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PDF LTC3808 Data sheet ( Hoja de datos )
| Número de pieza | LTC3808 | |
| Descripción | Synchronous DC/DC Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3808 (archivo pdf) en la parte inferior de esta página. Total 28 Páginas | ||
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FEATURES
■ Programmable Output Voltage Tracking
■ Sense Resistor Optional
■ Spread Spectrum Modulation for Low Noise
■ Constant Frequency Current Mode Operation for
Excellent Line and Load Transient Response
■ Wide VIN Range: 2.75V to 9.8V
■ Wide VOUT Range: 0.6V to VIN
■ 0.6V ±1.5% Reference
■ Low Dropout Operation: 100% Duty Cycle
■ True PLL for Frequency Locking or Adjustment
(Frequency Range: 250kHz to 750kHz)
■ Selectable Burst Mode®/Pulse Skipping/Forced
Continuous Operation
■ Auxiliary Winding Regulation
■ Internal Soft-Start Circuitry
■ Power Good Output Voltage Monitor
■ Output Overvoltage Protection
■ Micropower Shutdown: IQ = 9µA
■ Tiny Thermally Enhanced Leadless (4mm × 3mm)
DFN or 16-lead SSOP Package
U
APPLICATIO S
■ One or Two Cell Lithium-Ion Powered Devices
■ Notebook and Palmtop Computers, PDAs
■ Portable Instruments
■ Distributed DC Power Systems
LTC3808
No RSENSETM, Low EMI,
Synchronous DC/DC Controller
with Output Tracking
DESCRIPTIO
The LTC®3808 is a synchronous step-down switching
regulator controller that drives external complementary
power MOSFETs using few external components. The
constant frequency current mode architecture with MOSFET
VDS sensing eliminates the need for a current sense
resistor and improves efficiency.
Burst Mode operation provides high efficiency operation
at light loads. 100% duty cycle provides low dropout
operation, extending operating time in battery-powered
systems.
The switching frequency can be programmed up to 750kHz,
allowing the use of small surface mount inductors and
capacitors. For noise sensitive applications, the LTC3808
can be externally synchronized from 250kHz to 750kHz.
Burst Mode is inhibited during synchronization or when
the SYNC/MODE pin is pulled low to reduce noise and RF
interference. To further reduce EMI, the LTC3808 incorpo-
rates a novel spread spectrum frequency modulation
technique.
The LTC3808 is available in the tiny footprint thermally
enhanced DFN package or 16-lead SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode
is a registered trademark of Linear Technology Corporation. No RSENSE is a trademark of
Linear Technology Corporation. All other trademarks are the property of their respective
owners. Protected by U.S. Patents including 5481178, 5929620, 6580258, 6304066,
5847554, 6611131, 6498466. Other Patents pending.
TYPICAL APPLICATIO
High Efficiency, 550kHz Step-Down Converter
1M LTC3808 VIN
PLLLPF SENSE+
VIN
2.75V TO 9.8V
10µF
SYNC/MODE TG
59k PGOOD
187k
15k
220pF
VFB
ITH
RUN
SW
BG
IPRG
GND
2.2µH
+
47µF
VOUT
2.5V
2A
3808 TA01
Efficiency and Power Loss vs Load Current
100
EFFICIENCY
10k
VIN = 3.3V
90
1k
VIN = 5V
VIN = 4.2V
80
100
TYPICAL POWER
70 LOSS (VIN = 4.2V) 10
60
50
1
1
FIGURE 11 CIRCUIT
VOUT = 2.5V
0.1
10 100 1k 10k
LOAD CURRENT (mA)
3808 TA01b
3808f
1
1 page  
LTC3808
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Start-Up with Coincident Tracking
(VOUT = 0V at 0s)
Start-Up with Coincident Tracking
(VOUT = 0.8V at 0s)
Start-Up with Ratiometric
Tracking (VOUT = 0V at 0s)
Vx
2.5V
VOUT
1.8V
Vx
2.5V
VOUT
1.8V
500mV/DIV
500mV/DIV
Vx
2.5V
VOUT
1.8V
500mV/DIV
10ms/DIV
VIN = 4.2V
RTA = 590Ω
RTB = 1.18k
FIGURE 11 CIRCUIT
3808 G09
10ms/DIV
VIN = 4.2V
RTA = 590Ω
RTB = 1.18k
FIGURE 11 CIRCUIT
3808 G10
10ms/DIV
VIN = 4.2V
RTA = 590Ω
RTB = 1.69k
FIGURE 11 CIRCUIT
3808 G11
Regulated Feedback Voltage vs
Temperature
0.606
0.604
0.602
0.600
0.598
0.596
0.594
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3808 G12
Undervoltage Lockout Threshold
vs Temperature
2.55
2.50
VIN RISING
2.45
2.40
2.35
2.30
VIN FALLING
2.25
2.20
2.15
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3808 G13
Shutdown (RUN) Threshold vs
Temperature
1.20
1.15
1.10
1.05
1.00
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3808 G14
Maximum Current Sense
Threshold vs Temperature
135 IPRG = FLOAT
130
125
120
TRACK/SS Start-Up Current vs
Temperature
1.04
TRACK/SS = 0V
1.02
1.00
0.98
0.96
115
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3808 G16
0.94
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3808 G17
3808f
5
5 Page 
U
OPERATIO (Refer to Functional Diagram)
A phase-locked loop (PLL) is available on the LTC3808 to
synchronize the internal oscillator to an external clock
source that connects to the SYNC/MODE pin. In this case,
a series RC should be connected between the PLLLPF pin
and GND to serve as the PLL’s loop filter. The LTC3808
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of the external P-channel MOSFET to the
rising edge of the synchronizing signal.
The typical capture range of the LTC3808’s phase-locked
loop is from approximately 200kHz to 1MHz.
Spread Spectrum Modulation (SYNC/MODE and
PLLLPF Pins)
Connecting the SYNC/MODE pin to a DC voltage above
1.35V and several hundred mV below VIN enables spread
spectrum modulation (SSM) operation. An internal 2.6µA
pull-down current source at SYNC/MODE helps to set the
voltage at the SYNC/MODE pin for this operation by tying
a resistor with appropriate value between SYNC/MODE
and VIN. This mode of operation spreads the internal
oscillator frequency fOSC (= 550kHz) over a wider range
(460kHz to 635kHz), reducing the peaks of the harmonic
output on a spectral analysis of the output noise. In this
case, a 2.2nF filter cap should be connected between the
PLLLPF pin and GND and another 1000pF cap should be
connected between PLLLPF and the SYNC/MODE pin. The
controller operates in PWM pulse skipping mode at light
loads when spread spectrum modulation is selected. See
more discussions in the Spread Spectrum Modulation
with SYNC/MODE and PLLLPF Pins in the Applications
Information section.
LTC3808
Dropout Operation
When the input supply voltage (VIN) approaches the
output voltage, the rate of change of the inductor current
while the external P-channel MOSFET is on
(ON cycle) decreases. This reduction means that the
P-channel MOSFET will remain on for more than one
oscillator cycle if the inductor current has not ramped up
to the threshold set by the EAMP on the ITH pin. Further
reduction in the input supply voltage will eventually cause
the P-channel MOSFET to be turned on 100%; i.e., DC.
The output voltage will then be determined by the input
voltage minus the voltage drop across the P-channel
MOSFET and the inductor.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below
safe input voltage levels, an undervoltage lockout is
incorporated in the LTC3808. When the input supply
voltage (VIN) drops below 2.25V, the external P- and
N-channel MOSFETs and all internal circuits are turned
off except for the undervoltage block, which draws only
a few microamperes.
Peak Current Sense Voltage Selection and Slope
Compensation (IPRG Pin)
When the LTC3808 controller is operating below 20% duty
cycle, the peak current sense voltage (between the SENSE+
and SENSE–/SW pins) allowed across the external P-
channel MOSFET is determined by:
∆VSENSE(MAX)
=
A
•
VITH
– 0.7V
10
where A is a constant determined by the state of the IPRG
pin. Floating the IPRG pin selects A = 1; tying IPRG to VIN
selects A = 5/3; tying IPRG to GND selects A = 2/3. The
3808f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3808.PDF ] | |
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