RT8015 Datasheet PDF - Richtek Technology

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RT8015
Richtek Technology

Part Number RT8015
Description Synchronous Step-Down Regulator
Page 15 Pages


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Conceptual
RT8015www.DataSheet4U.com
2A, 4MHz, Synchronous Step-Down Regulator
General Description
The RT8015 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.
RT8015 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
RT8015
Package Type
SP : SOP-8 (Exposed Pad)
Operating Temperature Range
P : Pb Free with Commercial Standard
G : Green (Halogen Free with Comme-
rcial Standard)
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 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
GND
LX
PGND
8
27
GND
36
45
COMP
FB
VDD
PVDD
SOP-8 (Exposed Pad)
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.
DS8015-03C February 2007
www.richtek.com
1



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RT8015
Conceptual
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Typical Application Circuit
VIN
2.6V to 5.5V
ROSC
332k
RT8015
1 SHDN/RT
8
COMP
2 GND
3
LX
7
FB
6
VDD
4 PGND
PVDD 5
L1
1uH
Note : Using all Ceramic Capacitors
RCOMP
7.5k
CCOMP
1.5nF
R2 240k
CIN
22uF
R1
510k
COUT
22uF
VOUT
2.5V/2A
Layout Guide
CIN must be placed between
VDD and GND as closer as
possible
VIN CIN GND
PVDD
RT8015
54
Output capacitor must
be near RT8015
COUT VOUT
PGND
L1
LX should be
connected to
Inductor by wide
and short trace,
keep sensitive
compontents away
from this trace
VDD
FB
6
7
3 LX
2 GND
COMP
R1
CF R2 RCOMP
8
1 SHDN/RT
ROSC
VOUT
CCOMP
GND
Connect the FB pin directly to feedback resistors. The
resistor divider must be connected between VOUT and
GND.
Table 1
Component Supplier Series Inductance (uH) DCR (m) Current Rating (mA) Dimensions (mm)
TAIYO YUDEN
Sumida
GOTREND
ABC
NR 4018
CDRH4D28
GTSD53
SR0403
2.2
2.2
2.2
2.2
60
31.3
29
47
2700
2040
2410
2600
4x4x1.8
4.5x4.5x3
5x5x2.8
4.5x4x3.2
Component Supplier
TDK
TDK
Panasonic
Panasonic
Panasonic
TAIYO YUDEN
TAIYO YUDEN
TAIYO YUDEN
Table 2
Part No.
C3225X5R0J226M
C2012X5R0J106M
ECJ4YB0J226M
ECJ4YB1A226M
ECJ4YB1A106M
LMK325BJ226ML
JMK316BJ226ML
JMK212BJ106ML
Capacitance (uF)
22
10
22
22
10
22
22
10
Case Size
1210
0805
1210
1210
1210
1210
1206
0805
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DS8015-03C February 2007



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Conceptual
RT8015www.DataSheet4U.com
Functional Pin Description
Pin Number Pin Name
Pin Function
Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the
1 SHDN/RT
switching frequency. Forcing this pin to VDD causes the device to be shut down.
Signal Ground. All small-signal components and compensation components should
GND,
2 connect to this ground, which in turn connects to PGND at one point. Exposed pad
Exposed Pad
should be soldered to PCB board and connected to GND.
3 LX
Internal Power MOSFET Switches Output. Connect this pin to the inductor.
4 PGND
Power Ground. Connect this pin close to the () terminal of CIN and COUT.
5 PVDD
6 VDD
7 FB
8 COMP
Power Input Supply. Decouple this pin to PGND with a capacitor.
Signal Input Supply. Decouple this pin to GND with a capacitor. Normally VDD is equal
to PVDD.
Feedback Pin. Receives the feedback voltage from a resistive divider connected
across the output.
Error Amplifier Compensation Point. The current comparator threshold increases with
this control voltage. Connect external compensation elements to this pin to stabilize
the control loop.
Function Block Diagram
SHDN/RT
SD
COMP
0.8V
EA
FB
OSC
Output
Clamp
Slope
Com
ISEN
OC
Limit
Int-SS
POR
0.9V
0.7V
0.4V
Control
Logic
VREF OTP
Driver
NISEN
NMOS I Limit
VDD
PVDD
LX
PGND
GND
DS8015-03C February 2007
www.richtek.com
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RT8015
Conceptual
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Operation
Main Control Loop
The RT8015 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.
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 RT8015 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 RT8015 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 RT8015, 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.
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.
www.richtek.com
4
DS8015-03C February 2007



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