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PDF LTC3412A Data sheet ( Hoja de datos )
| Número de pieza | LTC3412A | |
| Descripción | 3A/ 4MHz/ Monolithic Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
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LTC3412Awww.DataSheet4U.com
3A, 4MHz, Monolithic
Synchronous Step-Down Regulator
FEATURES
■ High Efficiency: Up to 95%
■ 3A Output Current
■ Low Quiescent Current: 64μA
■ Low RDS(ON) Internal Switch: 77mΩ
■ 2.25V to 5.5V Input Voltage Range
■ Programmable Frequency: 300KHz to 4MHz
■ ±2% Output Voltage Accuracy
■ 0.8V Reference Allows Low Output Voltage
■ Selectable Forced Continuous/Burst Mode® Operation
with Adjustable Burst Clamp
■ Synchronizable Switching Frequency
■ Low Dropout Operation: 100% Duty Cycle
■ Power Good Output Voltage Monitor
■ Overtemperature Protected
■ Available in 16-Lead Exposed Pad TSSOP and
QFN Packages
U
APPLICATIO S
■ Point-of-Load Regulation
■ Notebook Computers
■ Portable Instruments
■ Distributed Power Systems
DESCRIPTIO
The LTC®3412A is a high efficiency monolithic synchro-
nous, step-down DC/DC converter utilizing a constant
frequency, current mode architecture. It operates from
an input voltage range of 2.25V to 5.5V and provides a
regulated output voltage from 0.8V to 5V while deliver-
ing up to 3A of output current. The internal synchronous
power switch with 77mΩ on-resistance increases efficiency
and eliminates the need for an external Schottky diode.
Switching frequency is set by an external resistor or can
be synchronized to an external clock. 100% duty cycle
provides low dropout operation extending battery life in
portable systems. OPTI-LOOP® compensation allows the
transient response to be optimized over a wide range of
loads and output capacitors.
The LTC3412A can be configured for either Burst Mode
operation or forced continuous operation. Forced continu-
ous operation reduces noise and RF interference while
Burst Mode operation provides high efficiency by reducing
gate charge losses at light loads. In Burst Mode operation,
external control of the burst clamp level allows the output
voltage ripple to be adjusted according to the application
requirements.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
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 5481178, 6580258, 6304066, 6127815, 6498466,
6611131, 6724174.
TYPICAL APPLICATIO
22μF
VIN
3.3V
2.2M
294k
PVIN SVIN
RT PGOOD
LTC3412A
SW
1000pF
820pF
12.1k
RUN/SS PGND
ITH SGND
SYNC/MODE VFB
69.8k
115k
0.47μH
392k
VOUT
2.5V AT 3A
COUT
100μF
×2
3412A F01a
Figure 1. 2.5V/3A Step-Down Regulator
Efficiency and Power Loss
100
95
90
85
80
75
70
65
60
55
50
0.01
EFFICIENCY
POWER LOSS
0.1 1
LOAD CURRENT (A)
100000
10000
1000
100
10
1
10
3412A F01b
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LTC3412Awww.DataSheet4U.com
TYPICAL PERFOR A CE CHARACTERISTICS
Load Step Transient Forced
Continuous
VOUT
100mV/DIV
INDUCTOR
CURRENT
2A/DIV
Start-Up Transient
VOUT
2V/DIV
RUN/SS
2V/DIV
INDUCTOR
CURRENT
2A/DIV
VIN = 3.3V
VOUT =2.5V
F = 1MHz
40μs/DIV
LOAD STEP = 0A TO 3A
FIGURE 4 CIRCUIT
3412A G10
VIN = 3.3V
1ms/DIV
VOUT =2.5V
LOAD STEP = 2A
FIGURE 4 CIRCUIT
3412A G11
VREF vs Temperature
0.7975
VIN = 3.3V
0.7970
0.7965
0.7960
0.7955
0.7950
0.7945
0.7940
0.7935
0.7930
–45 –25 –5 15 35 55 75 95 115
TEMPERATURE (°C)
3412A G12
Switch On-Resistance vs
Input Voltage
100
95
90
85
PFET
80
75
70
NFET
65
60
55
50
2.5
3.0 3.5 4.0 4.5
INPUT VOLTAGE (V)
5.0
3412A G13
Frequency vs ROSC
5000
4500
VIN = 3.3V
4000
3500
3000
2500
2000
1500
1000
500
0
40 140 240 340 440 540 640 740 840 940
ROSC (kΩ)
3412A G16
Switch On-Resistance vs
Temperature
120
VIN = 3.3V
100
80 PFET
60
NFET
40
20
0
–40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3412A G14
Frequency vs Input Voltage
1060
ROSC = 294k
1050
1040
1030
1020
1010
1000
990
2.5
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
3412A G17
Switch Leakage Current vs
Input Voltage
50
45
40
35
30
25
PFET
20
15
10
5
NFET
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
3412A G15
Frequency vs Temperature
1020
VIN = 3.3V
1015 ROSC = 294k
1010
1005
1000
995
990
985
980
975
970
–40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3412A G18
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LTC3412Awww.DataSheet4U.com
APPLICATIO S I FOR ATIO
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Different core materials and shapes will change the size/cur-
rent and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price verus size requirements and
any radiated field/EMI requirements. New designs for
surface mount inductors are available from Coiltronics,
Coilcraft, Toko, and Sumida.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the trapezoidal
wave current at the source of the top MOSFET. To prevent
large voltage transients from occurring, a low ESR input
capacitor sized for the maximum RMS current should be
used. The maximum RMS current is given by:
IRMS
= IOUT(MAX)
VOUT
VIN
VIN – 1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life which makes it advisable to further
derate the capacitor, or choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be paralleled to meet size or height requirements in the
design. For low input voltage applications, sufficient bulk
input capacitance is needed to minimize transient effects
during output load changes.
The selection of COUT is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later section.
The output ripple, ΔVOUT, is determined by:
VOUT
IL
ESR
+
1
8fCOUT
The output ripple is highest at maximum input voltage
since ΔIL increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and
RMS current handling requirements. Dry tantalum, special
polymer, aluminum electrolytic, and ceramic capacitors are
all available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density but it is important to only
use types that have been surge tested for use in switching
power supplies. Aluminum electrolytic capacitors have
significantly higher ESR, but can be used in cost-sensitive
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capacitors
have excellent low ESR characteristics but can have a high
voltage coefficient and audible piezoelectric effects. The
high Q of ceramic capacitors with trace inductance can
also lead to significant ringing.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the input,
VIN. At best, this ringing can couple to the output and be
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at VIN large enough to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
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| PDF Descargar | [ Datasheet LTC3412A.PDF ] | |
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