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PDF LTC3851 Data sheet ( Hoja de datos )
| Número de pieza | LTC3851 | |
| Descripción | Synchronous Step-Down Switching Regulator Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURESwww.datasheet4u.com
n Wide VIN Range: 4V to 38V Operation
n RSENSE or DCR Current Sensing
n ±1% Output Voltage Accuracy
n Phase-Lockable Fixed Frequency: 250kHz to 750kHz
n Dual N-Channel MOSFET Synchronous Drive
n Very Low Dropout Operation: 99% Duty Cycle
n Adjustable Output Voltage Soft-Start or Tracking
n Output Current Foldback Limiting
n Output Overvoltage Protection
n 5V Internal Regulator
n OPTI-LOOP® Compensation Minimizes COUT
n Selectable Continuous, Pulse-Skipping or
Burst Mode® Operation at Light Loads
n Low Shutdown IQ: 20μA
n VOUT Range: 0.8V to 5.5V
n 16-Lead Narrow SSOP or 3mm × 3mm QFN Package
APPLICATIONS
n Automotive Systems
n Telecom Systems
n Industrial Equipment
n Distributed DC Power Systems
LTC3851
Synchronous
Step-Down Switching
Regulator Controller
DESCRIPTION
The LTC®3851 is a high performance synchronous
step-down switching regulator controller that drives
an all N-channel synchronous power MOSFET stage. A
constant frequency current mode architecture allows a
phase-lockable frequency of up to 750kHz.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance
and ESR values. The LTC3851 features a precision 0.8V
reference that is compatible with a wide 4V to 38V input
supply range.
The TK/SS pin ramps the output voltage during start-up.
Current foldback limits MOSFET heat dissipation during
short-circuit conditions. The MODE/PLLIN pin selects
among Burst Mode operation, pulse skipping mode or
continuous inductor current mode at light loads and allows
the IC to be synchronized to an external clock.
, LT, LTC, LTM, Burst Mode and OPTI-LOOP are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5408150, 5481178, 5705919, 5929620, 6304066,
6498466, 6580258, 6611131.
TYPICAL APPLICATION
High Efficiency Synchronous Step-Down Converter
10nF
1k
ILIM VIN
FREQ/PLLFLTR TG
0.1μF
RUN SW
LTC3851
TK/SS BOOST
INTVCC
ITH
100pF
BG
GND
SENSE+
PLLIN/MODE
SENSE–
VFB
0.22μF
0.68μH
10k
150μF
VIN
4V TO 38V
VOUT
3.3V
10A
330μF
0.22μF 13k
62.5k
20k
3851 TA01a
Efficiency and Power Loss
vs Load Current
100
VIN = 12V
95 VOUT = 3.3V
90
85
80
EFFICIENCY
10000
1000
75
POWER LOSS
70
65 100
60
55
50
10
100 1000
LOAD CURRENT (mA)
10
10000
3851TA01b
3851f
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3851
Efficiency and Power Loss vs
Input Voltage
www.datasheet4u.com
100
VIN = 12V
VOUT = 3.3V
95 FIGURE 11 CIRCUIT
POWER LOSS,
IOUT = 5A
90 EFFICIENCY,
IOUT = 5A
85
10000
1000
80 POWER LOSS,
IOUT = 0.5A
75 EFFICIENCY,
IOUT = 0.5A
70 100
4 8 12 16 20 24 28 32 36 40
INPUT VOLTAGE (V)
Load Step
(Burst Mode Operation)
ILOAD
5A/DIV
0.2A TO 7.5A
IL
5A/DIV
VOUT
100mV/DIV
AC COUPLED
VOUT = 1.5V
100μs/DIV
VIN = 12V
FIGURE 11 CIRCUIT
3851 G04
Load Step
(Forced Continuous Mode)
ILOAD
5A/DIV
0.2A TO 7.5A
IL
5A/DIV
VOUT
100mV/DIV
AC COUPLED
3851 G05
VOUT = 1.5V
100μs/DIV
VIN = 12V
FIGURE 11 CIRCUIT
3851 G06
Load Step
(Pulse-Skip Mode)
Inductor Current at Light Load
Start-Up with Prebiased Output
at 2V
ILOAD
5A/DIV
0.2A TO 7.5A
IL
5A/DIV
VOUT
100mV/DIV
AC COUPLED
VOUT = 1.5V
100μs/DIV
VIN = 12V
FIGURE 11 CIRCUIT
3851 G07
FORCED
CONTINOUS
MODE
5A/DIV
Burst Mode
OPERATION
5A/DIV
PULSE SKIP
MODE
5A/DIV
VOUT = 1.5V
1μs/DIV
VIN = 12V
ILOAD = 1mA
FIGURE 11 CIRCUIT
VOUT
2V/DIV
VFB
0.5V/DIV
3851 G08
TK/SS
0.5V/DIV
20ms/DIV
3851 G09
Coincident Tracking with Master
Supply
Ratiometric Tracking with Master
Supply
VMASTER
0.5V/DIV
VOUT
2A LOAD
0.5V/DIV
VMASTER
0.5V/DIV
VOUT
2A LOAD
0.5V/DIV
10ms/DIV
3851 G10
10ms/DIV
3851 G11
Input DC Supply Current vs Input
Voltage
3.0
2.5
2.0
1.5
1.0
0.5
0
4 8 12 16 20 24 28 32 36 40
INPUT VOLTAGE (V)
3851 G12
3851f
5
5 Page 
LTC3851
OPERATION
the error amplifier, EA, will decrease the voltage on the ITH
pin. When the ITH voltage drops below 0.4V, the internal
wwws.dleateapshseiegt4nua.cl ogmoes high (enabling “sleep” mode) and both
external MOSFETs are turned off.
In sleep mode, the load current is supplied by the output
capacitor. As the output voltage decreases, the EA’s output
begins to rise. When the output voltage drops enough, the
sleep signal goes low, and the controller resumes normal
operation by turning on the top external MOSFET on the
next cycle of the internal oscillator. When a controller is
enabled for Burst Mode operation, the inductor current is
not allowed to reverse. The reverse current comparator,
IREV, turns off the bottom external MOSFET just before the
inductor current reaches zero, preventing it from revers-
ing and going negative. Thus, the controller operates in
discontinuous operation. In forced continuous operation,
the inductor current is allowed to reverse at light loads or
under large transient conditions. The peak inductor cur-
rent is determined by the voltage on the ITH pin, just as in
normal operation. In this mode the efficiency at light loads
is lower than in Burst Mode operation. However, continu-
ous mode has the advantages of lower output ripple and
less interference to audio circuitry.
When the MODE/PLLIN pin is connected to GND, the
LTC3851 operates in PWM pulse-skipping mode at light
loads. At very light loads the current comparator, ICMP, may
remain tripped for several cycles and force the external top
MOSFET to stay off for the same number of cycles (i.e.,
skipping pulses). The inductor current is not allowed to
reverse (discontinuous operation). This mode, like forced
continuous operation, exhibits low output ripple as well as
low audio noise and reduced RF interference as compared
to Burst Mode operation. It provides higher low current
efficiency than forced continuous mode, but not nearly as
high as Burst Mode operation.
Frequency Selection and Phase-Locked Loop
(FREQ/PLLFLTR and MODE/PLLIN Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency operation
increases efficiency by reducing MOSFET switching losses,
but requires larger inductance and/or capacitance to main-
tain low output ripple voltage. The switching frequency
of the LTC3851 can be selected using the FREQ/PLLFLTR
pin. If the MODE/PLLIN pin is not being driven by an ex-
ternal clock source, the FREQ/PLLFLTR pin can be used
to program the controller’s operating frequency from
250kHz to 750kHz.
A phase-locked loop (PLL) is available on the LTC3851
to synchronize the internal oscillator to an external clock
source that is connected to the MODE/PLLIN pin. The
controller operates in forced continuous mode of operation
when it is synchronized. A series RC should be connected
between the FREQ/PLLFLTR pin and GND to serve as the
PLL’s loop filter.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>10%) as well as other more serious con-
ditions that may overvoltage the output. In such cases,
the top MOSFET is turned off and the bottom MOSFET is
turned on until the overvoltage condition is cleared.
3851f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3851.PDF ] | |
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