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PDF LTC3850-2 Data sheet ( Hoja de datos )
| Número de pieza | LTC3850-2 | |
| Descripción | 2-Phase Synchronous Step-Down Switching Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURESwww.datasheet4u.com
n Dual, 180° Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
n High Efficiency: Up to 95%
n RSENSE or DCR Current Sensing
n ±1% 0.8V Output Voltage Accuracy
n Phase-Lockable Fixed Frequency 250kHz to 780kHz
n Supports Pre-Biased Output
n Dual N-Channel MOSFET Synchronous Drive
n Wide VIN Range: 4V to 30V Operation
n Adjustable Soft-Start Current Ramping or Tracking
n Foldback Output Current Limiting
n Output Overvoltage Protection
n Power Good Output Voltage Monitor
n 28-Pin Narrow SSOP Package
APPLICATIONS
n Notebook and Palmtop Computers
n Portable Instruments
n Battery-Operated Digital Devices
n DC Power Distribution Systems
L, LT, LTC, LTM, OPTI-LOOP and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners. Protected by U.S.
Patents, including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6580258.
LTC3850-2
Dual, 2-Phase
Synchronous Step-Down
Switching Controller
DESCRIPTION
The LTC®3850-2 is a high performance dual synchronous
step-down switching regulator controller that drives all
N-channel power MOSFET stages. A constant-frequency
current mode architecture allows a phase-lockable
frequency of up to 780kHz. Power loss and supply noise
are minimized by operating the two controller output
stages out of phase.
OPTI-LOOP® compensation allows the transient response
to be optimized over a wide range of output capacitance
and ESR values. The LTC3850-2 features a precision 0.8V
reference and a power good output indicator. A wide 4V
to 30V input supply range encompasses most battery
chemistries and intermediate bus voltages.
Independent TK/SS pins for each controller ramp the
output voltages during start-up. Current foldback limits
MOSFET heat dissipation during short-circuit condi-
tions. The MODE/PLLIN pin selects among Burst Mode®
operation, pulse-skipping mode, or continuous inductor
current mode and allows the IC to be synchronized to an
external clock.
The LTC3850-2 is identical to the LTC3850-1, except they
have different pin assignments.
TYPICAL APPLICATION
VOUT1
3.3V
5A
2.2μH
2.2k
63.4k
100μF
6V
High Efficiency Dual 3.3V/2.5V Step-Down Converter
0.1μF
VIN PGOOD INTVCC
TG1 TG2
BOOST1
BOOST2
SW1
LTC3850-2
BG1
SW2
BG2
500kHz MODE/PLLIN
PGND
EXTVCC FREQ/PLLFLTR
0.1μF
SENSE1+
RUN1
SENSE1–
VFB1
SENSE2+
RUN2
SENSE2–
VFB2
ITH1 ITH2
220pF
TK/SS1 SGND TK/SS2
20k 15k
0.1μF
0.1μF
4.7μF
0.1μF
22μF
50V
2.2μH
2.2k
0.1μF
10nF
10k
220pF
43.2k
15k 20k
VIN
7V TO
26V
VOUT2
2.5V
5A
100μF
6V
38502 TA01
Efficiency
100
VIN = 12V
95 VOUT = 3.3V
90
EFFICIENCY
85
80
75
70
65
POWER LOSS
60
55
50
10
100 1000
LOAD CURRENT (mA)
10000
1000
100
10
10000
38502 TA01b
38502f
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LTC3850-2
www.datasheet4u.com
Load Step
(Burst Mode Operation)
ILOAD
2A/DIV
200mA TO 2.5A
IL
2A/DIV
VOUT
100mV/DIV
AC COUPLED
40μs/DIV
CIRCUIT OF FIGURE 14
VIN = 12V, VOUT = 1.8V
38502 G04
Load Step
(Forced Continuous Mode)
ILOAD
2A/DIV
200mA TO 2.5A
IL
2A/DIV
VOUT
100mV/DIV
AC COUPLED
40μs/DIV
CIRCUIT OF FIGURE 14
VIN = 12V, VOUT = 1.8V
38502 G05
Load Step
(Pulse-Skipping Mode)
ILOAD
2A/DIV
200mA TO 2.5A
IL
2A/DIV
VOUT
100mV/DIV
AC COUPLED
40μs/DIV
CIRCUIT OF FIGURE 14
VIN = 12V, VOUT = 1.8V
38502 G06
Inductor Current at Light Load
FORCED
CONTINUOUS
MODE
2A/DIV
Burst Mode
OPERATION
2A/DIV
PULSE-SKIPPING
MODE
2A/DIV
1μs/DIV
CIRCUIT OF FIGURE 14
VIN = 12V, VOUT = 1.8V
ILOAD = 100μA
38502 G07
Prebiased Output at 2V
VOUT
2V/DIV
VFB
500mV/DIV
VTK/SS
500mV/DIV
2.5ms/DIV
38502 G08
Coincident Tracking
RUN1
2V/DIV
VOUT1, 3.3V
3Ω LOAD, 1V/DIV
VOUT2, 1.8V
1.5Ω LOAD
1V/DIV
1ms/DIV
38502 G09
38502f
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5 Page 
LTC3850-2
OPERATION
mode of operation, tie the MODE/PLLIN pin to INTVCC. To
select Burst Mode operation, float the MODE/PLLIN pin.
wwwW.dahtaesnheaetc4uo.ncotrmoller is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-third of the maximum sense voltage even though
the voltage on the ITH pin indicates a lower value. If the
average inductor current is higher than the load current,
the error amplifier EA will decrease the voltage on the ITH
pin. When the ITH voltage drops below 0.5V, 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 current
is determined by the voltage on the ITH pin, just as in nor-
mal operation. In this mode, the efficiency at light loads is
lower than in Burst Mode operation. However, continuous
mode has the advantages of lower output ripple and less
interference with audio circuitry.
When the MODE/PLLIN pin is connected to INTVCC, the
LTC3850-2 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 LTC3850-2’s controllers 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 780kHz.
A phase-locked loop (PLL) is available on the LTC3850-2
to synchronize the internal oscillator to an external clock
source that is connected to the MODE/PLLIN pin. The
controller is operating in forced continuous mode when
it is synchronized. A series R-C should be connected
between the FREQ/PLLFLTR pin and SGND to serve as
the PLL’s loop filter.
Power Good (PGOOD Pin)
The PGOOD pin is connected to an open drain of an internal
N-channel MOSFET. The MOSFET turns on and pulls the
PGOOD pin low when either VFB pin voltage is not within
±7.5% of the 0.8V reference voltage. The PGOOD pin is
also pulled low when either RUN pin is below 1.2V or when
the LTC3850-2 is in the soft-start or tracking phase. When
the VFB pin voltage is within the ±7.5% requirement, the
MOSFET is turned off and the pin is allowed to be pulled
up by an external resistor to a source of up to 6V. The
PGOOD pin will flag power good immediately when both
VFB pins are within the ±7.5% window. However, there is
an internal 17μs power bad mask when either VFB goes
out of the ±7.5% window.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (> 7.5%) 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.
38502f
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11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3850-2.PDF ] | |
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