RT8011/A
2A, 4MHz, Synchronous Step-Down Regulator
General Description
The RT8011/A is a high efficiency synchronous, step-down
DC/DC converter. Its input voltage range is from
2.6V to
5.5V and provides an adjustable regulated output voltage
from 0.8V to 5V while delivering up to 2A of output current.
The internal synchronous
low on-resistance power
switches increase efficiency and eliminate the need for
an external Schottky diode. Switching frequency is set
by an external resistor or can be synchronized to an
external clock. 100% duty cycle provides low dropout
operation extending battery life in portable systems.
Current mode operation with external compensation
allows the transient response to be optimized over a wide
range of loads and output
capacitors.
RT8011/A operation in forced continuous PWM
Mode which
minimizes ripple voltage and reduces the noise and RF
interference.
100% duty cycle in Low Dropout Operation further
maximize battery life.
Features
High Efficiency : Up to 95%
Low R
DS(ON)
Internal Switches : 110mΩ
Ω
Programmable Frequency : 300kHz to 4MHz
No Schottky Diode Required
0.8V Reference Allows Low Output Voltage
Forced Continuous Mode Operation
Low Dropout Operation : 100% Duty Cycle
Synchronizable Switching Frequency (For RT8011
Only)
Power Good Output Voltage Monitor (For RT8011
Only)
RoHS Compliant and 100% Lead (Pb)-Free
Applications
Portable Instruments
Battery-Powered Equipment
Notebook Computers
Distributed Power Systems
IP Phones
Digital Cameras
Ordering Information
RT8011/A
Package Type
F : MSOP-10 (RT8011)
QW : WDFN-10L 3x3 (RT8011)
QW : WDFN-8EL 3x3 (RT8011A)
Lead Plating System
P : Pb Free
G : Green (Halogen Free and Pb Free)
Without SYNC and PGOOD Function
With SYNC and PGOOD Function
Note :
Richtek products are :
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
WDFN-10L 3x3
WDFN-8EL 3x3
SHDN/RT
SYNC
GND
LX
PGND
1
2
3
4
5
10
9
8
7
9
Pin Configurations
(TOP VIEW)
SHDN/RT
SYNC
GND
LX
PGND
10
2
3
4
5
9
8
7
6
COMP
FB
PGOOD
VDD
PVDD
MSOP-10
COMP
FB
PGOOD
VDD
PVDD
SHDN/RT
GND
LX
PGND
1
2
3
4
8
7
6
5
COMP
FB
VDD
PVDD
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
DS8011/A-02 March 2011
www.richtek.com
1
RT8011/A
Typical Application Circuit
V
IN
2.6V to 5.5V
R
OSC
332k
RT8011
1 SHDN/RT
2 SYNC
3
4
5
GND
LX
PGND
COMP
FB
10
9
R
PGOOD
100k
R
COMP
13k
C
COMP
1000pF
R2 240k
PGOOD 8
VDD
PVDD
7
6
C
IN
22uF
R1
510k
C1
22pF
L1
2.2uH
C
OUT
22uF
V
OUT
2.5V/2A
Figure 1. Typical Application Circuit for RT8011
V
IN
2.6V to 5.5V
R
OSC
332k
RT8011A
1 SHDN/RT
2
3
4
GND
LX
PGND
COMP
FB
VDD
PVDD
8
7
6
5
C
IN
22uF
R1
510k
R
COMP
13k
C
COMP
1000pF
R2 240k
L1
2.2uH
C
OUT
22uF
V
OUT
2.5V/2A
Figure 2. Typical Application Circuit for RT8011A
Note : Using all Ceramic Capacitors
Table 1
Component Supplier Series
Inductance (μH) DCR (mΩ) Current Rating (mA) Dimensions (mm)
TAIYO YUDEN
NR 4018
2.2
60
2700
4x4x1.8
Sumida
CDRH4D28
2.2
31.3
2040
4.5x4.5x3
GOTREND
GTSD53
2.2
29
2410
5x5x2.8
ABC
SR0403
2.2
47
2600
4.5x4x3.2
Table 2
Component Supplier
TDK
TDK
TDK
Panasonic
Panasonic
Panasonic
TAIYO YUDEN
TAIYO YUDEN
TAIYO YUDEN
Part No.
C3225X5R0J226M
C3225X5R0J226M
C2012X5R0J106M
ECJ4YB0J226M
ECJ4YB1A226M
ECJ4YB1A106M
LMK325BJ226ML
JMK316BJ226ML
JMK212BJ106ML
Capacitance (μF)
22
22
10
22
22
10
22
22
10
Case Size
1210
1210
0805
1210
1210
1210
1210
1206
0805
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2
DS8011/A-02 March 2011
RT8011/A
Layout Guide
C
IN
must be placed between
V
DD
and GND as closer as
possible
V
IN
C
IN
GND
Output capacitor must
be near RT8011
C
OUT
V
OUT
RT8011
PVDD
VDD
PGOOD
FB
R1
C
F
V
OUT
R2
COMP
R
COMP
C
COMP
GND
Connect the FB pin directly to feedback resistors. The
resistor divider must be connected between V
OUT
and
GND.
6
7
8
9
10
5
4
3
2
1
L1
PGND
LX
GND
SYNC
SHDN/RT
LX should be
connected to
Inductor by wide
and short trace,
keep sensitive
compontents away
from this trace
C
IN
must be placed between
V
DD
and GND as closer as
possible
V
IN
C
IN
GND
Output capacitor must
be near RT8011A
C
OUT
V
OUT
RT8011A
PVDD
VDD
5
6
7
8
4
3
2
1
L1
PGND
LX
GND
SHDN/RT
LX should be
connected to
Inductor by wide
and short trace,
keep sensitive
compontents away
from this trace
V
IN
R
OSC
FB
R1
C
F
V
OUT
R2
COMP
R
COMP
C
COMP
R
OSC
GND
Connect the FB pin directly to feedback resistors. The
resistor divider must be connected between V
OUT
and
GND.
Figure 3. RT8011 Layout Guide
Figure 4. RT8011A Layout Guide
Functional Pin Description
Pin Number
RT8011
1
RT8011A
1
Pin Name
Pin Function
Oscillator Resistor Input. Connecting a resistor to ground from this pin sets
SHDN/RT
the switching frequency. Forcing this pin to V
DD
causes the device to be shut
down.
External Clock Synchronization Input. The oscillation frequency can be
2
--
SYNC
synchronized to an external oscillation applied to this pin. When tied to V
DD
,
internal oscillator is selected.
Signal Ground. All small-signal components and compensation components
should connect to this ground, which in turn connects to PGND at one point.
Internal Power MOSFET Switches Output. Connect this pin to the inductor.
Power Ground. Connect this pin close to the (−) terminal of C
IN
and C
OUT
.
Power Input Supply. Decouple this pin to PGND with a capacitor.
Signal Input Supply. Decouple this pin to GND with a capacitor. Normally V
DD
is equal to PVDD.
Power Good Indicator. Open-drain logic output that is pulled to ground when
the output voltage is not within
±12.5%
of regulation point.
Feedback Pin. Receives the feedback voltage from a resistive divider
connected across the output.
Error Amplifier Compensation Point. The current comparator threshold
10
8
COMP
increases with this control voltage. Connect external compensation elements
to this pin to stabilize the control loop.
3
4
5
6
7
8
9
2
3
4
5
6
--
7
GND
LX
PGND
PVDD
VDD
PGOOD
FB
DS8011/A-02 March 2011
www.richtek.com
3
RT8011/A
Function Block Diagram
SHDN/RT
SD
ISEN
SYNC
COMP
0.8V
FB
Output
Clamp
OC
Limit
OSC
Slope
Com
PVDD
EA
Int-SS
0.9V
0.7V
Control
Logic
Driver
LX
NISEN
POR
0.4V
NMOS I Limit
PGND
PGOOD
V
REF
OTP
GND
VDD
RT8011
SHDN/RT
SD
ISEN
OSC
COMP
0.8V
FB
Output
Clamp
OC
Limit
Slope
Com
PVDD
EA
Int-SS
0.9V
0.7V
Control
Logic
Driver
LX
NISEN
POR
0.4V
NMOS I Limit
PGND
V
REF
OTP
GND
VDD
RT8011A
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4
DS8011/A-02 March 2011
RT8011/A
Operation
Main Control Loop
The RT8011/A is a monolithic, constant-frequency, current
mode step-down DC/DC converter. During normal
operation, the internal top power switch (P-Channel
MOSFET) is turned on at the beginning of each clock
cycle. Current in the inductor increases until the peak
inductor current reach the value defined by the voltage on
the COMP pin. The error amplifier adjusts the voltage on
the COMP pin by comparing the feedback signal from a
resistor divider on the FB pin with an internal 0.8V
reference. When the load current increases, it causes a
reduction in the feedback voltage relative to the reference.
The error amplifier raises the COMP voltage until the
average inductor current matches the new load current.
When the top power MOSFET shuts off, the synchronous
power switch (N-Channel MOSFET) turns on until either
the bottom current limit is reached or the beginning of the
next clock cycle.
The operating frequency is set by an external resistor
connected between the RT pin and ground. The practical
switching frequency can range from 300kHz to 4MHz.
Power Good comparators will pull the PGOOD output low
if the output voltage comes out of regulation by 12.5%. In
an over-voltage condition, the top power MOSFET is turned
off and the bottom power MOSFET is switched on until
either the over-voltage condition clears or the bottom
MOSFET's current limit is reached.
Frequency Synchronization
The internal oscillator of the RT8011 can be synchronized
to an external clock connected to the SYNC pin. The
frequency of the external clock can be in the range of
300kHz to 4MHz. For this application, the oscillator timing
resistor should be chosen to correspond to a frequency
that is about 20% lower than the synchronization
frequency.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases toward the maximum
on-time. Further reduction of the supply voltage forces
the main switch to remain on for more than one cycle
eventually reaching 100% duty cycle.
DS8011/A-02 March 2011
www.richtek.com
5
The output voltage will then be determined by the input
voltage minus the voltage drop across the internal
P-Channel MOSFET and the inductor.
Low Supply Operation
The RT8011/A is designed to operate down to an input
supply voltage of 2.6V. One important consideration at
low input supply voltages is that the R
DS(ON)
of the
P-Channel and N-Channel power switches increases. The
user should calculate the power dissipation when the
RT8011/A is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant
frequency architectures by preventing sub-harmonic
oscillations at duty cycles greater than 50%. It is
accomplished internally by adding a compensating ramp
to the inductor current signal. Normally, the maximum
inductor peak current is reduced when slope compensation
is added. In the RT8011/A, however, separated inductor
current signals are used to monitor over current condition.
This keeps the maximum output current relatively constant
regardless of duty cycle.
Short-Circuit Protection
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. A
current runaway detector is used to monitor inductor
current. As current increasing beyond the control of current
loop, switching cycles will be skipped to prevent current
runaway from occurring.
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