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PDF LTC3720 Data sheet ( Hoja de datos )
| Número de pieza | LTC3720 | |
| Descripción | Single Phase VRM8.5 Current Mode Step-Down Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3720www.DataSheet4U.com
Single Phase VRM8.5
Current Mode Step-Down Controller
FEATURES
s 5-Bit Programmable Output Voltage:
1.05V to 1.825V (VRM8.5)
s No Sense Resistor Required
s 2% to 87% Duty Cycle at 200kHz
s tON(MIN) ≤ 100ns
s Supports Active Voltage Positioning
s True Current Mode Control
s Stable with Ceramic COUT
s Dual N-Channel MOSFET Synchronous Drive
s Power Good Output Voltage Monitor
s Wide VIN Range: 4V to 36V
s ±1% 0.8V Reference
s Adjustable Current Limit
s Adjustable Switching Frequency
s Forced Continuous Control Pin
s Programmable Soft-Start
s Output Overvoltage Protection
s Optional Short-Circuit Shutdown Timer
s Micropower Shutdown: IQ < 30µA
s Available in 28-Lead Narrow SSOP Package
U
APPLICATIO S
s Power Supplies for Pentium® Processors
s Notebook Computers and Servers
DESCRIPTIO
The LTC®3720 is a synchronous step-down switching
regulator controller for CPU power. An output voltage
between 1.05V and 1.825V is selected by a 5-bit code
(Intel VRM8.5 VID specification). The controller uses a
valley current control architecture to deliver very low duty
cycles without requiring a sense resistor. Operating fre-
quency is selected by an external resistor and is compen-
sated for variations in VIN and VOUT.
Discontinuous mode operation provides high efficiency
operation at light loads. A forced continuous control pin
reduces noise and RF interference and can assist second-
ary winding regulation by disabling discontinuous mode
operation when the main output is lightly loaded.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator and optional
short-circuit shutdown timer. Soft-start capability for sup-
ply sequencing is accomplished using an external timing
capacitor. The regulator current limit level is also user
programmable. Wide supply range allows operation from
4V to 36V at the input.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No RSENSE is a trademark of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation.
TYPICAL APPLICATIO
0.1µF
470pF
20k
5-BIT VID
LTC3720
PGOOD ION
RUN/SS
VIN
TG
ITH SW
SGND BOOST
VID4
VID3
VID2
VID1
VID0
VCC
INTVCC
SENSE+
BG
PGND
SENSE¯
VOSENSE
330k
0.33µF
CMDSH-3
+
4.7µF
IRF7811W
×2 L1
1µH
IRF7811W
×3
UPS840
10µF
×5
VIN
5V TO 24V
VOUT
1.05V TO 1.825V
+ COUT 20A
270µF
2V
×4
COUT: CORNELL DUBILIER
ESRE271M02B
L1: SUMIDA CEP125-IROMC
3720 F01a
Figure 1. High Efficiency Step-Down Converter
Efficiency vs Output Current
100
VOUT = 1.45V
95 L1 = 1µH
90
VIN = 5V
85
80 VIN = 15V
75
70
65
60
55
0.01
0.1 1 10
OUTPUT CURRENT (A)
100
3720 F01b
3720f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC3720www.DataSheet4U.com
Error Amplifier gm vs Temperature
2.0
1.8
1.6
1.4
1.2
1.0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3720 G10
EXTVCC Switch Resistance
vs Temperature
10
Input and Shutdown Currents
vs Input Voltage
1200
1000
EXTVCC OPEN
60
50
800 40
SHUTDOWN
600 30
400 20
200
EXTVCC = 5V
10
0
05
0
10 15 20 25 30 35
INPUT VOLTAGE (V)
3720 G11
FCB Pin Current vs Temperature
0
INTVCC Load Regulation
0
–0.1
–0.2
–0.3
–0.4
–0.5
0
10 20 30 40
INTVCC LOAD CURRENT (mA)
50
3720 G12
RUN/SS Pin Current
vs Temperature
3
8 –0.25
–0.50
6
–0.75
4
–1.00
2 –1.25
2
PULL-DOWN CURRENT
1
0
PULL-UP CURRENT
–1
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3720 G13
–1.50
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3720 G14
–2
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3720 G15
RUN/SS Latchoff Thresholds
vs Temperature
5.0
Undervoltage Lockout Threshold
vs Temperature
4.0
4.5
LATCHOFF ENABLE
4.0
3.5
3.0
3.5
LATCHOFF THRESHOLD
3.0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3720 G16
2.5
2.0
–50 –25
0 25 50 75
TEMPERATURE (C)
100 125
3720 G17
3720f
5
5 Page 
LTC3720www.DataSheet4U.com
APPLICATIO S I FOR ATIO
2.0
1.5
1.0
0.5
0
– 50
0 50 100
JUNCTION TEMPERATURE (°C)
150
3720 F02
Figure 2. RDS(ON) vs. Temperature
the load current. When the LTC3720 is operating in
continuous mode, the duty cycles for the MOSFETs are:
DTOP
=
VOUT
VIN
DBOT
=
VIN
– VOUT
VIN
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
PTOP = DTOP IOUT(MAX)2 ρT(TOP) RDS(ON)(MAX)
+ k VIN2 IOUT(MAX) CRSS f
PBOT = DBOT IOUT(MAX)2 ρT(BOT) RDS(ON)(MAX)
Both MOSFETs have I2R losses and the top MOSFET
includes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7A–1 can
be used to estimate the amount of transition loss. The
bottom MOSFET losses are greatest when the bottom duty
cycle is near 100%, during a short-circuit or at high input
voltage.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3720 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time tON of the top MOSFET switch. The on-time is set
by the current into the ION pin and the voltage at the VON
pin according to:
tON
=
VVON
IION
(10pF)
Tying a resistor RON from VIN to the ION pin yields an on-
time inversely proportional to VIN. For a step-down con-
verter, this results in approximately constant frequency
operation as the input supply varies:
[ ]f = VOUT
VVON RON(10pF)
Hz
To hold frequency constant during output voltage changes,
tie the VON pin to VOUT. The VON pin has internal clamps
that limit its input to the one-shot timer. If the pin is tied
below 0.7V, the input to the one-shot is clamped at 0.7V.
Similarly, if the pin is tied above 2.4V, the input is clamped
at 2.4V.
Because the voltage at the ION pin is about 0.7V, the
current into this pin is not exactly inversely proportional to
VIN, especially in applications with lower input voltages.
To correct for this error, an additional resistor RON2
connected from the ION pin to the 5V INTVCC supply will
further stabilize the frequency.
RON2
=
5V
0.7V
RON
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the ITH pin to the VON pin and VOUT. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the ITH pin to the VON pin as
3720f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC3720.PDF ] | |
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