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PDF LTC3778 Data sheet ( Hoja de datos )
| Número de pieza | LTC3778 | |
| Descripción | Wide Operating Range/ No RSENSE Step-Down Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3778 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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FEATURES
s Wide VIN Range: 4V to 36V
s Sense Resistor Optional
s True Current Mode Control
s 2% to 90% Duty Cycle at 200kHz
s tON(MIN) ≤ 100ns
s Stable with Ceramic COUT
s Dual N-Channel MOSFET Synchronous Drive
s Power Good Output Voltage Monitor
s ±1% 0.6V Reference
s Adjustable Current Limit
s Adjustable Switching Frequency
s Programmable Soft-Start
s Output Overvoltage Protection
s Optional Short-Circuit Shutdown Timer
s Micropower Shutdown: IQ ≤ 30µA
s Available in a 1mm 20-Lead TSSOP Package
U
APPLICATIO S
s Notebook and Palmtop Computers, PDAs
s Battery Chargers
s Distributed Power Systems
s DDR Memory Power Supply
s Automobile DC Power Supply
LTC3778
Wide Operating Range,
No RSENSETM Step-Down Controller
DESCRIPTIO
The LTC®3778 is a synchronous step-down switching
regulator controller for computer memory, automobile
and other DC/DC power supplies. 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 when the main output is lightly
loaded. SENSE+ and SENSE– pins provide true Kelvin
sensing across the optional sense resistor or the
sychronous MOSFET.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator, optional short-
circuit shutdown timer and input undervoltage lockout.
Soft-start capability for supply sequencing is accom-
plished using an external timing capacitor. The regulator
current limit level is user programmable. Wide supply
range allows operation from 4V to 36V at the input and
from 0.6V up to (0.9)VIN at the output.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No RSENSE is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
CC
500pF
CSS
0.1µF
RC
20k
RUN/SS
ION
VIN
TG
SW
ITH BOOST
LTC3778
SGND
INTVCC
DRVCC
BG
SENSE+
SENSE–
PGOOD PGND
RON
1.4M
CB 0.22µF
DB
CMDSH-3
+ CVCC
4.7µF
M1
Si4884 L1
1.8µH
M2
Si4874
D1
B340A
VFB
VIN
CIN 5V TO 28V
10µF
35V
×3 VOUT
2.5V
+ COUT 10A
180µF
4V
×2
R2
40.2k
R1
12.7k
3778 F01a
Figure 1. High Efficiency Step-Down Converter
Efficiency vs Load Current
100
VIN = 5V
90
VIN = 25V
80
70
60
0.01
0.1 1
LOAD CURRENT (A)
10
3778 F01b
3778f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC3778
On-Time vs ION Current
10k
VVON = 0V
1k
100
10
1
10
ION CURRENT (µA)
100
3778 G20
Current Limit Foldback
150
VRNG = 1V
125
100
75
50
25
0
0 0.15 0.30 0.45 0.60
VFB (V)
3778 G09
Maximum Current Sense
Threshold vs Temperature
150
VRNG = 1V
140
130
120
110
100
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3778 G11
On-Time vs VON Voltage
1000
IION = 30µA
800
600
400
200
0
0 123
VON VOLTAGE (V)
3778 G21
Maximum Current Sense
Threshold vs VRNG Voltage
300
250
200
150
100
50
0
0.5 0.75 1.0 1.25 1.5 1.75 2.0
VRNG VOLTAGE (V)
3778 G10
Feedback Reference Voltage
vs Temperature
0.62
0.61
0.60
0.59
0.58
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3778 G12
On-Time vs Temperature
300
IION = 30µA
VON = 0V
250
200
150
100
50
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3778 G22
Maximum Current Sense
Threshold vs RUN/SS Voltage
150
VRNG = 1V
125
100
75
50
25
0
1.5
2 2.5 3
RUN/SS VOLTAGE (V)
3.5
3778 G23
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
3778 G13
3778f
5
5 Page 
LTC3778
APPLICATIO S I FOR ATIO
The ρT term is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/°C as shown in
Figure 2. For a maximum junction temperature of 100°C,
using a value ρT = 1.3 is reasonable.
2.0
1.5
1.0
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 LTC3778 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:
0.5
0
– 50
0 50 100
JUNCTION TEMPERATURE (°C)
150
3778 F02
Figure 2. RDS(ON) vs. Temperature
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC3778 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.
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 [Hz]
(VVON) RON(10pF)
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. If output is above 2.4V, use a resistive divider from
VOUT to VON pin.
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
3778f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC3778.PDF ] | |
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