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PDF LTC3548-1 Data sheet ( Hoja de datos )
| Número de pieza | LTC3548-1 | |
| Descripción | Fixed Output 2.25MHz Step-Down DC/DC Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
n High Efficiency: Up to 95%
n 1.8V at 800mA/1.575V at 400mA
n Very Low Quiescent Current: Only 40μA
n 2.25MHz Constant Frequency Operation
n High Switch Current: 1.2A and 0.7A
n No Schottky Diodes Required
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n Current Mode Operation for Excellent Line
and Load Transient Response
n Short-Circuit Protected
n Low Dropout Operation: 100% Duty Cycle
n Ultralow Shutdown Current: IQ < 1μA
n Small Thermally Enhanced 3mm × 3mm
DFN Packages
APPLICATIONS
n PDAs/Palmtop PCs
n Digital Cameras
n Cellular Phones
n Portable Media Players
n PC Cards
n Wireless and DSL Modems
LTC3548-1
Dual Synchronous, Fixed
Output 2.25MHz Step-Down
DC/DC Regulator
DESCRIPTION
The LTC®3548-1 is a dual, fixed output, constant frequency,
synchronous step-down DC/DC converter. Intended for low
power applications, it operates from 2.5V to 5.5V input
voltage range and has a constant 2.25MHz switching
frequency, allowing the use of tiny, low cost capacitors
and inductors with a profile ≤1mm. The output voltage
for channel 1 is fixed at 1.8V and for channel 2 is fixed
at 1.575V. Internal synchronous 0.35Ω, 1.2A/0.7A power
switches provide high efficiency without the need for ex-
ternal Schottky diodes. Burst Mode® operation provides
high efficiency at light loads.
To further maximize battery runtime, the P-channel
MOSFETs are turned on continuously in dropout (100%
duty cycle), and both channels draw a total quiescent cur-
rent of only 40μA. In shutdown, the device draws <1μA.
The LTC3548-1 is available in both thin (0.75mm) and
ultra-thin (0.55mm) 3mm × 3mm DFN packages.
L, LT, LTC, LTM and Burst Mode 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.
TYPICAL APPLICATION
VIN
2.7V TO 5.5V
VOUT2
1.575V
400mA
COUT2
10μF
CER
CIN
10μF
CER
4.7μH
CFF2
330pF
VIN RUN1 RUN2
LTC3548-1
SW2 SW1
VOUT2
VOUT1
VFB2
GND
VFB1
2.2μH
CFF1
330pF
VOUT1
1.8V
800mA
COUT1
10μF
CER
3548-1 F01
Figure 1. 1.8V/1.575V at 800mA/400mA Step-Down Regulators
LTC3548-1 Efficiency Curve/Power Loss
100 1000
95
90
VOUT1 = 1.8V
85
100
80 VOUT2 = 1.575V
75
10
70 1
65 CHANNEL 1
CHANNEL 2
60 0.1
1 10 100 1000
LOAD CURRENT (mA)
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1 page  
LTC3548-1
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise specified.
Efficiency vs Load Current
100
95
90 VIN = 3.6V
VIN = 2.7V
85
VIN = 4.2V
80
75
70
www.DataSheet4U.com
65
60
1
VOUT1 = 1.8V
NO LOAD ON OTHER CHANNEL
CIRCUIT OF FIGURE 3
10 100
LOAD CURRENT (mA)
1000
3548-1 G16
Efficiency vs Load Current
100
95
VIN = 3.6V VIN = 2.7V
90
85
80 VIN = 4.2V
75
70
65
VOUT2 = 1.575V
NO LOAD ON OTHER CHANNEL
60 CIRCUIT OF FIGURE 3
1 10 100 1000
LOAD CURRENT (mA)
3548-1 G17
PIN FUNCTIONS
VFB1 (Pin 1): Output Feedback for Channel 1. Receives the
feedback voltage from internal resistive divider across the
output. Normal voltage for this pin is 0.6V.
VOUT1 (Pin 2): Output Voltage Feedback Pin for Channel 1.
An internal resistive divider divides the output voltage down
for comparison to the internal reference voltage.
VIN (Pin 3): Input Power Supply. Must be closely decoupled
to GND.
SW1 (Pin 4): Regulator 1 Switch Node Connection to the
Inductor. This pin swings from VIN to GND.
GND (Pin 5): Ground. Connect to the (–) terminal of COUT
and (–) terminal of CIN (see Figure 4).
RUN2 (Pin 6): Regulator 2 Enable. Forcing this pin to VIN en-
ables regulator 2, while forcing it to GND causes regulator 2
to shut down. This pin must be driven; do not float.
SW2 (Pin 7): Regulator 2 Switch Node Connection to the
Inductor. This pin swings from VIN to GND.
RUN1 (Pin 8): Regulator 1 Enable. Forcing this pin to VIN en-
ables regulator 1, while forcing it to GND causes regulator 1
to shut down. This pin must be driven; do not float.
VOUT2 (Pin 9): Output Voltage Feedback Pin for Channel 2.
An internal resistive divider divides the output voltage down
for comparison to the internal reference voltage.
VFB2 (Pin 10): Output Feedback for Channel 2. Receives
the feedback voltage from internal resistive divider across
the output. Normal voltage for this pin is 0.6V.
Exposed Pad (GND) (Pin 11): Ground. Connect to the
(–) terminal of COUT, and (–) terminal of CIN. Must be con-
nected to electrical ground on PCB (see Figure 4).
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LTC3548-1
APPLICATIONS INFORMATION
2) The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current re-
sults from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched from
low to high to low again, a packet of charge dQ moves
from VIN to ground. The resulting dQ/dt is a current
out of VIN that is typically much larger than the DC bias
current. In continuous mode, IGATECHG = fO(QT + QB),
where QT and QB are the gate charges of the internal
top and bottom MOSFET switches. The gate charge
www.DalotassSheeseat4rUe.cpormoportional to VIN and thus their effects
will be more pronounced at higher supply voltages.
3) I2R losses are calculated from the DC resistances of
the internal switches, RSW, and external inductor, RL.
In continuous mode, the average output current flows
through inductor L, but is “chopped” between the internal
top and bottom switches. Thus, the series resistance
looking into the SW pin is a function of both top and
bottom MOSFET RDS(ON) and the duty cycle (D) as
follows:
RSW = (RDS(ON)TOP)(D) + (RDS(ON)BOT)(1 – D)
The RDS(ON) for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
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 fre-
quency. Other losses including diode conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3548-1 does not
dissipate much heat due to its high efficiency. However,
in applications where the LTC3548-1 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 turn off and the SW node will
become high impedance.
To prevent the LTC3548-1 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 maximum
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 LTC3548-1 is
at an input voltage of 2.7V with a load current of 400mA
and 800mA and an ambient temperature of 70°C. From
the Typical Performance Characteristics graph of Switch
Resistance, the RDS(ON) resistance of the main switch is
0.425Ω. Therefore, power dissipated by each channel is:
PD = I2 • RDS(ON) = 272mW and 68mW
The DFN package junction-to-ambient thermal resistance,
θJA, is 40°C/W. Therefore, the junction temperature of
the regulator operating in a 70°C ambient temperature is
approximately:
TJ = (0.272 + 0.068) • 40 + 70 = 83.6°C
which is below the absolute maximum junction tempera-
ture of 125°C.
Design Example
As a design example, consider using the LTC3548-1 in
an portable application with a Li-Ion battery. The battery
provides a VIN = 2.8V to 4.2V. The load requires a maximum
of 800mA in active mode and 2mA in standby mode. The
output voltage is VOUT = 1.8V.
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| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3548-1.PDF ] | |
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