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PDF LTC3412 Data sheet ( Hoja de datos )
| Número de pieza | LTC3412 | |
| Descripción | Monolithic Synchronous Step-Down Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3412
2.5A, 4MHz, Monolithic
Synchronous Step-Down Regulator
FEATURES
DESCRIPTIO
■ High Efficiency: Up to 95%
■ 2.5A Output Current
■ Low Quiescent Current: 62μA
■ Low RDS(ON) Internal Switches: 85mΩ
■ Programmable Frequency: 300kHz to 4MHz
■ No Schottky Diode Required
■ ±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 Protection
■ Available in 16-Lead Thermally Enhanced TSSOP
and QFN Packages
U
APPLICATIO S
■ Portable Instruments
■ Battery-Powered Equipment
■ Notebook Computers
■ Distributed Power Systems
■ Cellular Telephones
■ Digital Cameras
The LTC®3412 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 2.625V to 5.5V and provides an
adjustable regulated output voltage from 0.8V to 5V while
delivering up to 2.5A of output current. The internal
synchronous power switch with 85mΩ on-resistance
increases efficiency and eliminates the need for an exter-
nal Schottky diode. Switching frequency is set by an
external resistor or can be sychronized 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 opti-
mized over a wide range of loads and output capacitors.
The LTC3412 can be configured for either Burst Mode®
operation or forced continuous operation. Forced con-
tinuous operation reduces noise and RF interference while
Burst Mode operation provides high efficiency by reduc-
ing 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
requirements of the application. To further maximize
battery life, the P-channel MOSFET is turned on continu-
ously in dropout (100% duty cycle).
, 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.
TYPICAL APPLICATIO
VIN
2.7V TO 5.5V
4.7M
470pF 15k
1000pF
309k
SVIN PVIN
RT PGOOD
SW
LTC3412
RUN/SS
PGND
SGND
ITH
100pF SYNC/MODE VFB
110k 392k
75k
22μF
1μH VOUT
2.5V
100μF 2.5A
3412 F01
Figure 1. 2.5V, 2.5A Step-Down Regulator
Efficiency vs Load Current
100
80
Burst Mode OPERATION
60
FORCED CONTINUOUS
40
20
0
0.001
VIN = 3.3V
VOUT = 2.5V
0.01 0.1
1
LOAD CURRENT (A)
10
3412 G01
3412fb
1
1 page  
LTC3412
TYPICAL PERFOR A CE CHARACTERISTICS
Switch On-Resistance
vs Temperature
120
VIN = 3.3V
110
100
PFET ON-RESISTANCE
90
80
70
60
50 NFET ON-RESISTANCE
40
30
20
–40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3412 G13
Frequency vs Input Voltage
1050
R = 309k
1040
1030
1020
1010
1000
990
2.5
3 3.5 4 4.5 5
INPUT VOLTAGE (V)
5.5
3412 G16
DC Supply Current vs Temperature
350
VIN = 3.3V
300
ACTIVE
250
200
150
100
SLEEP
50
0
–40 –20
0 20 40 60 80 100 120
TEMPERATURE (°C)
3412 G19
Switch Leakage vs Input Voltage
2.5
2.0
1.5
1.0
SYNCHRONOUS SWITCH
0.5
MAIN SWITCH
0
2.5 3 3.5 4 4.5
INPUT VOLTAGE (V)
5 5.5
3412 G14
Switching Frequency
vs Temperature
1010
1008
VIN = 3.3V
1006
1004
1002
1000
998
996
994
992
990
–40 –20 0 20 40 60 80 100 120
TEMPERATURE (°C)
3412 G17
Minimum Peak Inductor Current
vs Burst Clamp Voltage
4000
VIN = 3.3V
3500
3000
2500
2000
1500
1000
500
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
BURST CLAMP VOLTAGE (V)
3412 G20
Frequency vs ROSC
4500
4000
VIN = 3.3V
3500
3000
2500
2000
1500
1000
500
0
50 150 250 350 450 550 650 750 850 950
ROSC (kΩ)
3412 G15
DC Supply Current
vs Input Voltage
350
300
ACTIVE
250
200
150
100
50
2.5
SLEEP
3 3.5 4 4.5 5
INPUT VOLTAGE (V)
5.5
3412 G18
Current Limit vs Input Voltage
6.8
6.6
6.4
6.2
6.0
5.8
5.6
5.4
2.75
3.25 3.75 4.25 4.75
INPUT VOLTAGE (V)
5.25
3412 G21
3412fb
5
5 Page 
LTC3412
APPLICATIO S I FOR ATIO
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.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation:
VOUT = 0.8V ⎛⎝⎜1+ RR21⎞⎠⎟
The resistive divider allows the VFB pin to sense a fraction
of the output voltage as shown in Figure 2.
VOUT
VFB
LTC3412
SGND
3412 F02
R2
R1
Figure 2. Setting the Output Voltage
Burst Clamp Programming
If the voltage on the SYNC/MODE pin is less than VIN by 1V,
Burst Mode operation is enabled. During Burst Mode
operation, the voltage on the SYNC/MODE pin determines
the burst clamp level which sets the minimum peak
inductor current, IBURST, for each switching cycle accord-
ing to the following equation:
IBURST
=
(VBURST
−
0.2V)⎛⎝⎜
3.75A
0.8V
⎞⎠⎟
VBURST is the voltage on the SYNC/MODE pin. IBURST can
be programmed in the range of 0A to 3.75A. For values of
VBURST greater than 1V, IBURST is set at 3.75A. For values
of VBURST less than 0.2V, IBURST is set at 0A. As the output
load current drops, the peak inductor current decreases to
keep the output voltage in regulation. When the output
load current demands a peak inductor current that is less
than IBURST, the burst clamp will force the peak inductor
current to remain equal to IBURST regardless of further
reductions in the load current. Since the average inductor
current is greater than the output load current, the voltage
on the ITH pin will decrease. When the ITH voltage drops to
150mV, sleep mode is enabled in which both power
MOSFETs are shut off along with most of the circuitry to
minimize power consumption. All circuitry is turned back
on and the power MOSFETs begin switching again when
the output voltage drops out of regulation. The value for
IBURST is determined by the desired amount of output
voltage ripple. As the value of IBURST increases, the sleep
period between pulses and the output voltage ripple in-
crease. The burst clamp voltage, VBURST, can be set by a
resistor divider from the VFB pin to the SGND pin as shown
in Figure 1.
Pulse skipping, which is a compromise between low out-
put voltage ripple and efficiency, can be implemented by
connecting the SYNC/MODE pin to ground. This sets IBURST
to 0A. In this condition, the peak inductor current is limited
by the minimum on-time of the current comparator, and
the lowest output voltage ripple is achieved while still op-
erating discontinuously. During very light output loads,
pulse skipping allows only a few switching cycles to be
skipped while maintaining the output voltage in regulation.
Frequency Synchronization
The LTC3412’s internal oscillator can be synchronized to
an external clock signal. During synchronization, the top
MOSFET turn-on is locked to the falling edge of the
external frequency source. The synchronization frequency
range is 300kHz to 4MHz. Synchronization only occurs if
the external frequency is greater than the frequency set by
the external resistor. Because slope compensation is
generated by the oscillator’s RC circuit, the external fre-
quency should be set 25% higher than the frequency set
by the external resistor to ensure that adequate slope
compensation is present.
Soft-Start
The RUN/SS pin provides a means to shut down the
LTC3412 as well as a timer for soft-start. Pulling the
RUN/SS pin below 0.5V places the LTC3412 in a low
quiescent current shutdown state (IQ < 1μA).
3412fb
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3412.PDF ] | |
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