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PDF LTC1709 Data sheet ( Hoja de datos )
| Número de pieza | LTC1709 | |
| Descripción | 2-Phase/ 5-Bit Adjustable/ High Efficiency/ Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1709
FEATURES
2-Phase, 5-Bit Adjustable,
High Efficiency, Synchronous Step-Down
Switching Regulator
DESCRIPTIO
s Two Ouput Stages Operate Antiphase Reducing
Input Capacitance and Power Supply Noise
s 5-Bit VID Control (VRM 8.4 Compliant)
VOUT: 1.3V to 3.5V in 50mV/100mV Steps
s Current Mode Control Ensures Current Sharing
s True Remote Sensing Differential Amplifier
s OPTI-LOOPTM Compensation Minimizes COUT
s Programmable Fixed Frequency: 150kHz to
300kHz—Effective 300kHz to 600kHz Switching
Frequency
s ±1% Output Voltage Accuracy
s Wide VIN Range: 4V to 36V Operation
s Adjustable Soft-Start Current Ramping
s Internal Current Foldback
s Short-Circuit Shutdown Timer with Defeat Option
s Overvoltage Soft-Latch Eliminates Nuisance Trips
s Low Shutdown Current: 20µA
s Small 36-Lead Narrow (0.209") SSOP Package
U
APPLICATIO S
s Desktop Computers
s Internet/Network Servers
s Large Memory Arrays
s DC Power Distribution Systems
s Battery Chargers
The LTC®1709 is a 2-phase, VID programmable, synchro-
nous step-down switching regulator controller that drives
all N-channel external power MOSFET stages in a fixed
frequency architecture. The 2-phase controller drives its
two output stages out of phase at frequencies up to
300kHz to minimize the RMS ripple currents in both input
and output capacitors. The 2-phase technique effectively
multiplies the fundamental frequency by two, improving
transient response while operating each channel at a
optimum frequency for efficiency. Thermal design is also
simplified.
An internal differential amplifier provides true remote
sensing of the regulated supply’s positive and negative
output terminals as required in high current applications.
The RUN/SS pin provides soft-start and optional timed,
short-circuit shutdown. Current foldback limits MOSFET
dissipaton during short-circuit conditions when overcurrent
latchoff is disabled. OPTI-LOOP compensation allows the
transient response to be optimized for a wide range of
output capacitors and ESR values.
, LTC and LT are registered trademarks of Linear Technology Corporation.
OPTI-LOOP is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
0.1µF
VIN
RUN/SS
TG1
BOOST 1
15k 1.2nF
SW1
LTC1709
BG1
ITH PGND
S
SGND
SENSE1+
5 VID BITS VID0–VID4
SENSE1–
TG2
EAIN BOOST2
FBOUT
SW2
SENSEIN
BG2
VDIFFOUT
VOS–
VOS+
INTVCC
SENSE 2+
SENSE 2–
S
0.47µF
S
S
S
0.47µF
S
S
10µF
10µF ×4
Q1 35V
0.002Ω
1µH
Q2
Q3
0.002Ω
1µH
Q4
Q1–Q4 2× FAIRCHILD FDS7760A OR SILICONIX Si4874
Figure 1. High Current 2-Phase Step-Down Converter
VIN
5V TO 28V
VOUT
1.3V TO 3.5V
40A
+
COUT
1000µF
4V
×2
1709 TA01
Efficiency Curve
100 VIN = 5V
VOUT = 1.6V
90 fS = 200kHz
80
70
60
50
0 5 10 15 20 25 30 35 40 45
LOAD CURRENTS (A)
1709 TA01a
1
1 page  
LTC1709
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Input Voltage
and Mode
1000
800
600
ON
400
200
SHUTDOWN
0
0 5 10 15 20 25 30 35
INPUT VOLTAGE (V)
1709 G04
Internal 5V LDO Line Reg
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)
1709 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
1709 G10
EXTVCC Voltage Drop
250
200
150
100
50
0
0 10 20 30 40 50
CURRENT (mA)
1709 G05
Maximum Current Sense Threshold
vs Duty Factor
75
50
25
0
0 20 40 60 80 100
DUTY FACTOR (%)
1709 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
1709 G11
INTVCC and EXTVCC Switch
Voltage vs Temperature
5.05
INTVCC VOLTAGE
5.00
4.95
4.90
4.85
4.80
EXTVCC SWITCHOVER THRESHOLD
4.75
4.70
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
1709 G06
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 (%)
1709 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)
1709 G12
5
5 Page 
U
OPERATIO (Refer to Functional Diagram)
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both VOUT+ and VOUT– benefits regula-
tion in high current applications and/or applications hav-
ing electrical interconnection losses. The AMPMD pin
allows selection of internal, precision feedback resistors
for high common mode rejection differencing applica-
tions, or direct access to the actual amplifier inputs
without these internal feedback resistors for other applica-
tions. The AMPMD pin is grounded to connect the internal
precision resistors in a unity-gain differencing application,
or tied to the INTVCC pin to bypass the internal resistors
and make the amplifier inputs directly available. The
amplifier is a unity-gain stable, 2MHz gain-bandwidth,
>120dB open-loop gain design. The amplifier has an
output slew rate of 5V/µs and is capable of driving capaci-
tive loads with an output RMS current typically up to
35mA. The amplifier is not capable of sinking current and
therefore must be resistively loaded to do so.
LTC1709
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the controllers
have been given time, as determined by the capacitor on
the RUN/SS pin, to charge up the output capacitors and
provide full-load current, the RUN/SS capacitor is then
used as a short-circuit timeout circuit. If the output voltage
falls to less than 70% of its nominal output voltage the
RUN/SS capacitor begins discharging assuming that the
output is in a severe overcurrent and/or short-circuit
condition. If the condition 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 voltage
is recycled. This built-in latchoff can be overidden by
providing a current >5µA at a compliance of 5V to the
RUN/SS pin. This current shortens the soft-start period
but also prevents net discharge of the RUN/SS capacitor
during a severe overcurrent and/or short-circuit condi-
tion. Foldback current limiting is activated when the output
voltage falls below 70% of its nominal level whether or not
the short-circuit latchoff circuit is enabled.
APPLICATIO S I FOR ATIO
The basic LTC1709 application circuit is shown in Figure␣ 1
on the first page. External component selection begins
with the selection of the inductor(s) based on ripple
current requirements and continues with the RSENSE1, 2
resistor selection using the calculated peak inductor cur-
rent and/or maximum current limit. Next, the power
MOSFETs and D1 and D2 are selected. The operating
frequency and the inductor are chosen based mainly on
the amount of ripple current. Finally, CIN is selected for its
ability to handle the input ripple current (that PolyPhaseTM
operation minimizes) and COUT is chosen with low enough
ESR to meet the output ripple voltage and load step
specifications (also minimized with PolyPhase). Current
mode architecture provides inherent current sharing be-
tween output stages. The circuit shown in Figure␣ 1 can be
configured for operation up to an input voltage of 28V
(limited by the external MOSFETs).
RSENSE Selection For Output Current
RSENSE1, 2 are chosen based on the required peak output
current. The LTC1709 current comparator has a maxi-
mum threshold of 75mV/RSENSE and an input common
mode range of SGND to 1.1( INTVCC). The current com-
parator threshold sets the peak inductor current, yielding
a maximum average output current IMAX equal to the peak
value less half the peak-to-peak ripple current, ∆IL.
Allowing a margin for variations in the LTC1709 and
external component values yields:
RSENSE = 2(50mV/IMAX)
Operating Frequency
The LTC1709 uses a constant frequency, phase-lockable
architecture with the frequency determined by an internal
capacitor. This capacitor is charged by a fixed current plus
an additional current which is proportional to the voltage
applied to the PLLFLTR pin. Refer to Phase-Locked Loop
and Frequency Synchronization in the Applications Infor-
mation section for additional information.
PolyPhase is a registered trademark of Linear Technology Corporation.
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC1709.PDF ] | |
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