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PDF LTC3717-1 Data sheet ( Hoja de datos )
| Número de pieza | LTC3717-1 | |
| Descripción | Step-Down Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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LTC3717-1
Wide Operating Range,
No RSENSETM Step-Down Controller
for DDR/QDR Memory Termination
FEATURES
s VOUT = 1/2 VREF
s Adjustable and Symmetrical Sink/Source
Current Limit up to 20A
s True Current Mode Control with Optional Use of
Sense Resistor
s VON and ION Pins Allow Constant Frequency
Operation During Input and Output Voltage Changes
s ±0.65% Output Voltage Accuracy
s Up to 97% Efficiency
s Ultrafast Transient Response
s 2% to 90% Duty Cycle at 200kHz
s tON(MIN) ≤ 100ns
s Stable with Ceramic COUT
s Power Good Output Voltage Monitor
s Wide VIN Range: 4V to 36V
s Adjustable Switching Frequency up to 1.5MHz
s Output Overvoltage Protection
s Optional Short-Circuit Shutdown Timer
s Available in a 5mm × 5mm QFN Package
U
APPLICATIO S
s Bus Termination: DDR and QDR Memory, SSTL,
HSTL, ...
s Notebook Computers, Desktop Servers
s Tracking/Margining Power Supply
DESCRIPTIO
The LTC®3717-1 is a synchronous step-down switching
regulator controller for double data rate (DDR) and Quad
Data RateTM (QDRTM) memory termination. The controller
uses a valley current control architecture to deliver very
low duty cycles with or without a sense resistor. Operating
frequency is selected by an external resistor and is com-
pensated for variations in VIN and VOUT.
Forced continuous operation reduces noise and RF inter-
ference. Output voltage is internally set to half of VREF,
which is user programmable.
Fault protection is provided by an output overvoltage
comparator and optional short-circuit shutdown timer.
Soft-start capability for supply sequencing is accom-
plished using an external timing capacitor. The regulator
current limit level is symmetrical and user programmable.
Wide supply range allows operation from 4V to 36V at the
VCC input.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No RSENSE is a trademark of Linear Technology Corporation.
QDR RAMs and Quad Data Rate RAMs comprise a new family of products developed by Cypress
Semiconductor, Hitachi, IDT, Micron Technology, Inc. and Samsung.
TYPICAL APPLICATIO
VCC
5V TO 28V
1µF
0.1µF
470pF
20k
VCC ION
VREF
RUN/SS TG
LTC3717-1
SW
SENSE+
ITH BOOST
SGND
VON
DRVCC
INTVCC
BG
PGOOD PGND
SENSE–
VFB
715k
VDD = 2.5V
0.22µF
CMDSH-3
+
4.7µF
Si7840DP
Si7840DP
B320A
VIN
2.5V TO 5.5V
+ 150µF
6.3V
×2
VOUT
1.25V
0.68µH + 180µF ±10A
4V
×2
B320A
37171 F01a
Figure 1. High Efficiency DDR Memory Termination Supply
Efficiency vs Load Current
100
90
80
70
60
50
40
30
20
10
0
0
VIN = 5V
VIN = 2.5V
VOUT = 1.25V
2 4 6 8 10 12 14
LOAD CURRENT (A)
37171 F01b
sn37171 37171fs
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
On-Time vs ION Current
10k
VVON = 0V
1k
100
10
1
10
ION CURRENT (µA)
100
37171 G09
RUN/SS Latchoff Thresholds
vs Temperature
5.0
4.5
LATCHOFF ENABLE
4.0
3.5
LATCHOFF THRESHOLD
3.0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
37171 G12
Maximum Current Sense Threshold
vs RUN/SS Voltage, VRNG = 1V
160
140
120
100
80
60
40
20
0
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
RUN/SS (V)
37171 G15
INTVCC Load Regulation
0
–0.1
–0.2
–0.3
–0.4
–0.5
0
10 20 30 40 50
INTVCC LOAD CURRENT (mA)
37171 G10
Undervoltage Lockout Threshold
vs Temperature
4.0
3.5
3.0
2.5
2.0
–50 –25
0 25 50 75 100 125
TEMPERATURE (C)
37171 G13
Maximum Current Sense Threshold
vs Temperature, VRNG = 1V
180
160
140
120
100
80
60
40
20
0
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
37171 G16
LTC3717-1
RUN/SS Latchoff Thresholds
vs Temperature
3
2
PULL-DOWN CURRENT
1
0
PULL-UP CURRENT
–1
–2
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
37171 G11
Maximum Current Sense Threshold
vs VRNG Voltage
300
250
200
150
100
50
0
0.50 0.75 1.00 1.25 1.50
VRNG (V)
1.75 2.00
37171 G14
Error Amplifier gm
vs Temperature
1.50
1.40
1.30
1.20
1.10
1.00
0.90
0.80
0.70
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
37171 G17
sn37171 37171fs
5
5 Page 
LTC3717-1
APPLICATIO S I FOR ATIO
VOUT
RVON1
30k
RVON2
100k
RC
CC
CVON
0.01µF
VON
LTC3717-1
ITH
(3a)
VOUT
INTVCC
RVON1
3k
RVON2
10k 10k
Q1
2N5087
CVON
0.01µF
RC
CC
VON
LTC3717-1
ITH
37171 F03
(3b)
Figure 3. Adjusting Frequency Shift with Load Current Changes
25% of the voltage change at the ITH pin to the VON pin as
shown in Figure 3a. Place capacitance on the VON pin to
filter out the ITH variations at the switching frequency. The
resistor load on ITH reduces the DC gain of the error amp
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 3b.
Inductor L1 Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
∆IL
=
VOUT
fL
1−
VOUT
VIN
Lower ripple current reduces cores losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of IOUT(MAX). The largest ripple current
occurs at the highest VIN. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L
=
f
VOUT
∆IL(MAX)
1−
VOUT
VIN(MAX)
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ® cores. A variety of inductors designed for high
current, low voltage applications are available from manu-
facturers such as Sumida, Panasonic, Coiltronics, Coil-
craft and Toko.
Schottky Diode D1, D2 Selection
The Schottky diodes, D1 and D2, shown in Figure 1
conduct during the dead time between the conduction of
the power MOSFET switches. It is intended to prevent the
body diodes of the top and bottom MOSFETs from turning
on and storing charge during the dead time, which can
cause a modest (about 1%) efficiency loss. The diodes can
be rated for about one half to one fifth of the full load current
since they are on for only a fraction of the duty cycle. In
order for the diode to be effective, the inductance between
it and the bottom MOSFET must be as small as possible,
mandating that these components be placed adjacently.
The diodes can be omitted if the efficiency loss is tolerable.
CIN and COUT Selection
The input capacitance CIN is required to filter the square
wave current at the drain of the top MOSFET. Use a low
ESR capacitor sized to handle the maximum RMS current.
IRMS
≅
IOUT(MAX)
VOUT
VIN
VIN – 1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT(MAX)/ 2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief. Note that ripple
current ratings from capacitor manufacturers are often
Kool Mµ is a registered trademark of Magnetics, Inc.
sn37171 37171fs
11
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| PDF Descargar | [ Datasheet LTC3717-1.PDF ] | |
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