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PDF LTC3831 Data sheet ( Hoja de datos )
| Número de pieza | LTC3831 | |
| Descripción | High Power Synchronous Switching Regulator Controller for DDR Memory Termination | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s High Power Switching Regulator Controller
for DDR Memory Termination
s VOUT Tracks 1/2 of VIN or External VREF
s No Current Sense Resistor Required
s Low Input Supply Voltage Range: 3V to 8V
s Maximum Duty Cycle > 91% Over Temperature
s Drives All N-Channel External MOSFETs
s High Efficiency: Over 95% Possible
s Programmable Fixed Frequency Operation:
100kHz to 500kHz
s External Clock Synchronization Operation
s Programmable Soft-Start
s Low Shutdown Current: <10µA
s Overtemperature Protection
s Available in 16-Pin Narrow SSOP Package
U
APPLICATIO S
s DDR SDRAM Termination
s SSTL_2 Interface
s SSTL_3 Interface
LTC3831
High Power Synchronous
Switching Regulator Controller
for DDR Memory Termination
DESCRIPTIO
The LTC®3831 is a high power, high efficiency switching
regulator controller designed for DDR memory termina-
tion. The LTC3831 generates an output voltage equal to
1/2 of an external supply or reference voltage. The LTC3831
uses a synchronous switching architecture with N-chan-
nel MOSFETs. Additionally, the chip senses output cur-
rent through the drain-source resistance of the upper
N-channel FET, providing an adjustable current limit
without a current sense resistor.
The LTC3831 operates with input supply voltage as low as
3V and with a maximum duty cycle of > 91%. It includes a
fixed frequency PWM oscillator for low output ripple
operation. The 200kHz free-running clock frequency can
be externally adjusted or synchronized with an external
signal from 100kHz to above 500kHz. In shutdown mode,
the LTC3831 supply current drops to <10µA.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VDDQ
5V 2.5V
MBR0530T1
1µF
PVCC2 PVCC1
0.1µF
10k
Q1 MBRS340T3
+ CIN
330µF
×2
VCC TG
0.1µF
SS IMAX
+
4.7µF
0.01µF
130k
LTC3831 IFB
FREQSET
BG
SHDN
SHDN
PGND
C1
33pF
RC
15k
CC
1500pF
COMP
R–
GND
R+
FB
0.1µF
1k
LO
1.2µH
Q2 MBRS340T3
+
CIN: SANYO POSCAP 6TPB330M
COUT: SANYO POSCAP 4TPB470M
Q1, Q2: SILICONIX Si4410DY
COUT
470µF
×3
VTT
1.25V
±6A
3831 F01
Figure 1. Typical DDR Memory Termination Application
Efficiency vs Load Current
100
90
80
70
60
50
40
30
20 TA = 25°C
10
VIN = 2.5V
VOUT = 1.25V
0
0123 4
LOAD CURRENT (A)
5
6
2831 G01
3831f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum TG Duty Cycle
vs Temperature
100
VFB = 0V
99 REFER TO FIGURE 3
98
97
96
95
94
93
92
91
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3831 G11
Output Current Limit Threshold
vs Temperature
10
REFER TO FIGURE 1
9
8
7
6
5
4
3
2
1
0
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
3831 G14
Undervoltage Lockout Threshold
Voltage vs Temperature
3.0
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2.0
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
3831 G17
IMAX Sink Current
vs Temperature
20
18
16
14
12
10
8
6
4
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
3831 G12
Soft-Start Source Current
vs Temperature
–8
–9
–10
–11
–12
–13
–14
–15
–16
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
3831 G15
VCC Operating Supply Current
vs Temperature
1.6
FREQSET FLOATING
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3831 G18
LTC3831
Output Overcurrent Protection
1.4
1.2
1.0
0.8
0.6
0.4
0.2 TA = 25°C
REFER TO FIGURE 1
0
024
6
8
OUTPUT CURRENT (A)
10
3831 G13
Soft-Start Sink Current
vs (VIFB – VIMAX)
2.00
TA = 25°C
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
–150 –125 –100 –75 –50
VIFB – VIMAX (mV)
–25 0
3831 G16
PVCC Supply Current
vs Oscillator Frequency
90
TA = 25°C
80
70
60
TG AND BG LOADED
WITH 6800pF,
PVCC1,2 = 12V
50 TG AND BG
40
LOADED
WITH 1000pF,
30 PVCC1,2 = 5V
TG AND BG
LOADED
WITH 6800pF,
PVCC1,2 = 5V
20
10
0
0 100 200 300 400 500
OSCILLATOR FREQUENCY (kHz)
3831 G19
3831f
5
5 Page 
LTC3831
APPLICATIO S I FOR ATIO
SHDN
200kHz
FREE RUNNING
RAMP SIGNAL
TRADITIONAL
SYNC METHOD
WITH EARLY
RAMP
TERMINATION
RAMP SIGNAL
WITH EXT SYNC
RAMP AMPLITUDE
ADJUSTED
LTC3831
KEEPS RAMP
AMPLITUDE
CONSTANT
UNDER SYNC
3831 F04
Gate drive for the top N-channel MOSFET Q1 is supplied
from PVCC1. This supply must be above VIN (the main
power supply input) by at least one power MOSFET
VGS(ON) for efficient operation. An internal level shifter
allows PVCC1 to operate at voltages above VCC and VIN, up
to 14V maximum. This higher voltage can be supplied with
a separate supply, or it can be generated using a charge
pump.
Gate drive for the bottom MOSFET Q2 is provided through
PVCC2. This supply only need to be above the power
MOSFET VGS(ON) for efficient operation. PVCC2 can also be
driven from the same supply/charge pump for the PVCC1,
or it can be connected to a lower supply to improve
efficiency.
Figure 6 shows a doubling charge pump circuit that can be
used to provide 2VIN gate drive for Q1. The charge pump
consists of a Schottky diode from VIN to PVCC1 and a 0.1µF
capacitor from PVCC1 to the switching node at the drain of
Figure 4. External Synchronization Operation
decrease in ramp amplitude increases the open-loop gain
of the controller feedback loop. As a result, the loop
crossover frequency increases and it may cause the feed-
back loop to be unstable if the phase margin is insufficient.
To overcome this problem, the LTC3831 monitors the
peak voltage of the ramp signal and adjust the oscillator
charging current to maintain a constant ramp peak.
Input Supply Considerations/Charge Pump
The LTC3831 requires four supply voltages to operate: VIN
for the main power input, PVCC1 and PVCC2 for MOSFET
gate drive and a clean, low ripple VCC for the LTC3831
internal circuitry (Figure 5).
In many applications, VCC can be powered from VIN
through an RC filter. This supply can be as low as 3V. The
low quiescent current (typically 800µA) allows the use of
relatively large filter resistors and correspondingly small
filter capacitors. 100Ω and 4.7µF usually provide ad-
equate filtering for VCC. For best performance, connect the
4.7µF bypass capacitor as close to the LTC3831 VCC pin as
possible.
VCC PVCC2 PVCC1
VIN
INTERNAL
CIRCUITRY
LTC3831
TG
Q1
LO
VOUT
BG
Q2
+
COUT
3831 F05
Figure 5. Supplies Input
OPTIONAL
USE FOR VIN ≥ 7V
DZ
12V
1N5242
PVCC2
VIN
MBR0530T1
PVCC1
TG
0.1µF
Q1
LO
BG
Q2
VOUT
+
COUT
LTC3831
3831 F06a
Figure 6. Doubling Charge Pump
3831f
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3831.PDF ] | |
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