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PDF RT8011 Data sheet ( Hoja de datos )
| Número de pieza | RT8011 | |
| Descripción | Synchronous Step-Down Regulator | |
| Fabricantes | Richtek | |
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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
www.DataShseweitt4cUh.ecosmincrease 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.
Ordering Information
RT8011/A
Package Type
F : MSOP-10 (RT8011)
QW : WDFN-10L 3x3 (RT8011)
QW : WDFN-8E 3x3 (RT8011A)
Operating Temperature Range
P : Pb Free with Commercial Standard
G : Green (Halogen Free with Commer-
cial Standard)
Without SYNC and PGOOD Function
With SYNC and PGOOD Function
Note :
RichTek Pb-free and Green 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.
`100% matte tin (Sn) plating.
Features
z High Efficiency : Up to 95%
z Low RDS(ON) Internal Switches : 110mΩ
z Programmable Frequency : 300kHz to 4MHz
z No Schottky Diode Required
z 0.8V Reference Allows Low Output Voltage
z Forced Continuous Mode Operation
z Low Dropout Operation : 100% Duty Cycle
z Synchronizable Switching Frequency (For RT8011
Only)
z Power Good Output Voltage Monitor (For RT8011
Only)
z RoHS Compliant and 100% Lead (Pb)-Free
Applications
z Portable Instruments
z Battery-Powered Equipment
z Notebook Computers
z Distributed Power Systems
z IP Phones
z Digital Cameras
Pin Configurations
(TOP VIEW)
SHDN/RT
SYNC
GND
LX
PGND
10
29
38
47
56
MSOP-10
COMP
FB
PGOOD
VDD
PVDD
SHDN/RT 1
SYNC 2
GND 3
LX 4
PGND 5
10 COMP
9 FB
8 PGOOD
7 VDD
PVDD
SHDN/RT 1
GND 2
LX 3
PGND 4
8 COMP
7 FB
6 VDD
5 PVDD
WDFN-10L 3x3
WDFN-8E 3x3
Marking Information
For marking information, contact our sales representative
directly or through a RichTek distributor located in your
area, otherwise visit our website for detail.
DS8011/A-00 August 2006
www.richtek.com
1
1 page  
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
www.DataSheet4U.com
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-00 August 2006
RT8011/A
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 RDS(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.
www.richtek.com
5
5 Page 
Application Information
The basic RT8011/A application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by CIN and COUT.
Operating Frequency
Selection of the operating frequency is a tradeoff between
www.DataSheeffeict4iUen.ccoymand component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequency improves efficiency by
reducing internal gate charge and switching losses but
requires larger inductance and/or capacitance to maintain
low output ripple voltage.
The operating frequency of the RT8011/A is determined
by an external resistor that is connected between the RT
pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator. The RT resistor value
can be determined by examining the frequency vs. RT
curve. Although frequencies as high as 4MHz are possible,
the minimum on-time of the RT8011/A imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 110ns. Therefore, the minimum duty cycle is
equal to 100 x 110ns x f(Hz).
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
ΔIL
=
⎡ VOUT
⎢⎣ f × L
⎤
⎥⎦
⎢⎣⎡1−
VOUT
VIN
⎤
⎥⎦
Having a lower ripple current reduces the ESR losses in
the output capacitors and the output voltage ripple. Highest
efficiency operation is achieved at low frequency with small
ripple current. This, however, requires a large inductor. A
reasonable starting point for selecting the ripple current
is ΔI = 0.4(IMAX). The largest ripple current occurs at the
highest VIN. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation :
L
=
⎡
⎢
⎣
f
×
VOUT
ΔIL(MAX)
⎤⎡
⎥ ⎢1
−
⎦⎣
VOUT
VIN(MAX)
⎤
⎥
⎦
DS8011/A-00 August 2006
RT8011/A
The transition from low current operation begins when the
peak inductor current falls below the minimum peak
current. Lower inductor values result in higher ripple current
which causes this to occur at lower load currents. This
causes a dip in efficiency in the upper range of low current
operation.
4.5
4
3.5
3
2.5 RT = 154k for 2MHz
2
1.5 RT = 332k for 1MHz
1
0.5
0
0 100 200 300 400 500 600 700 800 900 10000
RRRTT (kٛΩ)
0
Figure 5
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or mollypermalloy
cores. Actual core loss is independent of core size for a
fixed inductor value but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design
current is exceeded.
This result in an abrupt increase in inductor ripple current
and consequent output voltage ripple.
Do not allow the core to saturate!
www.richtek.com
11
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| Páginas | Total 17 Páginas | |
| PDF Descargar | [ Datasheet RT8011.PDF ] | |
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