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PDF LTC3736EGN Data sheet ( Hoja de datos )
| Número de pieza | LTC3736EGN | |
| Descripción | Dual 2-Phase/ No RSENSE Synchronous Controller with Output Tracking | |
| Fabricantes | Linear Technology | |
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LTC3736
Dual 2-Phase, No RSENSETM,
Synchronous Controller
with Output Tracking
FEATURES
s No Current Sense Resistors Required
s Out-of-Phase Controllers Reduce Required
Input Capacitance
s Tracking Function
s Wide VIN Range: 2.75V to 9.8V
s Constant Frequency Current Mode Operation
s 0.6V ±1.5% Voltage Reference
s Low Dropout Operation: 100% Duty Cycle
s True PLL for Frequency Locking or Adjustment
s Selectable Burst Mode®/Forced Continuous Operation
s Auxiliary Winding Regulation
s Internal Soft-Start Circuitry
s Power Good Output Voltage Monitor
s Output Overvoltage Protection
s Micropower Shutdown: IQ = 9µA
s Tiny Low Profile (4mm × 4mm) QFN and Narrow
SSOP Packages
U
APPLICATIO S
s One or Two Lithium-Ion Powered Devices
s Notebook and Palmtop Computers, PDAs
s Portable Instruments
s Distributed DC Power Systems
DESCRIPTIO
The LTC®3736 is a 2-phase dual synchronous step-down
switching regulator controller with tracking that drives
external complementary power MOSFETs using few exter-
nal components. The constant frequency current mode
architecture 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
minimized by operating the two controllers out of phase.
Burst Mode 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
switching frequency can be externally synchronized from
250kHz to 850kHz. Burst Mode operation is inhibited dur-
ing synchronization or when the SYNC/FCB pin is pulled low
in order to reduce noise and RF interference. Automatic soft-
start is internally controlled.
The LTC3736 is available in the tiny thermally enhanced
(4mm × 4mm) QFN package or 24-lead SSOP narrow
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.
TYPICAL APPLICATIO
High Efficiency, 2-Phase, Dual Synchronous DC/DC Step-Down Converter
VIN
SENSE1+ SENSE2+
10µF
×2
VIN
2.75V TO 9.8V
2.2µH
TG1 TG2
SW1 SW2
LTC3736
BG1 BG2
2.2µH
VOUT1
2.5V
+
187k
47µF
220pF
59k
PGND
PGND
VFB1
VFB2
ITH1 ITH2
SGND
15k
220pF
15k
118k
+
59k
VOUT2
1.8V
47µF
3736 TA01a
Efficiency vs Load Current
100
95
VIN = 3.3V
90
85 VIN = 4.2V
80
75 VIN = 5V
70
65
60
55
50
1
FIGURE 16 CIRCUIT
VOUT = 2.5V
10 100 1000
LOAD CURRENT (mA)
10000
3736 TA01b
3736f
1
1 page  
LTC3736
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Maximum Current Sense Voltage
vs ITH Pin Voltage
100
Burst Mode OPERATION
(ITH RISING)
80 Burst Mode OPERATION
(ITH FALLING)
FORCED CONTINUOUS
60 MODE
PULSE SKIPPING
40 MODE
20
0
–20
0.5
1 1.5
ITH VOLTAGE (V)
2
3736 G09
Efficiency vs Load Current
100
Burst Mode
95 OPERATION
90 (SYNC/FCB = VIN)
85
80
FORCED
75 CONTINUOUS
(SYNC/FCB = 0V)
70
PULSE SKIPPING
65 MODE
(SYNC/FCB = 550kHz)
60
FIGURE 15 CIRCUIT
55 VIN = 5V
50 VOUT = 2.5V
1 10 100 1000 10000
LOAD CURRENT (mA)
3736 G10
Regulated Feedback Voltage
vs Temperature
0.609
0.607
0.605
0.603
0.601
0.599
0.597
0.595
0.593
0.591
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3736 G11
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)
3736 G12
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
3736 G13
Maximum Current Sense Threshold
vs Temperature
135 IPRG = FLOAT
130
125
120
115
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3736 G14
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)
3736 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
3736 G16
3736f
5
5 Page 
U
OPERATIO (Refer to Functional Diagram)
When a controller is enabled for Burst Mode operation, the
inductor current is not allowed to reverse. Hence, the
controller operates discontinuously. The reverse current
comparator (RICMP) senses the drain-to-source voltage
of the bottom external N-channel MOSFET. This MOSFET
is turned off just before the inductor current reaches zero,
preventing it from reversing and going negative.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by the
voltage on the ITH pin. The P-channel MOSFET is turned on
every cycle (constant frequency) regardless of the ITH pin
voltage. In this mode, the efficiency at light loads is lower
than in Burst Mode operation. However, continuous mode
has the advantages of lower output ripple and less inter-
ference with audio circuitry.
When the SYNC/FCB pin is clocked by an external clock
source to use the phase-locked loop (see Frequency
Selection and Phase-Locked Loop), the LTC3736 operates
in PWM pulse skipping mode at light loads. In this mode,
the current comparator ICMP may remain tripped for
several cycles and 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 as compared to Burst Mode
operation. However, it provides low current efficiency
higher than forced continuous mode, but not nearly as
high as Burst Mode operation. During start-up or a short-
circuit condition (VFB1 or VFB2 ≤ 0.54V), the LTC3736
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
LTC3736
(VOUT1 = VFB1 = 0V), then the LTC3736 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,
but requires larger inductance and/or capacitance to main-
tain low output ripple voltage.
The switching frequency of the LTC3736’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 to
synchronize the internal oscillator to an external clock
source that 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
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 synchronizing 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’s phase-locked
loop is from approximately 200kHz to 1MHz, with a
guarantee over all variations and temperature to be be-
tween 250kHz and 850kHz. In other words, the LTC3736’s
PLL is guaranteed to lock to an external clock source
whose frequency is between 250kHz and 850kHz.
3736f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3736EGN.PDF ] | |
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