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PDF LTC3544 Data sheet ( Hoja de datos )
| Número de pieza | LTC3544 | |
| Descripción | Quad Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3544
Quad Synchronous
Step-Down Regulator: 2.25MHz,
300mA, 200mA, 200mA, 100mA
FEATURES
■ High Efficiency: Up to 95%
■ Four Independent Regulators Provide Up to 300mA,
200mA, 200mA and 100mA Output Current
■ 2.25V to 5.5V Input Voltage Range
■ 2.25MHz Constant-Frequency Operation
■ No Schottky Diodes Required
■ Extremely Low Channel-to-Channel Transient
Crosstalk
■ Low Ripple (20mVP-P) Burst Mode Operation:
IQ = 70µA (All Channels On)
■ 0.8V Reference Allows Low Output Voltages
■ Shutdown Mode Draws <1µA Supply Current
■ Current Mode Operation for Excellent Line and Load
Transient Response
■ Overtemperature Protected
■ Low Profile (3mm × 3mm) 16-Lead QFN Package
APPLICATIONS
■ Cellular Telephones
■ Personal Information Appliances
■ Wireless and DSL Modems
■ Digital Still Cameras
■ Media Players
■ Portable Instruments
DESCRIPTION
The LTC®3544 is a quad, high efficiency, monolithic
synchronous buck regulator using a constant-frequency,
current mode architecture. The four regulators operate
independently with separate run pins. The 2.25V to 5.5V
input voltage range makes the LTC3544 well suited for
single Li-Ion/polymer battery-powered applications.
100% duty cycle provides low dropout operation, ex-
tending battery runtime in portable systems. Low ripple
Burst Mode® operation increases efficiency at light loads,
further extending battery runtime with typically only 20mV
of ripple.
Switching frequency is internally set to 2.25MHz, allowing
the use of small surface mount inductors and capacitors.
The internal synchronous switches increase efficiency
and eliminate the need for external Schottky diodes. Low
output voltages are easily supported with the 0.8V feedback
reference voltage.
The LTC3544 is available in a low profile (0.75mm) (3mm
× 3mm) QFN package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents,
including 5481178, 6580258, 6304066, 6127815, 6498466, 6611131, 5994885.
TYPICAL APPLICATION
High Efficiency Quad Step-Down Converter
VIN
2.25V TO 5.5V
VOUT2
1.5V
200mA
4.7µF
CER
4.7µF
CER
4.7µH
93.1k
107k
VOUT4
1.8V
300mA
4.7µF
CER
3.3µH
133k
107k
VCC
RUN200B
SW200B
VFB200B
PVIN
RUN200A
SW200A
VFB200A
LTC3544
RUN300
SW300
VFB300
GNDA
RUN100
SW100
VFB100
PGND
3.3µH
100k
VOUT3
0.8V
200mA
4.7µF
CER
10µH
59k
118k
VOUT1
1.2V
100mA
4.7µF
CER
3544B TA01a
Efficiency vs Load Current, 300mA
Channel, All Other Channels Off
100 1
90
80 EFFICIENCY
70
0.1
60 LOSS
50 VOUT = 2.5V
40 TA = 25°C
0.01
30
20
10
0
0.0001
0.001
0.001
0.01
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V 0.0001
0.1 1
LOAD CURRENT (A)
3544 TA01b
3544f
1
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TYPICAL PERFORMANCE CHARACTERISTICS
LTC3544
Efficiency vs Load Current 300mA
Channel. All Other Channels Off
100
90
80
70
60
50
40
30
20
10
0
0.0001
VIN = 2.25V
VIN = 3.6V
VIN = 4.2V
VOUT = 1.8V
TA = 25°C
ALL OTHER
CHANNELS OFF
0.001
0.01
0.1
LOAD CURRENT (A)
1
3544 G07
Efficiency vs Load Current 200mA
Channel A. All Other Channels Off
100
VIN = 2.25V
90 VIN = 3.6V
80 VIN = 4.2V
70
60
50
40
30
20
10
0
0.0001
VOUT = 0.8V
TA = 25°C
ALL OTHER CHANNELS OFF
0.001
0.01
0.1
LOAD CURRENT (A)
1
3544 G08
Efficiency vs Load Current 200mA
Channel B. All Other Channels Off
100
90
80
70
60
50
40
30
20
10
0
0.0001
VIN = 2.25V
VIN = 3.6V
VIN = 4.2V
VOUT = 1.5V
TA = 25°C
ALL OTHER
CHANNELS OFF
0.001
0.01
0.1
LOAD CURRENT (A)
1
3544 G09
Efficiency vs Load Current 100mA
Channel. All Other Channels Off
100
90
80
70
60
50
40
30
20
10
0
0.0001
VIN = 2.25V
VIN = 3.6V
VIN = 4.2V
VOUT = 1.2V
TA = 25°C
ALL OTHER
CHANNELS OFF
0.001
0.01
0.1
LOAD CURRENT (A)
1
3544 G10
Load Regulation
All Channels
1.2
100mA = 1.2V
1.0
200mA (A) = 0.8V
200mA (B) = 1.5V
300mA = 1.8V
0.8
VIN = 3.6V
TA = 25°C
0.6
Burst Mode OPERATION
0.4 RIPPLE
0.2
0
–0.2
0
100 200 300
LOAD (mA)
400
3544 G11
Start-Up Curves
All Channels
VOUT100
VOUT200A
VOUT200B
VOUT300
RUNx
VIN = 3.6V
200µs/DIV
TA = 25°C
ALL CHANNELS UNLOADED
Load Step Response
300mA Channel
VOUT300
100mV/DIV
AC
COUPLED
IL
250mA/DIV
ILOAD
250mA/DIV
VIN = 3.6V
20µs/DIV
VOUT = 1.8V
TA = 25°C
ILOAD = 600µA TO 300mA
Load Step Response
200mA Channel A
VOUT200A
50mV/DIV
AC
COUPLED
IL
250mA/DIV
ILOAD
250mA/DIV
3544 G13
VIN = 3.6V
20µs/DIV
VOUT = 0.8V
TA = 25°C
ILOAD = 600µA TO 200mA
Load Step Response
200mA Channel B
VOUT200B
50mV/DIV
AC COUPLED
IL
250mA/DIV
ILOAD
250mA/DIV
3544 G14
VIN = 3.6V
20µs/DIV
VOUT = 1.5V
TA = 25°C
ILOAD = 600µA TO 200mA
3544 G12
3544 G15
3544f
5
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LTC3544
APPLICATIONS INFORMATION
voltage, the output ripple is highest at maximum input
voltage since ΔIL increases with input voltage.
Using Ceramic Input and Output Capacitors
Higher value, lower cost, ceramic capacitors are now
widely available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them
ideal for switching regulator applications. Because the
LTC3544’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors
are used at the input and the output. When a ceramic
capacitor is used at the 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 best, this
ringing can couple to the output and be mistaken as loop
instability. 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.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
Output Voltage Programming
The output voltage is set by tying VFB to a resistive divider
according to the following formula:
VOUT
=
0.8V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
The external resistive divider is connected to the output
allowing remote voltage sensing as shown in Figure 2.
0.8V ≤ VOUT ≤ 5.5V
VFB
LTC3544
GND
R2 CF
R1
3544 F02
Figure 2. Setting the LTC3544 Output Voltage
Keeping the current in the resistors small maximizes the
efficiency, but making them too small may allow stray
capacitance to cause noise problems or reduce the phase
margin of the control loop. It is recommended that the
total feedback resistor string be kept to under 100k.
To improve the frequency response of the control loop, a
feed forward capacitor, CF, may be used. Great care should
be taken to route the feedback line away from noise sources
such as the inductor of the SW line.
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 circuit produce
losses, two main sources usually account for most of the
losses in LTC3544 circuits: VIN quiescent current and I2R
losses. VIN quiescent current loss dominates the efficiency
loss at low load currents, whereas the I2R loss dominates
the efficiency loss at medium to high load currents.
1. The 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 is switched
from high to low to high again, a packet of charge, dQ,
moves from PVIN to ground. The resulting dQ/dt is the
current out of PVIN that is typically larger than the DC
bias current and proportional to frequency. Both the
DC bias and gate charge losses are proportional to
PVIN and thus their effects will be more pronounced
at higher supply voltages.
2. I2R losses are 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 SW pin is a function of both
3544f
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| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3544.PDF ] | |
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