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PDF LTC3851A Data sheet ( Hoja de datos )
| Número de pieza | LTC3851A | |
| Descripción | Synchronous Step-Down Switching Regulator Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Features
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 Thermally Enhanced 16-Lead MSOP, 16-Lead Narrow
SSOP or 3mm × 3mm QFN Package
Applications
n Automotive Systems
n Telecom Systems
n Industrial Equipment
n Distributed DC Power Systems
LTC3851A
Synchronous
Step-Down Switching
Regulator Controller
Description
The LTC®3851A 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 LTC3851A 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 al-
lows the IC to be synchronized to an external clock. The
LTC3851A contains an improved PLL compared to the
LTC3851.
The LTC3851A-1 is a version with a power good output
signal instead of adjustable current limit.
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Linear Technology and the Linear logo are registered
trademarks and No RSENSE, UltraFast are 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
82.5k
0.1µF
0.1µF
ILIM VIN
FREQ/PLLFLTR TG
RUN SW
LTC3851A
TK/SS BOOST
2200pF
15k
INTVCC
ITH
330pF
BG
GND
SENSE+
MODE/PLLIN
SENSE–
VFB
0.1µF
4.7µF
0.68µH
3.01k
22µF
VIN
4.5V TO 36V
VOUT
3.3V
15A
330µF
×2
0.047µF
30.1k
154k
48.7k
3851A TA01a
Efficiency and Power Loss
vs Load Current
100
95
VVIONU=T
12V
= 3.3V
90 EFFICIENCY
85
80
10000
1000
75 POWER LOSS
70
65 100
60
55
50
10
100
1000
10000
LOAD CURRENT (mA)
10
100000
3851A TA01b
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1
1 page  
Typical Performance Characteristics
LTC3851A
Efficiency and Power Loss
vs Input Voltage
100
EFFICIENCY,
95 IOUT = 5A
90
POWER LOSS,
IOUT = 5A
10000
85
80
75
70
4
EFFICIENCY,
IOUT = 0.5A
1000
POWER LOSS,
IOUT = 0.5A
8 12 16
VIN = 12V
VOUT = 3.3V
FIGURE 11 CIRCUIT
100
20 24 28 32
INPUT VOLTAGE (V)
3851A G04
Load Step
(Forced Continuous Mode)
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
3851A G06
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
3851A G05
Load Step
(Pulse-Skipping Mode)
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
3851A G07
Inductor Current at Light Load
FORCED
CONTINOUS
MODE
5A/DIV
Burst Mode
OPERATION
5A/DIV
PULSE-SKIPPING
MODE
5A/DIV
VOUT = 1.5V
1µs/DIV
VIN = 12V
ILOAD = 1mA
FIGURE 11 CIRCUIT
3851A G08
Start-Up with Prebiased Output
at 2V
VOUT
2V/DIV
VFB
0.5V/DIV
TK/SS
0.5V/DIV
20ms/DIV
3851A G09
Coincident Tracking with Master
Supply
VMASTER
0.5V/DIV
VOUT
2A LOAD
0.5V/DIV
10ms/DIV
3851A G10
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LTC3851A
Operation
the error amplifier, EA, will decrease the voltage on the ITH
pin. When the ITH voltage drops below 0.4V, the internal
sleep signal goes 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
LTC3851A 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 opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage. The switching fre-
quency of the LTC3851A can be selected using the FREQ/
PLLFLTR pin. If the MODE/PLLIN pin is not being driven
by an external 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 LTC3851A
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. It is suggested that the external clock be
applied before enabling the controller unless a second
resistor is connected in parallel with the series RC network.
The second resistor prevents very low switching frequency
operation if the controller is enabled before the clock.
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.
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11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3851A.PDF ] | |
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