compensating the output voltage drop to keep a constant
voltage at load, whatever the load current is. The output
voltage is compensated by feeding a current to the top
feedback resistance R1. The load line compensation gain
can be programmed according to R
SENSE
and Rtrace values.
R
SENSE
Rtrace1
Rtrace2
CSP
CSN
FB
R1
Vload
+
I
OUT
_
R2
I
OUT
R
SENSE
20μ
R1
½
I
OUT
Rtrace
R1
½
Rtrace
20μ
R
SENSE
Output Over Voltage Protection (OVP)
The V
OUT
Over Voltage is sensed by CSN pin. When CSN
> 5.8V, the high side switch will be turned off immediately.
When CSN < 5.5V, the driver will recovers to normal state
automatically.
External Current Limit Protection
The external current limit is set by outside resistance
(R
SENSE
). The average current is limited according to the
following equation :
Average Current_Limit (A)
½
100mV
R
SENSE
Internal Current Limit Protection
When the external R
SENSE
is
too small and the external
peak current is higher than 5.8A, the high-side switch will
turn off immediately and then turn on at the next clock
cycle. The inductor’s peak current will be limited at 5.8A
by internal current limit.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6268-00
September 2015
www.richtek.com
3
RT6268
Output Short-Circuit Protection
When V
OUT
is short (V
FB
< 0.3V), the short-circuit
protection function can be started that restart the regulator
cycle by cycle. The cycle time is set by the driver internally.
The internal current limit time is t1 and the regulator off
time is t2. The typically t1 = 5ms, t2 = 200ms.
I
OUT
(A)
Time
t1
t2
Under Voltage Lockout (UVLO)
To avoid mis-operation
at low input voltage, when input
voltage falls below 6.1V, the under voltage lockout is
induced and the device is disabled.
Thermal Shutdown
The over temperature protection function will shut down
the switching operation when the junction temperature
exceeds 150°C. Once the junction temperature cools
down by approximately 30° C, the converter will
automatically resume switching.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6268-00
September 2015
RT6268
Absolute Maximum Ratings
(Note 1)
−0.3V
to 40V
−0.3V
to 40V
−0.3V
to 6V
−0.3V
to 6V
2.041W
Supply Input Voltage ------------------------------------------------------------------------------------------------------
Switch Voltage, SW ------------------------------------------------------------------------------------------------------
V
BOOT
- V
SW
------------------------------------------------------------------------------------------------------------------
EN, FB, CSP, CSN -------------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
SOP-8 (Exposed Pad) ---------------------------------------------------------------------------------------------------
Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad),
θ
JA
---------------------------------------------------------------------------------------------- 49°C/W
SOP-8 (Exposed Pad),
θ
JC
--------------------------------------------------------------------------------------------- 15C/W
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------- 260°C
Junction Temperature ----------------------------------------------------------------------------------------------------- 150°C
Storage Temperature Range --------------------------------------------------------------------------------------------
−65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) ---------------------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Input Voltage ------------------------------------------------------------------------------------------------------ 7V to 36V
Junction Temperature Range --------------------------------------------------------------------------------------------
−40°C
to 125°C
Ambient Temperature Range --------------------------------------------------------------------------------------------
−40°C
to 85°C
Electrical Characteristics
(V
IN
= 12V, V
OUT
= 5V, T
A
= 25°C, Load Current = 0A,
unless otherwise specified)
Parameter
V
OUT
OVP Detect Voltage
V
OUT
OVP Hysteresis
Shutdown Supply Current
Supply Current
Feedback Voltage
High-Side Switch On-Resistance
Low-Side Switch On-Resistance
Upper Switch Current Limit
Load Line Compensation Gain
Current Sense Voltage
Oscillation Frequency
Minimum Off-Time
Minimum On-Time
Symbol
V
OVP
VOVP
I
SD
I
Q
V
FB
R
DS(ON),U
R
DS(ON),L
Test Conditions
Normal Operation.
Normal Operation.
V
EN
= 0V
V
EN
= 3 V, V
FB
= 0.9V
7V = V
IN
= 36V
Min
--
--
--
--
0.792
--
--
Typ
5.8
0.3
1.5
0.8
0.8
60
20
0
5.8
20
100
100
20
200
150
Max
--
--
3
1.2
0.808
--
--
10
--
25
102
115
--
--
--
Unit
V
V
A
mA
V
m
A
A
A/V
mV
kHz
kHz
ns
ns
High-Side Switch Leakage Current I
SWLEAK
I
LIM
G
LC
V
SENSE
f
OSC1
V
EN
= 0V, V
SW
= 0V
V
CSP
V
CSN
= 100mV, check I
FB
V
CSP
V
CSN
V
FB
= 0V
--
--
15
98
85
--
--
--
Short Circuit Oscillation Frequency f
OSC2
t
OFF
t
ON
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6268-00
September 2015
www.richtek.com
5
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