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PDF LTC3858-1 Data sheet ( Hoja de datos )
| Número de pieza | LTC3858-1 | |
| Descripción | Dual 2-Phase Synchronous Step-Down Controller | |
| Fabricantes | Linear Technology Corporation | |
| Logotipo |  |
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FEATURES
n Low Operating IQ: 170μA (One Channel On)
n Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 24V
n Wide VIN Range: 4V to 38V
n RSENSE or DCR Current Sensing
n Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
n OPTI-LOOP® Compensation Minimizes COUT
n Phase-Lockable Frequency (75kHz-850kHz)
n Programmable Fixed Frequency (50kHz-900kHz)
n Selectable Continuous, Pulse-Skipping or
Burst Mode® Operation at Light Loads
n Very Low Dropout Operation: 99% Duty Cycle
n Adjustable Output Voltage Soft-Start
n Power Good Output Voltage Monitor
n Output Overvoltage Protection
n Output Latch-Off Protection During Short Circuit
n Low Shutdown IQ: 8μA
n Internal LDO Powers Gate Drive from VIN or EXTVCC
n No Current Foldback During Start-Up
n Tiny 4mm × 5mm QFN and Narrow SSOP Packages
APPLICATIONS
n Automotive Systems
n Battery Operated Digital Devices
n Distributed DC Power Systems
LTC3858-1
Low IQ, Dual
2-Phase Synchronous
Step-Down Controller
DESCRIPTION
The LTC®3858-1 is a high performance dual step-down
switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant frequency
current mode architecture allows a phase-lockable fre-
quency of up to 850kHz. Power loss and noise due to the
input capacitor ESR are minimized by operating the two
controller outputs out of phase.
The 170μA no-load quiescent current extends operating
life in battery powered systems. OPTI-LOOP compensa-
tion allows the transient response to be optimized over
a wide range of output capacitance and ESR values. The
LTC3858-1 features a precision 0.8V reference and a power
good output indicator. A wide 4V to 38V input supply range
encompasses a wide range of intermediate bus voltages
and battery chemistries.
Independent soft-start pins for each controller ramp the
output voltages during start-up. The output latch-off feature
protects the circuit in short-circuit conditions.
For a leadless 32-pin QFN package with additional features
of adjustable current limit, clock out, phase modulation
and two PGOOD outputs, see the LTC3858 data sheet.
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Linear Technology and the Linear logo 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.
TYPICAL APPLICATION
VOUT1
3.3V
5A
High Efficiency Dual 8.5V/3.3V Step-Down Converter
3.3μH
0.1μF
VIN
TG1
BOOST1
SW1
INTVCC
TG2
BOOST2
SW2
4.7μF
VIN
22μF 9V TO 38V
50V
0.1μF
7.2μH
BG1 BG2
LTC3858-1
PGND
SENSE1+
SENSE2+
0.007Ω
62.5k
150μF
680pF
SENSE1–
VFB1
ITH1
SS1
SGND
SENSE2–
VFB2
ITH2
SS2
20k 15k
0.1μF
0.1μF
680pF
15k
0.01Ω
193k
20k
VOUT2
8.5V
3.5A
150μF
38581 TA01
Efficiency and Power Loss
vs Load Current
100 10000
90
80 1000
70
EFFICIENCY
60
100
50
POWER LOSS
40 10
30
20
10
0
0.0001
0.001 0.01
VIN = 12V
1
VOUT = 3.3V
FIGURE 12 CIRCUIT
0.1
0.1 1 10
OUTPUT CURRENT (A)
38581 TA01b
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TYPICAL PERFORMANCE CHARACTERISTICS
LTC3858-1
Efficiency and Power Loss vs
Output Current
100
FIGURE 12 CIRCUIT
90 VIN = 12V
80 VOUT = 3.3V
10000
1000
70
60 100
50
40
30
20
10
0
0.0001
0.001 0.01
Burst Mode
OPERATION 10
PULSE-
SKIPPING
MODE
FORCED
1
CONTINUOUS
MODE
0.1
0.1 1 10
OUTPUT CURRENT (A)
3858 G01
Efficiency vs Load Current
100
90
VIN = 5V
80
70
VIN = 12V
60
50
40
30
20
10
0
0.0001
VOUT = 3.3V
FIGURE 12 CIRCUIT
0.001 0.01 0.1
1
OUTPUT CURRENT (A)
10
3858 G02
Efficiency vs Input Voltage
98
FIGURE 12 CIRCUIT
96 VOUT = 3.3V
IOUT = 4A
94
92
Load Step (Burst Mode Operation)
VOUT
100mV/DIV
AC
COUPLED
90
88
86 IL
2A/DIV
84
82
80
0 5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
3858 G03
VOUT = 3.3V
20μs/DIV
FIGURE 12 CIRCUIT
3858 G04
Load Step
(Forced Continuous Mode)
VOUT
100mV/DIV
AC
COUPLED
IL
2A/DIV
VOUT = 3.3V
20μs/DIV
FIGURE 12 CIRCUIT
3858 G05
Load Step (Pulse-Skipping Mode)
Inductor Current at Light Load
Soft-Start
VOUT
100mV/DIV
AC
COUPLED
IL
2A/DIV
VOUT = 3.3V
20μs/DIV
FIGURE 12 CIRCUIT
FORCED
CONTINUOUS
MODE
Burst Mode
OPERATION
2A/DIV
3858 G06
PULSE
SKIPPING
MODE
VOUT = 3.3V
2μs/DIV
ILOAD = 200μA
FIGURE 12 CIRCUIT
VOUT2
2V/DIV
VOUT1
2V/DIV
3858 G07
20ms/DIV
FIGURE 12 CIRCUIT
3858 G08
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LTC3858-1
OPERATION (Refer to the Functional Diagram)
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin. When
the EXTVCC pin is left open or tied to a voltage less than
4.7V, the VIN LDO (low dropout linear regulator) supplies
5.1V from VIN to INTVCC. If EXTVCC is taken above 4.7V,
the VIN LDO is turned off and the EXTVCC LDO is turned on.
Once enabled, the EXTVCC LDO supplies 5.1V from EXTVCC
to INTVCC. Using the EXTVCC pin allows the INTVCC power
to be derived from a high efficiency external source such
as one of the LTC3858-1 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, CB, which normally recharges during each
switching cycle through an external diode when the top
MOSFET turns off. If the input voltage VIN decreases to
a voltage close to VOUT, the loop may enter dropout and
attempt to turn on the top MOSFET continuously. The
dropout detector detects this and forces the top MOSFET
off for about one-twelfth of the clock period every tenth
cycle to allow CB to recharge.
Shutdown and Start-Up (RUN1, RUN2 and
SS1, SS2 Pins)
The two channels of the LTC3858-1 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either
of these pins below 1.2V shuts down the main control loop
for that controller. Pulling both pins below 0.7V disables
both controllers and most internal circuits, including the
INTVCC LDOs. In this state, the LTC3858-1 draws only 8μA
of quiescent current.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low impedance
source, do not exceed the absolute maximum rating of
8V on this pin. The RUN pin has an internal 11V voltage
clamp that allows the RUN pin to be connected through a
resistor to a higher voltage (for example, VIN), so long as
the maximum current into the RUN pin does not exceed
100μA.
The start-up of each controller’s output voltage VOUT is
controlled by the voltage on the SS pin for that channel.
When the voltage on the SS pin is less than the 0.8V
internal reference, the LTC3858-1 regulates the VFB volt-
age to the SS pin voltage instead of the 0.8V reference.
This allows the SS pin to be used to program a soft-start
by connecting an external capacitor from the SS pin to
SGND. An internal 1μA pull-up current charges this ca-
pacitor creating a voltage ramp on the SS pin. As the SS
voltage rises linearly from 0V to 0.8V (and beyond up to
the absolute maximum rating of 6V), the output voltage
VOUT rises smoothly from zero to its final value.
Short-Circuit Latch-Off
After the controller has been started and been given
adequate time to ramp up the output voltage, the SS
capacitor is used in a short-circuit time-out circuit. Spe-
cifically, once the voltage on the SS pin rises above 2V
(the arming threshold), the short-circuit timeout circuit is
enabled (see Figure 1). If the output voltage falls below
70% of its nominal regulated voltage, the SS capacitor
begins discharging with a net 9μA pull-down current on
the assumption that the output is in an overcurrent and/or
short-circuit condition. If the condition lasts long enough
to allow the SS pin voltage to fall below 1.5V (the latchoff
threshold) , the controller will shut down (latch off) until
the RUN pin voltage or the VIN voltage is recycled.
The delay time from when an short-circuit occurs until
the controller latches off can be calculated using the fol-
lowing equation:
tLATCH
≈
CSS
VSS – 1.5V
9μA
where VSS is the initial voltage (must be greater than 2V)
on the SS pin at the time the short-circuit occurs. Normally
the SS pin voltage will have been pulled up to the INTVCC
voltage (5.1V) by the internal 1μA pull-up current.
Note that the two controllers on the LTC3858-1 have sepa-
rate, independent short-circuit latchoff circuits. Latchoff
can be overridden/defeated by connecting a resistor 150k
or less from the SS pin to INTVCC. This resistor provides
enough pull-up current to overcome the 9μA pull-down
current present during a short-circuit. Note that this resis-
tor also shortens the soft-start period.
38581f
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11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3858-1.PDF ] | |
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