|
|
PDF LTC3878 Data sheet ( Hoja de datos )
| Número de pieza | LTC3878 | |
| Descripción | Wide Operating Range No RSENSE Step-Down Controller | |
| Fabricantes | Linear Technology Corporation | |
| Logotipo |  |
|
Hay una vista previa y un enlace de descarga de LTC3878 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
|
No Preview Available !
FEATURESwww.datasheet4u.com
n Wide VIN Range: 4V to 38V
n ±1% 0.8V Voltage Reference
n Extremely Fast Transient Response
n tON(MIN): 43ns
n Valley Current Mode Control
n Stable for Low ESR Ceramic COUT
n No Sense Resistor Required
n Optimized for High Step-Down Ratios
n Pin Compatible with the LTC1778 (No EXTVCC Pin)
n Power Good Output Voltage Monitor
n Dual N-Channel MOSFET Synchronous Drive
n Adjustable Switching Frequency
n Programmable Current Limit with Foldback
n Programmable Soft-Start
n Output Overvoltage Protection
n Small 16-Pin Narrow SSOP Package
APPLICATIONS
n Distributed Power Systems
n Embedded Computing
n Communications Infrastructure
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
No RSENSE is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5481178, 6100678, 6580258, 5847554, 6304066.
LTC3878
Fast, Wide Operating
Range No RSENSETM
Step-Down Controller
DESCRIPTION
The LTC®3878 is a synchronous step-down switching
regulator controller optimized for high switching frequency
and fast transient response. The constant on-time valley
current mode architecture allows for a wide input range,
including very low duty factor operation. No external sense
resistor or slope compensation is required. The LTC3878
is pin compatible with the LTC1778 in applications that do
not use EXTVCC while offering better efficiency. Consult
with the factory to verify compatibility.
Operating frequency is set by an external resistor and
compensated for variations in VIN to offer excellent line
stability. Discontinuous mode operation provides high
efficiency during light load conditions. A forced continu-
ous control pin allows the user to reduce noise and RF
interference. Safety features include output overvoltage
protection and programmable current limit with foldback.
Soft-start capability for supply sequencing is accomplished
through an external timing capacitor. The current limit is
user programmable.
The LTC3878 allows operation from 4V to 38V at the input
and from 0.8V to 90% VIN at the output. The LTC3878 is
available in a small 16-pin narrow SSOP package.
TYPICAL APPLICATION
High Efficiency Step-Down Converter
0.1μF
ION
RUN/SS VIN
120pF
20k
ITH
SGND
TG
SW
BOOST
LTC3878
FCB INTVCC
BG
PGOOD PGND
VRNG
VFB
432k
RJK0305
0.56μH
0.22μF
4.7μF
RJK0330
5.11k
VIN
4.5V TO 28V
10μF
VOUT
1.2V
15A
750μF
10k
3878 TA01a
Efficiency vs Load Current
100
90 DISCONTINUOUS
MODE
80
70
60 CONTINUOUS
MODE
50
40
30
20
10
0
0.01
VIN = 12V
VOUT = 1.2V
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
0.1 1
10
LOAD CURRENT (A)
100
3878 G07
3878f
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3878
Frequency vs Load Current
410
www.datasheCeOt4NuTI.NcUoOmUS MODE
370
330
DISCONTINUOUS MODE
290
250
210
170
130
90 VIN = 15V
50 VOUT = 1.2V
FIGURE 7 CIRCUIT
10
0 2 4 6 8 10 12 14
ILOAD (A)
3878 G10
Current Sense Voltage
vs ITH Voltage
300
250
200
150
100
50
0
–50
–100
–150
0
VRNG = 0.2V
VRNG = 0.5V
VRNG = 1.0V
VRNG = 1.5V
VRNG = 2.0V
0.5 1 1.5
ITH VOLTAGE (V)
2
2.5
3878 G13
Maximum VDS Current Sense
Threshold vs VRNG Voltage
300
250
200
150
100
50
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
VRNG VOLTAGE (V)
3878 G16
Load Regulation FCM
0
–0.02
–0.04
VIN = 15V
VOUT = 1.2V
FIGURE 7 CIRCUIT
–0.06
–0.08
–0.10
–0.12
–0.14
–0.16
0
5 10
LOAD CURRENT (A)
15
3878 G11
On-Time vs ION Current
10000
FIGURE 7 CIRCUIT
1000
100
10
1
10
ION CURRENT (A)
100
3878 G14
Maximum VDS Current Sense
Threshold vs RUN/SS Voltage
140
120
100
80
60
40
20
0
1.5 2.0 2.5 3.0
RUN/SS VOLTAGE (V)
3878 G17
ITH Voltage vs Load Current
2.5
2.3
2.1
1.9
1.7
1.5
1.3
1.1 VIN = 15V
VOUT = 1.2V
0.9 FIGURE 7 CIRCUIT
0.7 CONTINUOUS MODE
DISCONTINUOUS MODE
0.5
0 5 10 15 20 25
LOAD CURRENT (A)
3878 G12
Current Limit Foldback
140
VRNG = 1V
FIGURE 7 CIRCUIT
120
100
80
60
40
20
0
0 0.2 0.4 0.6 0.8
VFB (V)
3878 G15
Foldback Reference Voltage vs
Temperature
0.808
0.806
0.804
0.802
0.800
0.798
0.796
0.794
0.792
–50 –30 –10 10 30 50 70 90 110
TEMPERATURE (°C)
3878 G18
3878f
5
5 Page 
LTC3878
APPLICATIONS INFORMATION
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
www.datPaTsOhePe=t4Du.TcOoPm•IOUT(MAX)2 • ρτ(TOP) • RDS(ON)(MAX)
( )+VIN2
⎛
⎝⎜
IOUT(MAX)
2
⎞
⎠⎟
CMILLER
⎡
⎢
⎣
DRTGHIGH
VINTVCC – VMILLER
+
DRTGLOW
VMILLER
⎤
⎥
⎦
fOSC
PBOT = DBOT •IOUT(MAX)2 • ρτ(BOT) •RDS(ON)(MAX)
DRTGHIGH is pull-up driver resistance and DRTGLOW is the
TG driver pull-down resistance. VMILLER is the Miller ef-
fect VGS voltage and is taken graphically from the power
MOSFET data sheet.
MOSFET input capacitance is a combination of several
components but can be taken from the typical “gate charge”
curve included on the most data sheets (Figure 2). The
curve is generated by forcing a constant input current
into the gate of a common source, current source, loaded
stage and then plotting the gate versus time. The initial
slope is the effect of the gate-to-source and gate-to-drain
capacitance. The flat portion of the curve is the result of the
Miller multiplication effect of the drain-to-gate capacitance
as the drain drops the voltage across the current source
load. The upper sloping line is due to the drain-to-gate
accumulation capacitance and the gate-to-source capaci-
tance. The Miller charge (the increase in coulombs on the
horizontal axis from a to b while the curve is flat) is speci-
fied from a given VDS drain voltage, but can be adjusted
for different VDS voltages by multiplying by the ratio of
the application VDS to the curve specified VDS values. A
way to estimate the CMILLER term is to take the change in
gate charge from points a and b or the parameter QGD on
a manufacturers data sheet and divide by the specified
VDS test voltage, VDS(TEST).
CMILLER
QGD
VDS(TEST)
CMILLER is the most important selection criteria for deter-
mining the transition loss term in the top MOSFET but is
not directly specified on MOSFET data sheets.
VIN
MILLER EFFECT
VGS
ab
QIN
CMILLER = (QB – QA)/VDS
V
+
VGS
–
+
VDS
–
3878 F02
Figure 2. Gate Charge Characteristic
Both MOSFETs have I2R power loss, and the top MOSFET
includes an additional term for transition loss, which are
highest at high input voltages. For VIN < 20V, the high cur-
rent efficiency generally improves with larger MOSFETs,
while for VIN > 20V, the transition losses rapidly increase
to the point that the use of a higher RDS(ON) device with
lower CMILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Lowering the operating fre-
quency improves efficiency by reducing MOSFET switching
losses but requires larger inductance and/or capacitance
to maintain low output ripple voltage. Conversely, raising
the operating frequency degrades efficiency but reduces
component size.
The operating frequency of LTC3878 applications is de-
termined 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 according to:
( )tON
=
0.7V
IION
10pF
Tying a resistor RON from VIN to the ION pin yields an
on-time inversely proportional to VIN. For a step-down
converter, this results in pseudo fixed frequency operation
as the input supply varies.
( )fOP
=
0.7V
VOUT
• RON
10pF
[Hz]
3878f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC3878.PDF ] | |
Hoja de datos destacado
| Número de pieza | Descripción | Fabricantes |
| LTC3870 | PolyPhase Step-Down Slave Controller |  Linear Technology |
| LTC3872 | No RSENSE Current Mode Boost DC/DC Controller | Linear Technology |
| LTC3873 | No RSENSETM Constant Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller | Linear Technology |
| LTC3873-5 | No RSENSETM Constant Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller | Linear Technology |
| Número de pieza | Descripción | Fabricantes |
| SLA6805M | High Voltage 3 phase Motor Driver IC. |
 Sanken |
| SDC1742 | 12- and 14-Bit Hybrid Synchro / Resolver-to-Digital Converters. |
 Analog Devices |
|
DataSheet.es es una pagina web que funciona como un repositorio de manuales o hoja de datos de muchos de los productos más populares, |
| DataSheet.es | 2020 | Privacy Policy | Contacto | Buscar |