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PDF LTC3736-2 Data sheet ( Hoja de datos )
| Número de pieza | LTC3736-2 | |
| Descripción | Synchronous Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3736-2 (archivo pdf) en la parte inferior de esta página. Total 28 Páginas | ||
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FEATURES
■ No Current Sense Resistors Required
■ Out-of-Phase Controllers Reduce Required
Input Capacitance
■ Tracking Function
■ Wide VIN Range: 2.75V to 9.8V
■ 0.6V ±1% Voltage Reference
■ High Current Limit
■ Constant Frequency Current Mode Operation
■ Low Dropout Operation: 100% Duty Cycle
■ True PLL for Frequency Locking or Adjustment
■ Selectable 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 Low Profile (4mm × 4mm) QFN and Narrow
SSOP Packages
U
APPLICATIO S
■ One or Two Lithium-Ion Powered Devices
■ Notebook and Palmtop Computers, PDAs
■ Portable Instruments
■ Distributed DC Power Systems
LTC3736-2www.DataSheet4U.com
Dual 2-Phase, No RSENSETM,
Synchronous Controller
with Output Tracking
DESCRIPTIO
The LTC®3736-2 is a 2-phase dual synchronous step-down
switching regulator controller with tracking that drives ex-
ternal complementary power MOSFETs using few external
components. The constant frequency current mode archi-
tecture with MOSFET VDS sensing eliminates the need for
sense resistors and improves efficiency. Power loss and
noise due to the ESR of the input capacitance are mini-
mized by operating the two controllers out of phase.
Pulse-skipping operation provides high efficiency at light
loads. 100% duty cycle capability 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 ca-
pacitors. For noise sensitive applications, the LTC3736-2
switching frequency can be externally synchronized from
250kHz to 850kHz. An internal soft-start, which can be
lengthened externally, smoothly ramps the output voltage
during start-up.
The LTC3736-2 is available in the tiny thermally enhanced
(4mm × 4mm) QFN and 24-lead narrow SSOP packages.
, LTC and LT are registered trademarks 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, 6144194, 6580258, 6304066, 6611131, 6498466.
TYPICAL APPLICATIO
High Efficiency, 2-Phase, Dual Synchronous DC/DC Step-Down Converter
VOUT1
2.5V
2.2µH
187k
47µF
220pF
59k
VIN
SENSE1+ SENSE2+
TG1 TG2
SW1 SW2
LTC3736-2
BG1 BG2
PGND
PGND
VFB1
VFB2
ITH1 ITH2
SGND
15k
10µF
×2
VIN
2.75V TO 9.8V
2.2µH
118k
220pF
15k
59k
VOUT2
1.8V
47µF
37362 TA01a
Efficiency and Power Loss
vs Load Current (Figure 15 Circuit)
100
VOUT = 2.5V
95
10
90
EFFICIENCY
85
1
80
75 0.1
POWER LOSS
70
65
0.01
60
55
50 0.001
1 10 100 1000 10000
LOAD CURRENT (mA)
37362 TA01b
37362fa
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1 page  
LTC3736-2www.DataSheet4U.com
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Maximum Current Sense Voltage
vs ITH Pin Voltage
100
FORCED CONTINUOUS
MODE
80 PULSE-SKIPPING
MODE
60
40
20
0
–20
0.5
1 1.5
ITH VOLTAGE (V)
2
37362 G09
Shutdown (RUN) Threshold
vs Temperature
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
–60 –40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
37362 G12
Efficiency vs Load Current
100
FIGURE 15 CIRCUIT
95 VIN = 3.3V
90 VOUT = 2.5V
85
80
PULSE-SKIPPING
MODE
75 (SYNC/FCB = VIN)
70
65
60
FORCED
CONTINUOUS
55 (SYNC/FCB = 0V)
50
1 10 100 1000 10000
LOAD CURRENT (mA)
37362 G10
RUN/SS Pull-Up Current
vs Temperature
1.0
0.9
0.8
0.7
0.6
0.5
0.4
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
37362 G13
Regulated Feedback Voltage
vs Temperature
0.606
0.605
0.604
0.603
0.602
0.601
0.600
0.599
0.598
0.597
0.596
0.595
0.594
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
37362 G14
Maximum Current Sense Threshold
vs Temperature
180
IPRG = GND
175
170
165
160
155
150
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
37362 G11
Oscillator Frequency
vs Temperature
10
8
6
4
2
0
–2
–4
–6
–8
–10
–60 –40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
37362 G15
Undervoltage Lockout Threshold
vs Temperature
2.50
2.45
VIN RISING
2.40
2.35
2.30
VIN FALLING
2.25
2.20
2.15
2.10
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
37362 G16
37362fa
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OPERATIO (Refer to Functional Diagram)
force the external P-channel MOSFET to stay off for the same
number of cycles. The inductor current is not allowed to
reverse, though (discontinuous operation). This mode, like
forced continuous operation, exhibits low output ripple as
well as low audio noise and reduced RF interference.
However, it provides low current efficiency higher than
forced continuous mode. During start-up or a short-circuit
condition (VFB1 or VFB2 ≤ 0.54V), the LTC3736-2 operates
in pulse-skipping mode (no current reversal allowed),
regardless of the state of the SYNC/FCB pin.
Short-Circuit Protection
When an output is shorted to ground (VFB < 0.12V), the
switching frequency of that controller is reduced to 1/5 of
the normal operating frequency. The other controller is
unaffected and maintains normal operation.
The short-circuit threshold on VFB2 is based on the smaller
of 0.12V and a fraction of the voltage on the TRACK pin.
This also allows VOUT2 to start up and track VOUT1 more
easily. Note that if VOUT1 is truly short-circuited
(VOUT1 = VFB1 = 0V), then the LTC3736-2 will try to
regulate VOUT2 to 0V if a resistor divider on VOUT1 is
connected to the TRACK pin.
Output Overvoltage Protection
As a further protection, the overvoltage comparator (OV)
guards against transient overshoots, as well as other more
serious conditions that may overvoltage the output. When
the feedback voltage on the VFB pin has risen 13.33%
above the reference voltage of 0.6V, the external P-chan-
nel MOSFET is turned off and the N-channel MOSFET is
turned on until the overvoltage is cleared.
Frequency Selection and Phase-Locked Loop
(PLLLPF and SYNC/FCB Pins)
The selection of switching frequency is a tradeoff between
efficiency and component size. Low frequency operation
increases efficiency by reducing MOSFET switching losses,
LTC3736-2www.DataSheet4U.com
but requires larger inductance and/or capacitance to main-
tain low output ripple voltage.
The switching frequency of the LTC3736-2’s controllers
can be selected using the PLLLPF pin.
If the SYNC/FCB is not being driven by an external clock
source, the PLLLPF can be floated, tied to VIN or tied to
SGND to select 550kHz, 750kHz or 300kHz respectively.
A phase-locked loop (PLL) is available on the LTC3736-2
to synchronize the internal oscillator to an external
clock source that is connected to the SYNC/FCB pin. In
this case, a series RC should be connected between the
PLLLPF pin and SGND to serve as the PLL’s loop filter. The
LTC3736-2 phase detector adjusts the voltage on the
PLLLPF pin to align the turn-on of controller 1’s external
P-channel MOSFET to the rising edge of the synchroniz-
ing signal. Thus, the turn-on of controller 2’s external
P-channel MOSFET is 180 degrees out of phase with the
rising edge of the external clock source.
The typical capture range of the LTC3736-2’s phase-
locked loop is from approximately 200kHz to 1MHz, and is
guaranteed over temperature to be between 250kHz and
850kHz. In other words, the LTC3736-2’s PLL is guaran-
teed to lock to an external clock source whose frequency
is between 250kHz and 850kHz.
Dropout Operation
When the input supply voltage (VIN) decreases towards
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.
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| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3736-2.PDF ] | |
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