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PDF LTC1628-SYNC Data sheet ( Hoja de datos )
| Número de pieza | LTC1628-SYNC | |
| Descripción | 2-Phase Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear | |
| Logotipo |  |
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LTC1628-SYNC
High Efficiency, 2-Phase
Synchronous Step-Down Switching Regulator
FEATURES
■ Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
■ OPTI-LOOP® Compensation Minimizes COUT
■ Dual N-Channel MOSFET Synchronous Drive
■ ±1% Output Voltage Accuracy
■ Power Good Output Voltage Monitor
■ Phase-Lockable Fixed Frequency 150kHz to 300kHz
■ Wide VIN Range: 3.5V to 36V Operation
■ Very Low Dropout Operation: 99% Duty Cycle
■ Adjustable Soft-Start Current Ramping
■ Foldback Output Current Limiting
■ Latched Short-Circuit Shutdown with Defeat Option
■ Output Overvoltage Protection
■ Remote Output Voltage Sense
■ Low Shutdown IQ: 20µA
■ 5V and 3.3V Standby Regulators
■ Selectable Constant Frequency or Burst Mode®
Operation
■ Small 28-Lead SSOP Package
U
APPLICATIO S
■ Notebook and Palmtop Computers, PDAs
■ Telecom Systems
■ Battery-Operated Digital Devices
■ DC Power Distribution Systems
DESCRIPTIO
The LTC®1628-SYNC is a high performance dual step-
down switching regulator controller that drives all
N-channel synchronous power MOSFET stages. A con-
stant frequency current mode architecture allows phase-
lockable frequency of up to 300kHz. Power loss and noise
due to the ESR of the input capacitors are minimized by
operating the two controller output stages out of phase.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance and
ESR values. The precision 0.8V reference and power good
output indicator are compatible with future microproces-
sor generations, and a wide 3.5V to 30V (36V maximum)
input supply range encompasses all battery chemistries.
A RUN/SS pin for each controller provides both soft-start
and optional timed, short-circuit shutdown. Current
foldback limits MOSFET dissipation during short-circuit
conditions when overcurrent latchoff is disabled. Output
overvoltage protection circuitry latches on the bottom
MOSFET until VOUT returns to normal. The FCB mode pin
can select among Burst Mode, constant frequency mode
and continuous inductor current mode or regulate a
secondary winding.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by U.S. Patents,
including 5481178, 5929620, 6177787, 6144194, 6100678, 5408150, 6580258, 6304066,
5705919.
TYPICAL APPLICATIO
L1
6.3µH
D1
VOUT1
5V
5A
RSENSE1
0.01Ω
+
COUT1
47µF
6V
SP
R2
105k
1%
+
4.7µF
M1
CB1, 0.1µF
M2
D3
VIN PGOOD INTVCC
TG1 TG2
BOOST1
BOOST2
SW1 SW2
LTC1628-SYNC
BG1 BG2
1µF
D4 CERAMIC
CB2, 0.1µF
M3
M4
fIN
1000pF
PLLIN
SENSE1+
SENSE1–
PGND
SENSE2+
SENSE2–
1000pF
VOSENSE1
VOSENSE2
ITH1 ITH2
R1
CC1
220pF
RUN/SS1 SGND RUN/SS2
CC2
220pF
20k
1%
RC1
15k
CSS1
0.1µF
CSS2
0.1µF
RC2
15k
R3
20k
1%
VIN
CIN 5.2V TO 28V
22µF
50V
CERAMIC
L2
6.3µH
D2
R4
63.4k
1%
RSENSE2
0.01Ω
VOUT2
3.3V
5A
COUT
56µF
6V
SP
+
M1, M2, M3, M4: FDS6680A
Figure 1. High Efficiency Dual 5V/3.3V Step-Down Converter
1628 F01
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1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1628-SYNC
Internal 5V LDO Line Regulation
5.1
ILOAD = 1mA
5.0
4.9
4.8
4.7
4.6
4.5
4.4
0
5 10 15 20 25 30 35
INPUT VOLTAGE (V)
1628 G07
Maximum Current Sense Threshold
vs VRUN/SS (Soft-Start)
80
VSENSE(CM) = 1.6V
60
40
20
0
01
2 34
VRUN/SS (V)
56
1628 G10
Load Regulation
0.0
–0.1
FCB = 0V
VIN = 15V
FIGURE 1
–0.2
–0.3
–0.4
0
1 2 34
LOAD CURRENT (A)
5
1628 G13
Maximum Current Sense Threshold
vs Duty Factor
75
50
25
0
0 20 40 60 80 100
DUTY FACTOR (%)
1628 G08
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
80
76
72
68
64
60
0
1 2 34
COMMON MODE VOLTAGE (V)
5
1628 G11
VITH vs VRUN/SS
2.5
VOSENSE = 0.7V
2.0
1.5
1.0
0.5
0
012 34
VRUN/SS (V)
56
1628 G14
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
80
70
60
50
40
30
20
10
0
0 25 50 75 100
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
1628 G09
Current Sense Threshold
vs ITH Voltage
90
80
70
60
50
40
30
20
10
0
–10
–20
–30
0 0.5 1 1.5 2 2.5
VITH (V)
1628 G12
SENSE Pins Total Source Current
100
50
0
–50
–100
0
246
VSENSE COMMON MODE VOLTAGE (V)
1628 G15
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5 Page 
U
OPERATIO (Refer to Functional Diagram)
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious condi-
tions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned on
until the overvoltage condition is cleared.
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an internal
MOSFET. The MOSFET turns on and pulls the pin low when
either output is not within ±7.5% of the nominal output
level as determined by the resistive feedback divider.
When both outputs meet the ±7.5% requirement, the
MOSFET is turned off within 10µs and the pin is allowed to
be pulled up by an external resistor to a source of up to 7V.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff
The RUN/SS capacitors are used initially to limit the inrush
current of each switching regulator. After the controller
has been started and been given adequate time to charge
up the output capacitors and provide full load current, the
RUN/SS capacitor is used in a short-circuit time-out
circuit. If the output voltage falls to less than 70% of its
nominal output voltage, the RUN/SS capacitor begins
discharging on the assumption that the output is in an
overcurrent and/or short-circuit condition. If the condi-
tion lasts for a long enough period as determined by the
size of the RUN/SS capacitor, the controller will be shut
down until the RUN/SS pin(s) voltage(s) are recycled.
This built-in latchoff can be overridden by providing a
>5µA pull-up at a compliance of 5V to the RUN/SS pin(s).
This current shortens the soft start period but also pre-
vents net discharge of the RUN/SS capacitor(s) during an
overcurrent and/or short-circuit condition. Foldback cur-
rent limiting is also activated when the output voltage falls
below 70% of its nominal level whether or not the short-
circuit latchoff circuit is enabled. Even if a short is present
and the short-circuit latchoff is not enabled, a safe, low
output current is provided due to internal current foldback
and actual power wasted is low due to the efficient nature
of the current mode switching regulator.
LTC1628-SYNC
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC1628-SYNC dual high efficiency DC/DC controller
brings the considerable benefits of 2-phase operation to
portable applications for the first time. Notebook comput-
ers, PDAs, handheld terminals and automotive electronics
will all benefit from the lower input filtering requirement,
reduced electromagnetic interference (EMI) and increased
efficiency associated with 2-phase operation.
Why the need for 2-phase operation? Up until the LTC1628
family, constant-frequency dual switching regulators op-
erated both channels in phase (i.e., single-phase opera-
tion). This means that both switches turned on at the same
time, causing current pulses of up to twice the amplitude
of those for one regulator to be drawn from the input
capacitor and battery. These large amplitude current pulses
increased the total RMS current flowing from the input
capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two channels of the dual-
switching regulator are operated 180 degrees out of
phase. This effectively interleaves the current pulses
drawn by the switches, greatly reducing the overlap time
where they add together. The result is a significant reduc-
tion in total RMS input current, which in turn allows less
expensive input capacitors to be used, reduces shielding
requirements for EMI and improves real world operating
efficiency.
Figure 3 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the new
LTC1628-SYNC 2-phase dual switching regulator. An
actual measurement of the RMS input current under these
conditions shows that 2-phase operation dropped the
input current from 2.53ARMS to 1.55ARMS. While this is an
impressive reduction in itself, remember that the power
losses are proportional to IRMS2, meaning that the actual
power wasted is reduced by a factor of 2.66. The reduced
input ripple voltage also means less power is lost in the
input power path, which could include batteries, switches,
trace/connector resistances and protection circuitry. Im-
provements in both conducted and radiated EMI also
directly accrue as a result of the reduced RMS input
current and voltage.
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| PDF Descargar | [ Datasheet LTC1628-SYNC.PDF ] | |
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