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PDF LTC3700 Data sheet ( Hoja de datos )
| Número de pieza | LTC3700 | |
| Descripción | Constant Frequency Step-Down DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3700
Constant Frequency
Step-Down DC/DC Controller
with LDO Regulator
FEATURES
DESCRIPTIO
s Dual Output Regulator in Tiny 10-Pin MSOP
s High Efficiency: Up to 94%
s Wide VIN Range: 2.65V to 9.8V
s Constant Frequency 550kHz Operation
s 150mA LDO Regulator with Current Limit and
Thermal Shutdown Protection
s High Output Currents Easily Achieved
s Burst Mode® Operation at Light Load
s Low Dropout: 100% Duty Cycle
s Current Mode Operation for Excellent Line and Load
Transient Response
s 0.8V Reference Allows Low Output Voltages
s Low Quiescent Current: 260µA Total
s Shutdown Mode Draws Only 10µA Supply Current
s Common Power Good Output for Both Supplies
U
APPLICATIO S
s Notebook Computers
s Portable Instruments
s One or Two Li-Ion Battery-Powered Applications
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
The LTC®3700 is a constant frequency current mode step-
down (buck) DC/DC controller with excellent AC and DC
load and line regulation. The on-chip 150mA low dropout
(LDO) linear regulator can be powered from the buck
controller’s input supply, its own independent input supply
or the buck regulator’s output. The buck controller incor-
porates an undervoltage lockout feature that shuts down the
controller when the input voltage falls below 2.1V.
The buck regulator provides a ±2.5% output voltage accu-
racy. It consumes only 210µA of quiescent current in nor-
mal operation with the LDO consuming an additional 50µA.
In shutdown, a mere 10µA (combined) is consumed.
For applications where efficiency is a prime consideration,
the buck controller is configured for Burst Mode operation
which enhances efficiency at low output current. To fur-
ther maximize the life of a battery source, the external
P-channel MOSFET is turned on continuously in dropout
(100% duty cycle). High constant operating frequency of
550kHz allows the use of a small external inductor.
The LDO is protected by both current limit and thermal
shutdown circuits.
The LTC3700 is available in a tiny 10-pin MSOP.
TYPICAL APPLICATIO
VIN1
5V
VOUT1
1.8V
AT 1A
C2 +
47µF
6V
C1
10µF
10V
R1
0.068Ω
VIN
SENSE–
VIN2
LDO
L1 M1
10µH
PGATE
VFB2
100k D1 LTC3700
80.6k
10k
220pF
VFB PGOOD
ITH/RUN GND
VIN2
C3 3.3V
10µF
10V
VOUT2
2.5V AT
169k C4 150mA
2.2µF
78.7k
16V
C1, C3: TAIYO YUDEN EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
C4: MURATA GRM42-6X7R225K016AL
D1: MOTOROLA MBRM120T3
L1: COILTRONICS UP1B-100
M1: Si3443DV
R1: DALE 0.25W
3700 F01
Figure 1. High Efficiency 5V to 1.8V/1A Buck with 3.3V to 2.5V/150mA LDO
Buck Efficiency vs Load Current
90
VOUT = 1.8V
86 RSENSE = 0.068Ω
82
VIN = 3.3V
78
74 VIN = 5V
70 VIN = 4.2V
66
62
58
54
50
1
10 100
LOAD CURRENT (mA)
1000
3700 F01a
3700f
1
1 page  
LTC3700
TYPICAL PERFOR A CE CHARACTERISTICS
LDO REGULATOR
VFB2 Voltage vs Temperature
850
840
VIN2 = 4.2V
LDO = VFB2
830
820
810 ILOAD = 10µA
ILOAD = 1mA
800
790 ILOAD = 10mA
780
ILOAD = 100mA
770
760
750
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
3700 G06
LDO Line Regulation (VFB2
Voltage vs Supply)
850
840
TA = 25°C
LDO = VFB2
830
820
810 ILOAD = 10µA
ILOAD = 1mA
800
790
ILOAD = 10mA
780
ILOAD = 100mA
770
760
750
2.4 2.85 3.3 3.75 4.2 4.65 5.1 5.55 6
VIN2 INPUT VOLTAGE (V)
3700 G07
LDO Pass FET RON vs Input
Voltage
4.0
3.7
VIN = 0
ILDO = 100mA
3.4 TA = 25°C
3.1
2.8
2.5
2.2
1.9
1.6
1.3
1.0
2 2.5 3 3.5 4 4.5 5 5.5 6
VIN2 INPUT VOLTAGE (V)
3700 G08
PGOOD RON vs Input Voltage
300
270 VIN2 = 0V
VPGOOD = 100mV
240 TA = 25°C
210
180
150
120
90
60
30
0
2 3 4 5 6 7 8 9 10
VIN INPUT VOLTAGE (V)
3700 G09
LDO Supply Current
vs Input Voltage
120
LDO = VFB2
110 ILDO = 10µA
100 TA = 25°C
VIN = 0V
90
80
70
60
VIN = 9.8V
50
40
30
20
2 2.5 3 3.5 4 4.5 5 5.5
VIN2 INPUT VOLTAGE (V)
6
3700 G11
Load Transient Response
150
ILDO (mA) 100
50mA/DIV 50
0
∆VLDO
20mV/DIV
AC COUPLED
0
TA = 25°C
VIN2 = 3.3V
VLDO = 2.5V
CLDO = 10µF
20µs/DIV
3700 G12
3700f
5
5 Page 
LTC3700
APPLICATIONS INFORMATION
In Burst Mode operation on the LTC3700, the ripple
current is normally set such that the inductor current is
continuous during the burst periods. Therefore, the peak-
to-peak ripple current must not exceed:
IRIPPLE≤
0.03
RSENSE
This implies a minimum inductance of:
LMIN
=
VIN − VOUT
f
0.03
RSENSE
VOUT + VD
VIN + VD
(Use VIN(MAX) = VIN)
A smaller value than LMIN could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ® cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires
more turns of wire and therefore copper losses will in-
crease. Ferrite designs have very low core losses and are
preferred 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 cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mµ. Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
not increase the height significantly are available.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC3700. The main selection criteria for
the power MOSFET are the threshold voltage VGS(TH) and
the “on” resistance RDS(ON), reverse transfer capacitance
CRSS and total gate charge.
Since the LTC3700 is designed for operation down to low
input voltages, a sublogic level threshold MOSFET (RDS(ON)
guaranteed at VGS = 2.5V) is required for applications that
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the buck is less than the
absolute maximum VGS rating, typically 8V.
The required minimum RDS(ON) of the MOSFET is gov-
erned by its allowable power dissipation. For applications
that may operate the LTC3700 in dropout, i.e., 100% duty
cycle, at its worst case the required RDS(ON) is given by:
( ) ( )RDS(ON)
=
DC=100%
PP
IOUT(MAX) 2 1+ δp
where PP is the allowable power dissipation and δp is the
temperature dependency of RDS(ON). (1 + δp) is generally
given for a MOSFET in the form of a normalized RDS(ON) vs
temperature curve, but δp = 0.005/°C can be used as an
approximation for low voltage MOSFETs.
Kool Mµ is a registered trademark of Magnetics, Inc.
3700f
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3700.PDF ] | |
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