RT6266 - Richtek

RT6266
2.4A, 36V, 100kHz Asynchronous Step-Down Converter with
Load Line Compensation
General Description
Features
The RT6266 is a high-efficiency, monolithic
asynchronous step-down DC/DC converter that can
deliver up to 2.4A output current from a 7.5V to 36V
input supply. The RT6266’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
RT6266 also provides output over voltage protection
output under voltage protection and thermal shutdown
protection. The low current (<3A) shutdown mode
provides output disconnect, enabling easy power
management in battery-powered systems. The RT6266
is available in a SOP-8 (Exposed Pad) package.
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Ordering Information

2% High Accuracy Feedback Voltage
7.5V to 36V Input Voltage Range
2.4A Continuous Output Current (2.7A Peak)
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
110m Internal Power MOSFET Switch
Lowe EMI signature
Up to 95% Efficiency
Cycle-by-Cycle Over Current Protection
Input Under Voltage Lockout
Output Under Voltage Protection
Thermal Shutdown Protection
RT6266
Package Type
SP: SOP-8(Exposed Pad-Option 2)
Applications

Lead Plating System
G : Green (Halogen Free and Pb Free)


Note :

USB Power Supplies
Automotive Cigarette Lighter Adapters
Power Supply for Linear Chargers
DC/DC Converters with Current Limited
Richtek products are :

RoHS compliant and compatible with the current
Marking Information
requirements of IPC/JEDEC J-STD-020.

Suitable for use in SnPb or Pb-free soldering processes.
RT6266
GSPYMDNN
RT6266GSP : Product Number
YMDNN : Date Code
Simplified Application Circuit
CBOOT
L1
RSENSE
COUT
VOUT
D1
VIN
VIN
SW
RT6266
BOOT
GND
CIN
REN
EN
CSP
FB
CSN
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DS6266-00
March 2015
R1
R2
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RT6266
Pin Configurations
(TOP VIEW)
8
VIN
BOOT
2
EN
3
FB
4
GND
SW
7
GND
6
CSP
5
CSN
9
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
VIN
Input Supply Voltage, 7.5V to 36V. Must bypass with a suitably large ceramic
capacitor.
2
BOOT
Bootstrap for High-Side Gate Driver. Connect 0.1F or greater ceramic
capacitor from BOOT to SW pins.
3
EN
Enable Input Pin. A logic high enables the converter; a logic low forces the
RT6266 into shutdown mode reducing the supply current to less than 3A.
Attach this pin to VIN with a 100k pull up resistor for automatic startup.
4
FB
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.
5
CSN
Current Sense Negative Input. It is used for load current limiting and load line
drop compensation.
6
CSP
Current Sense Positive Input. It is used for load current limiting and load line
drop compensation.
7,
9 (Exposed Pad)
GND
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)
SW
Switch Output -- Connect to external L-C filter and Schottky diode.
8
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March 2015
RT6266
Function Block Diagram
EN
VIN
PVCC
Internal
Regulator
VCC
VIBIAS
OT
EN
VREF
Oscillator
Current
Sense
Ramp
Foldback
Control
PVCC
BOOT
OC
GND
UV&OV
+
UGATE
Current
Control
Comp
SW
Driver
-
SS
Circuit
0.8V
2Meg
+
+EA
-
80p
LGATE
Current
Limit
1p
Line
Comp
100mV
+
CSP
-
CSN
1Meg
1Meg
10p
+
EA
-
FB
Operation
The RT6266 is a constant frequency, current mode
Oscillator
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.
The internal oscillator runs at fixed frequency 100kHz.
In short circuit condition, the frequency is reduced to
20kHz for low power consumption.
Error Amplifier
The error amplifier adjusts its output voltage by
comparing the feedback signal (VFB) 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.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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March 2015
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 1.4V and turned off when the EN pin is lower than
0.4V. Attach this pin to VIN with a 100k pull up
resistor for automatic startup.
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RT6266
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 RT6266 output terminator to the load is
too long, there will be a voltage drop on the long trace
which is variable with load current. RT6266 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
RSENSE and Rtrace values.
RSENSE
CSP
Rtrace1
R1
CSN
Rtrace2
Internal Current Limit Protection
When the external RSENSE is too small and the external
peak current is higher than 4.4A, the high-side switch
will turn off immediately and then turn at the next clock
cycle. The inductor’s peak current will be limited at 4.4A
by internal current limit.
Output Short-Circuit Protection
When VOUT is short (VFB < 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.
IOUT (A)
Vload
+
IOUT
_
FB
Time
R2
t1
t2
Under Voltage Lockout (UVLO)
IOUT  RSENSE  20μ  R1  IOUT  Rtrace
Rtrace
R1 
20μ  RSENSE
To avoid mis-operation at low input voltage, when input
voltage falls below 6.2V, and under voltage lockout is
induced and the device is disabled.
Output Over Voltage Protection (OVP)
Thermal Shutdown
The VOUT 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.
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.
External Current Limit Protection
The external current limit is set by outside resistance
(RSENSE). The average current is limited according to
the following equation :
Average Current_Limit (A) 
100mV
RSENSE
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is a registered trademark of Richtek Technology Corporation.
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March 2015
RT6266
Absolute Maximum Ratings
(Note 1)

Supply Input Voltage ----------------------------------------------------------------------------------------- 0.3V to 40V

Switch Voltage, SW ------------------------------------------------------------------------------------------ 0.3V to (VIN + 0.3V)

VBOOT - VSW --------------------------------------------------------------------------------------------------- 0.3V to 6V

EN, FB, CSP, CSN ------------------------------------------------------------------------------------------- 0.3V to 6V

Power Dissipation, PD @ TA = 25C

SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------- 2.041W

Package Thermal Resistance
(Note 2)
SOP-8 (Exposed Pad), JA --------------------------------------------------------------------------------- 49C/W
SOP-8 (Exposed Pad), JC --------------------------------------------------------------------------------- 15C/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 ----------------------------------------------------------------------------------------- 7.5V to 36V

Ambient Temperature Range ------------------------------------------------------------------------------ 40C to 85C

Junction Temperature Range ------------------------------------------------------------------------------ 40C to 125C
Electrical Characteristics
(VIN = 12V, VOUT = 5V, TA = 25C, Load Current = 0A, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
VOUT OVP Detect Voltage
VOVP
Normal Operation.
--
5.8
--
V
VOUT OVP Hysteresis
VOVP
Normal Operation.
--
0.3
--
V
Shutdown Supply Current
ISD
VEN = 0V
--
1.5
3
A
Supply Current
IQ
VEN = 3 V, VFB = 0.9V
--
0.8
1.2
mA
Feedback Voltage (*)
VFB
7.5V ≤ VIN ≤ 36V
0.784
0.8
0.816
V
High-Side Switch On-Resistance
RDS(ON),U
--
100
--
m
Low-Side Switch On-Resistance
RDS(ON),L
--
15
--

--
0
10
A
--
4.4
--
A
High-Side Switch Leakage Current ISWLEAK
VEN = 0V, VSW = 0V
Upper Switch Current Limit
ILIM
Load Line Compensation Gain
GLC
VCSP  VCSN = 100mV, check IFB
15
20
25
A/V
Current Sense Voltage (*)
VSENSE
VCSP  VCSN
98
100
102
mV
Oscillation Frequency
fOSC1
85
100
115
kHz
--
20
--
kHz
Short Circuit Oscillation Frequency fOSC2
VFB = 0V
Minimum Off-Time
tOFF
--
200
--
ns
Minimum On-Time
tON
--
150
--
ns
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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March 2015
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RT6266
Parameter
EN Input Threshold
Voltage
Symbol
Test Conditions
Min
Typ
Max
Unit
Logic-High
VIH
2.7
--
--
Logic-Low
VIL
--
--
0.4
6.3
6.7
7.2
V
V
Input Under Voltage Lockout
Threshold
VUVLO
Input Under Voltage Lockout
Hysteresis
VUVLO
--
0.5
1
V
Soft-Start Period
tSS
--
3.5
--
ms
Thermal Shutdown
TSD
--
150
--

VIN Rising
C
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect
device reliability.
Note 2. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. JC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Typical Application Circuit
CBOOT
100n
VIN
7.5V to 36V
1
20µF
D1
8
SW
RT6266
7,
2
0.1µF
BOOT
GND 9 (Exposed Pad)
REN 3
EN
CSP 6
100k 4
FB
CSN 5
VIN
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RSENSE
33m
L1
39µH
470µF
VOUT
5V/2.4A
10µF
R1
180k
R2
34.3k
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March 2015
RT6266
Typical Operating Characteristics
Efficiency vs. Output Current
Stanby Supply Current vs. Input Voltage
100
10
VOUT = 5V
80
VIN = 12V
70
VIN = 24V
8
Stanby Current (mA)
Efficiency (%)
90
60
50
40
30
6
4
2
20
10
VOUT = 5V
0
0.00
0.01
0.10
1.00
0
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
10.00
Input Voltage (V)
Output Current (A)
Output Current vs. Temperature
5.0
3.4
4.5
3.3
4.0
Output Current (A)
Over Current Limit (A)
Over Current Limit vs. Input Voltage
3.5
3.2
3.1
3.0
2.9
2.8
3.5
3.0
2.5
2.0
1.5
2.7
1.0
2.6
0.5
2.5
0.0
6
8
10
12
14
16
18
20
22
24
VIN = 12V, VOUT = 5V
0
25
50
75
100
125
Temperature (°C)
Input Voltage (V)
UVLO Threshold vs. Temperature
Frequency vs. Input Voltage
8.0
110
108
7.5
UVLO Threshold (V)
Frequency (kHz)
106
104
102
100
98
96
94
6.5
Falling
6.0
5.5
92
VOUT = 5V
VOUT = 5V, IOUT = 2.4A
5.0
90
8
12
16
20
24
28
32
Input Voltage (V)
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Rising
7.0
March 2015
36
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT6266
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March 2015
RT6266
Application Information
Output Voltage Setting
The resistive divider allows the FB pin to sense the
output voltage as shown in Figure 1.
VOUT
R1
FB
RT6266
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive
 VOUT  
VOUT 
L =
  1 VIN(MAX) 
f


I
L(MAX)

 

The inductor's current rating (caused a 40C
temperature rising from 25C ambient) should be
greater than the maximum load current and its
saturation current should be greater than the short
circuit peak current limit. Please see Table 2 for the
inductor selection reference.
Table 2. Suggested Inductors for Typical
Application Circuit
Component
Dimensions
Series
Supplier
(mm)
MAG.LAYERS MCD110C-390K-LV 10 x 6.5 x 10.5
voltage divider according to the following equation :
CIN and COUT Selection
VOUT = VREF  1 R1 
 R2 
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the high side
Where VREF is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 0.1F low ESR ceramic capacitor between
the BOOT pin and SW pin. This capacitor provides the
gate driver voltage for the high side MOSFET.
Inductor Selection
The inductor value and operating frequency determine
the ripple current according to a specific input and
output voltage. The ripple current IL increases with
higher VIN and decreases with higher inductance.
V
V
IL =  OUT   1 OUT 
VIN 
 f L  
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output
voltage ripple. High frequency with small ripple current
can achieve the highest efficiency operation. However,
it requires a large inductor to achieve this goal.
For the ripple current selection, the value of IL =
0.24(IMAX) will be a reasonable starting point. The
largest ripple current occurs at the highest VIN. To
guarantee that the ripple current stays below the
specified maximum, the inductor value should be
chosen according to the following equation :
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March 2015
MOSFET. To prevent large ripple current, a low ESR
input capacitor sized for the maximum RMS current
should be used. The approximate RMS current is
given :
IRMS = IOUT(MAX)
VOUT
VIN
VIN
1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst case condition is
commonly used for design because even significant
deviations do not offer much relief. Choose a capacitor
rated at a higher temperature than required. Several
capacitors may also be paralleled to meet size or
height requirements in the design. For the input
capacitor, two 10F low ESR ceramic capacitors are
Suggested. For the Suggested capacitor, please refer
to Table 3 for more details. The selection of COUT is
determined by the required ESR to minimize voltage
ripple. Moreover, the amount of bulk capacitance is
also a key for COUT selection to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later
section.
The output ripple, VOUT , is determined by :
1 
VOUT  IL ESR 
8fCOUT 

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RT6266
The output ripple will be the highest at the maximum
input voltage since IL increases with input voltage.
Multiple capacitors placed in parallel may be needed to
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature.
The maximum power dissipation can be calculated by
Maximum Power Dissipation (W)1
meet the ESR and RMS current handling requirement.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high
ripple current, high voltage rating and low ESR make
them ideal for switching regulator applications.
However, care must be taken when these capacitors
are used at input and output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output
can induce ringing at the input, VIN. At best, this ringing
can couple to the output and be mistaken as loop
instability. At worst, a sudden inrush of current through
the long wires can potentially cause a voltage spike at
VIN large enough to damage the part.
3.2
JEDEC 51-7 four-layer thermal test board. The
maximum power dissipation at TA = 25C can be
calculated by the following formula :
PD(MAX) = (125C  25C) / (49C/W) = 2.041W for
SOP-8 (Exposed Pad) package
The maximum power dissipation depends on the
operating ambient temperature for fixed TJ(MAX) and
thermal resistance, JA. The derating curve in Figure 2
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
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2.0
1.6
1.2
0.8
0.4
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 2. Derating Curve of Maximum Power
Dissipation
Layout Consideration
Follow the PCB layout guidelines
performance of the RT6266.

for
optimal
Keep the traces of the main current paths as short
and wide as possible.

Put the input capacitor as close as possible to the
device pins (VIN and GND).

SW node is with high frequency voltage swing and
should be kept at small area. Keep analog
PD(MAX) = (TJ(MAX)  TA) / JA
For recommended operating condition specifications,
the maximum junction temperature is 125C. The
junction to ambient thermal resistance, JA, is layout
dependent. For SOP-8 (Exposed Pad) package, the
thermal resistance, JA, is 49C/W on a standard
2.4
0.0
the following formula :
where TJ(MAX) is the maximum junction temperature,
TA is the ambient temperature, and JA is the junction to
ambient thermal resistance.
Four-Layer PCB
2.8
components away from
the SW node to prevent
stray capacitive noise pick-up.

Connect feedback network behind the output
capacitors. Keep the loop area small. Place the
feedback components near the RT6266.

An example of PCB layout guide is shown in Figure 3
for reference.
LX should be connected to inductor by
wide and short trace. Keep sensitive
components away from this trace.
Input capacitor must be placed
as close to the IC as possible.
VOUT
BOOT
CBOOT
VIN
8
VIN
CIN
CIN
GND
R2
BOOT
REN
EN
2
FB
4
3
GND
SW
7
GND
6
CSP
5
CSN
9
L
RSENSE
D1
COUT
COUT
R1
VOUT
GND
The feedback and must be connected
as lose to the device as possible. Keep
sensitive component away.
Figure 3. PCB Layout Guide
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March 2015
RT6266
Outline Dimension
Dimensions In Millimeters
Symbol
Dimensions In Inches
Min
Max
Min
Max
A
4.801
5.004
0.189
0.197
B
3.810
4.000
0.150
0.157
C
1.346
1.753
0.053
0.069
D
0.330
0.510
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.170
0.254
0.007
0.010
I
0.000
0.152
0.000
0.006
J
5.791
6.200
0.228
0.244
M
0.406
1.270
0.016
0.050
X
2.000
2.300
0.079
0.091
Y
2.000
2.300
0.079
0.091
X
2.100
2.500
0.083
0.098
Y
3.000
3.500
0.118
0.138
Option 1
Option 2
8-Lead SOP (Exposed Pad) Plastic Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume
responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and
reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may
result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS6266-00
March 2015
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