POWER LP78081 600ma high efficiency synchronous buck with 350ma ldo Datasheet

Preliminary Datasheet
LP78081
600mA High Efficiency Synchronous Buck with 350mA LDO
General Description
Features
The LP78081 are PMU, and contain a 600mA Buck
600mA Buck High Efficiency: 96% 
DC/DC and a 350mA Linear Regulator, Buck DC/DC is
1.5MHz Fixed-Frequency PWM Operation
a constant frequency, current mode, PWM step-down
Adjustable Output From 0.6V to VIN
converter.The device integrates a main switch and a
350mA LDO: 3.0V and 3.3V Fixed output
synchronous rectifier for high efficiency. The 2.7V to
100% Duty Cycle Low Dropout Operation
6.5V input voltage range makes the LP78081 is ideally
Available in SOT23-6 Package
suited for portable electronic devices that are powered
Low than 1µA Shutdown Current
from 1-cell Li-ion battery or from other power sources
within the range such as cellular phones, PDAs and
handy-terminals.
Internal synchronous rectifier with
low RDS(ON) dramatically reduces conduction loss at
PWM
mode.
The
internal
synchronous
switch
increases efficiency while eliminate the need for an
external Schottky diode.The switching ripple is easily
smoothed-out by small package filtering elements due
to a fixed operation frequency of 1.5MHz. This along
Applications
Portable Media Players/MP3 players
Cellular and Smart mobile phone
PDA
DSC
Wireless Card
Pin Configurations
with small SOT-23-6 package provide small PCB area
(TOP View)
application. Other features include soft start, lower
internal reference voltage with 2% accuracy, over
temperature protection, and over current protection。
Ordering Information
LP78081 - □ □ □ □
□
F: Pb-Free
PIN NO.
Function
1
GND
2
VDD
3
SW
4
FB
5
EN
Package Type
B6: SOT23-6
LDO Output Voltage Type
30: 3.0V
33: 3.3V
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
Marking Information
Please see website.
Page 1 of 8
6
OUT
Preliminary Datasheet
LP78081
Typical Application Circuit
VIN
2
LP78081
VDD
SW
FB
4
EN
OUT
6
2.2uH
Vout1
Cf
R1
R2
Vout2
1
GND
5
3
Figure 1. LP78081 High Efficiency Step-Down Converter
Functional Pin Description
Pin Number
Pin Name
Pin Function
1
GND
Ground.Chip Enable(Active High).
2
VDD
Power Input.
3
SW
Pin For Switching
4
FB
Feedback Input Pin,Reference voltage is 0.612 V
5
EN
Chip Enable.
6
OUT
LDO Output.
Function Block Diagram
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
Page 2 of 8
Preliminary Datasheet
Absolute Maximum Ratings
■
■
■
■
■
■
■
■
■
■
Input Supply Voltage
LDO Current
P-Channel Switch Source Current(DC)
N-Channel Switch Current(DC)
Peak SW Sink and Source Current
Operation Temperature Range
Junction Temperature
Storage Temperature
Lead Temp(Soldering,10sec)
ESD Rating(HBM)
LP78081
-0.3V to 6V
350mA
600mA
600mA
800mA
--40℃ to 85℃
125℃
--65℃ to 150℃
260℃
2KV
Electrical Characteristics
(VIN = 3.6V, VOUT = 2.5V, VREF = 0.6V, L = 2.2µH, CIN= 4.7µF, COUT= 10µF, TA= 25°C, IMAX = 600mA
unless otherwise specified)
Parameter
Symbol
Input Voltage Range
VIN
Quiescent Current
IQ
Shutdown Current
ISHDN
IOUT = 0mA, VFB =0.5V
IOUT = 0mA, VFB =0.7V
EN = GND
Reference Voltage
VREF
For adjustable output voltage
Adjustable Output Range
VOUT
ΔVOUT
ΔVOUT
Output
Voltage
Accuracy
Fixed
ΔVOUT
ΔVOUT
ΔVOUT
Adjustable
LDO Output
current
PMOSFET
RON
NMOSFET
RON
P-Channel
Current Limit
EN
Threshold
EN Leakage
Current
ΔVOUT
ΔVOUT
Test Conditions
NRDS(ON)
IP(LM)
Typ
Max
Units
V
270
25
0.1
6.5
350
35
1
2.7
0.602
0.625
VREF
VIN = 2.2 to 5.5V, VOUT = 1.2V 0A <
IOUT < 600mA
VIN = 2.2 to 5.5V, VOUT = 1.5V 0A <
IOUT < 600mA
VIN = 2.2 to 5.5V, VOUT = 1.8V 0A <
IOUT < 600mA
VIN = 2.8 to 5.5V, VOUT = 2.5V 0A <
IOUT < 600mA
VIN = 3.5 to 5.5V, VOUT = 3.3V 0A <
IOUT < 600mA
VIN = VOUT + 0.2V to 5.5V, VIN ≧
3.5V 0A < IOUT < 600mA
VIN = VOUT + 0.4V to 5.5V, VIN ≧
2.2V 0A < IOUT < 600mA
uA
V
V
+3
%
−3
+3
%
−3
+3
%
−3
+3
%
−3
+3
%
−3
+3
%
−3
+3
%
350
400
mA
0.53
Ω
IOUT =
200mA
VIN = 3.6V
0.45
IOUT =
200mA
VIN = 3.6V
0.45
VIN =2.2
to 5.5V
0.632
VIN −
0.2
uA
−3
300
ILDO
PRDS(ON)
Min
Ω
600
800
1000
mA
VEN
0.8
1.2
1.5
V
VENL
--
2
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
uA
Page 3 of 8
Preliminary Datasheet
LP78081
Typical Operating Characteristics
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
Page 4 of 8
Preliminary Datasheet
Applications Information
LP78081
The basic LP78081 applicaton circuit is shown inTypical
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.
series resistance(ESR) that is required to minimize
voltage ripple and load step transients, an well as the
amount or bulk capacitance that is necessary to ensure
that the control loop is stable. Loop stability can be
checked by viesing the load transient response as
described in later section.the output ripple, △ VOUT, is
determined by:
Inductor selection
The output inductor is selected to limit the ripple
current to some predetermined value. typically 20%~40%
of the full load current at the maximum input voltage.
Large value inductors lower ripple currents. Higher Vin or
VOUT also increases the ripple current as shown in
equation. A reasonable starting point for setting ripple
current is △IL=180mA(40% of 600mA).
Using ceramic input and output capacitors
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720Ma rated
Inductor should be enough for most applications
(600mA+120mA). For better efficiency, choose a low
DC-resistance inductor.
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 the
input and output. When a ceramic capacitor is use at
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 worst,a sudden inrush of current through
the long wires can potentially cause a voltage spike at VIN
large enough to damage the part.
Output voltage programming
CIN and COUT Selection
The output voltage is set by a resistive divider according
to the
The input capacitance, CIN ,is needed to filter the
trapezoidal current at the source of the top MOSFET. To
prevent large ripple voltage, a low ESR input capacitor
Sized for the maximum RMS current should be used.
RMS current is given by:
Following formula:
The external resistive divider is connected to the output,
allowing Remote voltage sensing as shown in figure3.
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. Note that ripple
current ratings from capacitor manufacturers are often
based on only 2000 hours of life which makes it advisable
to further derate the Capacitor, or 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.
The selection of COUT is determined by the effective
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
Page 5 of 8
Preliminary Datasheet
Efficiency considerations
The efficiency of a switching regulator is equal to the output
Power divided by the input power times 100%.it is often useful
to analyze individual losses to determine what is limiting the
efficiency and which change would produce the most
improvement efficiency can be expressed as :
Efficiency= 100%- (L1+L2+L3…)
Where L1、L2, etc. are the individual losses as a percentage of
Input power .although all dissipative elements in the for most
of losses: VIN quiescent current and 12R loss dominates the
efficiency loss at medium to high load currents. In a typical
efficiency plot, the efficiency curve at very low load currents
can be misleading since the actual power lost is of no
consequence.
LP78081
2. 12Rlosses tae calculated from the resistances of the internal
switches, RSW and external inductor RL. in continuous mode
the average output current flowing through inductor L is
“chopped” between the main switch and the synchronous
switch. Thus, the series resistance looking into the LX pin is a
function of both top and bottom MOSFER RDS(ON) and the duty
cycle (DC) as follows:
RSW=RDS(ON)TOP×DC+RDS(ON)BOT×(1-DC)
The RDS(ON) for both the top and bottom MOSFETS can be
obtained from the typical performance characteristics curves.
thus, to obtain 12R losses, simply add RSW to RL and multiply
the square of the average output current.
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for
less than 2% of the total loss.
1.The VIN quiescent current is due to two components:
the DC Bias current as given in the electrical
characteristics and the Internal main switch and
synchronous switch gate charge currents. the gate
charge current results from switching the gate
capacitance of the internal power MOSFET
switches .Each time the gate charge current.results
from switching the gate capacitance of the internal
power MOSFET switches. Each time the gate is
switches from high to low to high again, a packet of
charge △Q moves from VIN to ground.
The resulting △Q/△t is the current out of VIN that is typically
larger than the DC bias current. In continuous mode.
LGATCHG=f(QT+QB)
Where QT and QB are the gate charges of the internal top and
bottom switches. Both the DC bias and gate charge losses are
proportional to VIN and thus their effects will be more
pronounced at higher supply voltages.
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
Page 6 of 8
Preliminary Datasheet
LP78081
Checking Transient Response
The regulator loop response can be checked by looking at the load transient response. Switching
regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT
immediately shifts by an amount equal to ∆ILOAD (ESR), where ESR is the effective series
resistance of COUT. ∆ILOAD also begins to charge or discharge COUT generating a feedback error
signal used by the regulator to return VOUT to its steady-state value. During this recovery time,
VOUT can be monitored for overshoot or ringing that would indicate a stability problem.
Layout Considerations
Follow the PCB layout guidelines for optimal performance of LP78081.
For the main current paths as indicated in bold lines, keep their traces short and wide.
Put the input capacitor as close as possible to the device pins (VIN and GND).
LX node is with high frequency voltage swing and should be kept small area. Keep analog
components away from LX 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 LP78081.
Connect all analog grounds to a command node and then connect the command node to the
power ground behind the output capacitors.
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
Page 7 of 8
Preliminary Datasheet
Packaging Information
LP78081 – Ver. 1.0 Datasheet
Feb.-2008
LP78081
Page 8 of 8
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