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PDF LTC3561 Data sheet ( Hoja de datos )
| Número de pieza | LTC3561 | |
| Descripción | Synchronous Step-Down DC/DC Converter | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ Uses Tiny Capacitors and Inductor
■ High Frequency Operation: Up to 4MHz
■ High Switch Current: 1.4A
■ Low RDS(ON) Internal Switches: 0.110Ω
■ High Efficiency: Up to 95%
■ VIN: 2.63V to 5.5V
■ Stable with Ceramic Capacitors
■ Current Mode Operation for Excellent Line
and Load Transient Response
■ Short-Circuit Protected
■ Low Dropout Operation: 100% Duty Cycle
■ Low Shutdown Current: IQ ≤ 1µA
■ Low Quiescent Current: 240µA
■ Output Voltages from 0.8V to 5V
■ Low Noise Pulse-Skipping Operation
■ Small 8-Pin DFN Package
U
APPLICATIO S
■ Wireless LAN Power
■ Notebook Computers
■ Digital Cameras
■ Cellular Phones
■ Board Mounted Power Supplies
LTC3561www.DataSheet4U.com
1A, 4MHz, Synchronous
Step-Down DC/DC Converter
DESCRIPTIO
The LTC®3561 is a constant-frequency, synchronous,
step-down DC/DC converter. Intended for medium power
applications, it operates from a 2.63V to 5.5V input voltage
range and has a user configurable operating frequency up
to 4MHz, allowing the use of tiny, low cost capacitors and
inductors 2mm or less in height. The output voltage is
adjustable from 0.8V to 5V. Internal synchronous 0.11Ω
power switches with 1.4A peak current ratings provide
high efficiency. The LTC3561’s current mode architecture
and external compensation allow the transient response
to be optimized over a wide range of loads and output
capacitors.
To further maximize battery life, the P-channel MOSFET is
turned on continuously in dropout (100% duty cycle). The
no-load quiescent current is only 240µA. In shutdown, the
device draws <1µA.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by U.S. Patents,
including 5481178, 6127815, 6304066, 6498466, 6580258, 6611131.
TYPICAL APPLICATIO
13k
1000pF
Step-Down 2.5V/1A Regulator
VIN
2.63V TO 5.5V
PVIN
SVIN
ITH LTC3561 SW
SHDN/RT
SGND
324k
VFB
PGND
22µF
2.2µH
887k
412k
VOUT
2.5V/1A
22µF
NOTE: IN DROPOUT, THE OUTPUT TRACKS
THE INPUT VOLTAGE.
3561 F01
Efficiency and Power Loss vs Load Current
100
95
90
85
80
75
70
65
60
55
50
10
EFFICIENCY
1000
100
POWER LOSS
10
VIN = 3.3V
VOUT = 2.5V
fO = 1MHz 1
100 1000
LOAD CURRENT (mA)
3561 TA01
3561f
1
1 page  
BLOCK DIAGRA
SVIN
6
SGND
2
ITH
8
0.8V VOLTAGE
REFERENCE
VFB 7
+
–
ERROR
AMPLIFIER
–
+
VB
OSCILLATOR
LTC3561www.DataSheet4U.com
ITH
LIMIT
PVIN
5
PMOS CURRENT
COMPARATOR
+
–
SLOPE
COMPENSATION
3 SW
U
OPERATIO
1
SHDN/RT
The LTC3561 uses a constant frequency, current mode
architecture. The operating frequency is determined by
the value of the RT resistor.
The output voltage is set by an external divider returned to
the VFB pin. An error amplifier compares the divided
output voltage with a reference voltage of 0.8V and adjusts
the peak inductor current accordingly.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the VFB voltage is below the reference voltage. The
current into the inductor and the load increases until the
current limit is reached. The switch turns off and energy
stored in the inductor flows through the bottom switch (N-
channel MOSFET) into the load until the next clock cycle.
The peak inductor current is controlled by the voltage on
the ITH pin, which is the output of the error amplifier. This
amplifier compares the VFB pin to the 0.8V reference.
When the load current increases, the VFB voltage de-
creases slightly below the reference. This decrease causes
the error amplifier to increase the ITH voltage until the
average inductor current matches the new load current. At
+
LOGIC
NMOS
COMPARATOR
–
+
REVERSE –
COMPARATOR
4 PGND
3561 BD
low load currents, the inductor current becomes discontinu-
ous, and pulses may be skipped to maintain regulation.
The main control loop is shut down by pulling the SHDN/RT
pin to SVIN. A digital soft-start is enabled after shutdown,
which will slowly ramp the peak inductor current up over
1024 clock cycles or until the output reaches regulation,
whichever is first. Soft-start can be lengthened by ramping
the voltage on the ITH pin (see Applications Information
section).
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which is
the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being
equal to the input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Low Supply Operation
The LTC3561 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.5V to prevent unstable operation.
3561f
5
5 Page 
LTC3561www.DataSheet4U.com
APPLICATIO S I FOR ATIO
looking into the SW pin is a function of both top and
bottom MOSFET 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 Characteris-
tics curves. Thus, to obtain I2R losses:
I2R losses = IOUT2(RSW + RL)
4) Other “hidden” losses such as copper trace and internal
battery resistances can account for additional efficiency
degradations in portable systems. It is very important to
include these “system” level losses in the design of a
system. The internal battery and fuse resistance losses
can be minimized by making sure that CIN has adequate
charge storage and very low ESR at the switching
frequency. Other losses including diode conduction
losses during dead-time and inductor core losses gen-
erally account for less than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3561 does not dissi-
pate much heat due to its high efficiency. However, in
applications where the LTC3561 is running at high ambient
temperature with low supply voltage and high duty cycles,
such as in dropout, the heat dissipated may exceed the
maximum junction temperature of the part. If the junction
temperature reaches approximately 150°C, both power
switches will be turned off and the SW node will become
high impedance.
To avoid the LTC3561 from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the maxi-
mum junction temperature of the part. The temperature
rise is given by:
TRISE = PD • θJA
where PD is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, TJ, is given by:
TJ = TRISE + TAMBIENT
As an example, consider the case when the LTC3561 is in
dropout at an input voltage of 3.3V with a load current of
1A. From the Typical Performance Characteristics graph
of Switch Resistance, the RDS(ON) resistance of the
P-channel switch is 0.11Ω. Therefore, power dissipated
by the part is:
PD = I2 • RDS(ON) = 110mW
The DD8 package junction-to-ambient thermal resistance,
θJA, will be in the range of about 43°C/W. Therefore, the
junction temperature of the regulator operating in a 70°C
ambient temperature is approximately:
TJ = 0.11 • 43 + 70 = 74.7°C
Remembering that the above junction temperature is
obtained from an RDS(ON) at 25°C, we might recalculate
the junction temperature based on a higher RDS(ON) since
it increases with temperature. However, we can safely
assume that the actual junction temperature will not
exceed the absolute maximum junction temperature of
125°C.
Design Example
As a design example, consider using the LTC3561 in a
portable application with a Li-Ion battery (refer to Figure 4
for reference designation). The battery provides a VIN =
2.5V to 4.2V. The load requires a maximum of 1A in active
mode and 10mA in standby mode. The output voltage is
VOUT = 2.5V.
First, calculate the timing resistor:
RT = 9.78 • ( )1011 1MHz −1.08 = 323.8k
Use a standard value of 324k. Next, calculate the inductor
value for about 40% ripple current at maximum VIN:
L
=
2.5V
1MHz • 400mA
•
⎛
⎝⎜
1−
2.5V
4.2V
⎞
⎠⎟
=
2.5µH
Choosing the closest inductor from a vendor of 2.2µH,
results in a maximum ripple current of:
∆IL
=
2.5V
1MHz • 2.2µ
• ⎛⎝⎜1−
2.5V
4.2V
⎞⎠⎟
=
460mA
3561f
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3561.PDF ] | |
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