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PDF LTC3448 Data sheet ( Hoja de datos )
| Número de pieza | LTC3448 | |
| Descripción | 1.5MHz/2.25MHz 600mA Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3448 (archivo pdf) en la parte inferior de esta página. Total 20 Páginas | ||
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FEATURES
■ High Efficiency: Up to 96%
■ Very Low Quiescent Supply Current: 32µA During
Linear Regulator Operation
■ 600mA Output Current (Buck Converter)
■ Optionally Operates as Linear Regulator Below
3mA—External or Automatic ON/OFF
■ 2.5V to 5.5V Input Voltage Range
■ 1.5MHz or 2.25MHz Constant Frequency Operation
or External Synchronization
■ No Schottky Diode Required
■ Low Dropout Operation: 100% Duty Cycle
■ 0.6V 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) 8-Lead DFN and 8-Lead
MSOP Packages
U
APPLICATIO S
■ Cellular Telephones
■ Personal Information Appliances
■ Wireless and DSL Modems
■ Digital Still Cameras
■ MP3 Players
■ Portable Instruments
, LTC and LT are registered trademarks 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. Others pending.
LTC3448www.DataSheet4U.com
1.5MHz/2.25MHz, 600mA
Synchronous Step-Down
Regulator with LDO Mode
DESCRIPTIO
The LTC®3448 is a high efficiency, monolithic, synchronous
buck regulator using a constant frequency, current mode
architecture. Supply current during operation is only 32µA
(linear regulator mode) and drops to <1µA in shutdown. The
2.5V to 5.5V input voltage range makes the LTC3448 ide-
ally suited for single Li-Ion battery-powered applications.
100% duty cycle provides low dropout operation, extend-
ing battery life in portable systems. At moderate output load
levels, PWM pulse skipping mode operation provides very
low output ripple voltage for noise sensitive applications.
The LTC3448 automatically switches into linear regulator
operation at very low load currents to maintain <5mVP-P
output voltage ripple. Supply current in this mode is
typically 32µA. The switch to linear regulator mode occurs
at a threshold of 3mA. Linear regulator operation can be set
to on, off or automatic turn on/off.
Switching frequency is selectable at either 1.5MHz or
2.25MHz, allowing the use of small surface mount induc-
tors and capacitors.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.6V feed-
back reference voltage. The LTC3448 is available in a low
profile 3mm × 3mm DFN package or thermally enhanced
8-lead MSOP.
TYPICAL APPLICATIO
1.5V High Efficiency Regulator with Automatic LDO Mode
VIN
2.5V TO 5.5V
CIN
4.7µF
VIN SW
RUN VOUT
LTC3448
MODE
FREQ VFB
SYNC
GND
2.2µH
22pF 474k
316k
COUT
4.7µF
VOUT
1.5V
3448 TA01a
Efficiency and Power Loss vs Load Current
100 VIN = 3.6V
90 VOUT = 1.5V
80 TA = 25°C
70
EFFICIENCY
60
50
POWER LOSS
1
0.1
0.01
40
30
0.001
20
10
0
0.0001
0.001
0.01
0.1
0.0001
1
LOAD CURRENT (A)
23448 TA01b
3448f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure1a Except for the Resistive Divider Resistor Values)
LTC3448www.DataSheet4U.com
RDS(ON) vs Temperature
0.6
0.5
0.4
0.3
0.2
MAIN SWITCH SYNCH SWITCH
0.1 2.5V
2.5V
3.6V 3.6V
4.2V 4.2V
0
–50 –25 0 25 50 75
TEMPERATURE (°C)
100 125
3448 G10
Switch Leakage vs Temperature
350 VIN = 5.5V
RUN = 0V
300
250
200
MAIN
SWITCH
150
100
SYNCHRONOUS
SWITCH
50
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3448 G13
Load Step
Dynamic Supply Current
vs Supply Voltage
340
ILOAD = 0A
320 TA = 25°C
300
280
2.25MHz
260
1.5MHz
240
220
Dynamic Supply Current
vs Temperature
320
VIN = 3.6V
ILOAD = 0A
300
280
2.25MHz
260
1.5MHz
240
220
200
2
345
SUPPLY VOLTAGE (V)
6
3448 G11
200
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3448 G12
Switch Leakage vs Input Voltage
10 RUN = 0V
TA = 25°C
1
SYNCHRONOUS
SWITCH
MAIN
SWITCH
Start-Up from Shutdown
RUN
5V/DIV
VOUT
1V/DIV
0.1
IL
500mA/DIV
0.01
0.001
0
12 345
INPUT VOLTAGE (V)
6
3448 G14
VIN = 3.6V
VOUT = 1.5V
ILOAD = 600mA
40µs/DIV
3448 G15
Load Step
VOUT
200mV/DIV
AC COUPLED
ILOAD
100mA/DIV
IL
500mA/DIV
VIN = 3.6V
10µs/DIV
VOUT = 1.5V
ILOAD = 100µA TO 200mA
COUT = 10µF
3448 G16
VOUT
100mV/DIV
AC COUPLED
ILOAD
250mA/DIV
IL
500mA/DIV
VIN = 3.6V
10µs/DIV
VOUT = 1.5V
ILOAD = 50mA TO 600mA
COUT = 10µF
3448 G17
3448f
5
5 Page 
LTC3448www.DataSheet4U.com
APPLICATIO S I FOR ATIO
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
=
⎛
0.6V ⎝⎜ 1+
R2⎞
R1⎠⎟
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 5.
VFB
LTC3448
GND
0.6V ≤ VOUT ≤ 5.5V
R2
R1
3448 F05
Figure 5. Setting the LTC3448 Output Voltage
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 LTC3448 circuits: VIN quiescent current and I2R
losses. When in switching mode, 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. At very low load currents
with the part operating in LDO mode, efficiency can be
dominated by I2R losses in the pass transistor and is a
strong function of (VIN – VOUT). In a typical efficiency plot,
the efficiency curve at very low load currents can be
misleading since the actual power lost is of little conse-
quence as illustrated in Figure 6.
1
VIN = 3.6V
FREQ = 0V
LDOCNTRL = VOUT(AUTO)
0.1
0.01
0.001
0.0001
0.0001
0.001
0.01
0.1
LOAD CURRENT (A)
1.2V
1.5V
1.8V
1
3448 F06
Figure 6. Power Loss vs Load Current
1. The VIN quiescent current is due to two components:
the DC bias current as given in the Electrical Character-
istics 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 VIN to ground. The resulting
dQ/dt is the current out of VIN that is typically larger than
the DC bias current and proportional to frequency. Both
the DC bias and gate charge losses are proportional to
VIN 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
3448f
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11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3448.PDF ] | |
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