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PDF LTC1778 Data sheet ( Hoja de datos )
| Número de pieza | LTC1778 | |
| 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 LTC1778 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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LTC1778/LTC1778-1
Wide Operating Range,
No RSENSETM Step-Down Controller
FEATURES
s No Sense Resistor Required
s True Current Mode Control
s Optimized for High Step-Down Ratios
s tON(MIN) ≤ 100ns
s Extremely Fast Transient Response
s Stable with Ceramic COUT
s Dual N-Channel MOSFET Synchronous Drive
s Power Good Output Voltage Monitor (LTC1778)
s Adjustable On-Time (LTC1778-1)
s Wide VIN Range: 4V to 36V
s ±1% 0.8V Voltage 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 16-Pin Narrow SSOP Package
U
APPLICATIO S
s Notebook and Palmtop Computers
s Distributed Power Systems
DESCRIPTIO
The LTC®1778 is a synchronous step-down switching
regulator controller optimized for CPU power. The con-
troller uses a valley current control architecture to deliver
very low duty cycles with excellent transient response
without requiring a sense resistor. Operating frequency is
selected by an external resistor and is compensated for
variations in VIN.
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 opera-
tion 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 user program-
mable. Wide supply range allows operation from 4V to 36V
at the input and from 0.8V 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
CSS
0.1µF
CC
500pF
RC
20k
RUN/SS
ION
VIN
TG
SW
ITH BOOST
LTC1778
SGND INTVCC
BG
PGOOD PGND
RON
1.4MΩ
CB 0.22µF
DB
CMDSH-3
+ CVCC
4.7µF
M1
Si4884 L1
1.8µH
M2
Si4874
D1
B340A
CIN
10µF
VIN
5V TO 28V
50V
×3 VOUT
2.5V
+ COUT 10A
180µF
4V
×2
R2
30.1k
VFB
R1
14k
1778 F01a
Figure 1. High Efficiency Step-Down Converter
Efficiency vs Load Current
100
VOUT = 2.5V
VIN = 5V
90
VIN = 25V
80
70
60
0.01
0.1 1
LOAD CURRENT (A)
10
1778 F01b
1778fa
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1778/LTC1778-1
Current Sense Threshold
vs ITH Voltage
300
VRNG =
2V
200 1.4V
1V
100 0.7V
0.5V
0
–100
–200
0
0.5 1.0 1.5 2.0
ITH VOLTAGE (V)
2.5 3.0
1778 G08
On-Time vs Temperature
300
IION = 30µA
VVON = 0V
250
200
150
100
50
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1778 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
1778 G23
On-Time vs ION Current
10k
VVON = 0V
1k
100
10
1
10
ION CURRENT (µA)
100
1778 G20
Current Limit Foldback
150
VRNG = 1V
125
100
75
50
25
0
0 0.2 0.4 0.6 0.8
VFB (V)
1778 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
1778 G11
On-Time vs VON Voltage
1000
IION = 30µA
800
600
400
200
0
0 123
VON VOLTAGE (V)
1778 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)
1778 G10
Feedback Reference Voltage
vs Temperature
0.82
0.81
0.80
0.79
0.78
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1778 G12
1778fa
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5 Page 
LTC1778/LTC1778-1
APPLICATIO S I FOR ATIO
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.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC1778 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 LTC1778 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 (LTC1778-1) according to:
tON
=
VVON (10pF)
IION
VON defaults to 0.7V in the LTC1778.
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
=
VVON
VOUT
RON(10pF)
[HZ ]
To hold frequency constant during output voltage changes,
tie the VON pin to VOUT or to a resistive divider from VOUT
when VOUT > 2.4V. 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. Simi-
larly, if the pin is tied above 2.4V, the input is clamped at
2.4V. In high VOUT applications, tying VON to INTVCC so
that the comparator input is 2.4V results in a lower value
for RON. Figures 3a and 3b show how RON relates to
switching frequency for several common output voltages.
1000
VOUT = 1.5V
VOUT = 3.3V
VOUT = 2.5V
100
100
1000
RON (kΩ)
10000
1778 F03a
Figure 3a. Switching Frequency vs RON
for the LTC1778 and LTC1778-1 (VON = 0V)
1000
VOUT = 3.3V
VOUT = 12V
VOUT = 5V
100
100
1000
RON (kΩ)
10000
1778 F03b
Figure 3b. Switching Frequency vs RON
for the LTC1778-1 (VON = INTVCC)
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11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC1778.PDF ] | |
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