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PDF LTC3772B Data sheet ( Hoja de datos )
| Número de pieza | LTC3772B | |
| Descripción | Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3772B
Micropower No RSENSE
Constant Frequency Step-Down
DC/DC Controller
FEATURES
■ No Current Sense Resistor Required
■ High Output Currents Easily Achieved
■ Internal Soft-Start Ramps VOUT
■ Wide VIN Range: 2.75V to 9.8V
■ Low Dropout: 100% Duty Cycle
■ Constant Frequency 550kHz Operation
■ Low Ripple Pulse Skipping Operation at Light Load
■ Output Voltage as Low as 0.8V
■ ±1.5% Voltage Reference Accuracy
■ Current Mode Operation for Excellent Line and Load
Transient Response
■ Only 8µA Supply Current in Shutdown
■ Low Profile 8-Lead SOT-23 (1mm) and
(3mm × 2mm) DFN (0.75mm) Packages
U
APPLICATIO S
■ 1- or 2-Cell Li-Ion Battery-Powered Applications
■ Wireless Devices
■ Portable Computers
■ Distributed Power Systems
DESCRIPTIO
The LTC®3772B is a constant frequency current mode
step-down DC/DC controller in a low profile 8-lead SOT-23
(ThinSOTTM) and a 3mm × 2mm DFN package. The No
RSENSETM architecture eliminates the need for a current
sense resistor, improving efficiency and saving board
space.
The LTC3772B automatically switches into pulse skipping
operation at light loads. It consumes only 200µA of quies-
cent current under a no-load condition.
The LTC3772B incorporates an undervoltage lockout fea-
ture that shuts down the device when the input voltage
falls below 2V. To maximize the runtime from a battery
source, the external P-channel MOSFET is turned on
continuously in dropout (100% duty cycle). High switch-
ing frequency of 550kHz allows the use of a small inductor
and capacitors. An internal soft-start smoothly ramps the
output voltage from zero to its regulation point.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
ThinSOT and No RSENSE are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by
U.S. Patents including 5731694, 6127815.
www.DataSheet4U.com
TYPICAL APPLICATIO
550kHz Micropower Step-Down DC/DC Converter
680pF
20k
82.5k
ITH/RUN VIN
LTC3772B
GND PGATE
VFB SW
22pF 174k
3.3µH
VIN
2.75V TO 9.8V
10µF
47µF
VOUT
2.5V
2A
3772B TA01
Efficiency and Power Loss vs Load Current
(Figure 5 Circuit)
100 10
90
80
EFFICIENCY
70
1
60
50 0.1
40
POWER LOSS
30
0.01
20
10 VIN = 3.3V
VIN = 5V
0 0.001
1 10 100 1000 10000
LOAD CURRENT (mA)
3772B TA01b
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1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC3772B
Maximum Current Sense
Threshold vs Temperature
300
250
IPRG = VIN
200
150 IPRG = FLOAT
100
IPRG = GND
50
0
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3772B G13
Soft-Start Time vs Temperature
1100
1000
900
800
700
600
500
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3772B G14
Foldback Frequency
vs Temperature
230 VFB = 0V
220
210
200
190
180
170
160
150
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3772B G15
Efficiency vs Load Current
100
VIN = 3.3V
90
VIN = 4.2V
80 VIN = 7V
VIN = 5V
70
Efficiency vs Load Current
100
90 VOUT = 3.3V
VOUT = 2.5V
80
VOUT = 1.8V
70
60 60
50
40
www.DataSheet4U.com 1
VOUT = 2.5V
FIGURE 5 CIRCUIT
10 100 1000
LOAD CURRENT (mA)
10000
3772B G16
Start-Up
50
40
10
VIN = 5V
FIGURE 5 CIRCUIT
100 1000
LOAD CURRENT (mA)
10000
3772B G17
Load Step
VOUT
1V/DIV
ITH/RUN
1V/DIV
INDUCTOR
CURRENT
2A/DIV
VIN = 5V
500µs/DIV
VOUT = 2.5V
RLOAD = 1.5Ω
FIGURE 5 CIRCUIT
3772B G18
VOUT
100mV/DIV
(AC)
IL
2A/DIV
ILOAD
2A/DIV
VIN = 5V
20µs/DIV
VOUT = 2.5V
ILOAD = 100mA TO 1.5A
FIGURE 5 CIRCUIT
3772B G19
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5 Page 
LTC3772B
APPLICATIO S I FOR ATIO
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.
In applications where the maximum duty cycle is less than
100% and the LTC3772B is in continuous mode, the RDS(ON)
is governed by:
RDS(ON)
≅
PP
(DC)IOUT2 (1+
δP)
where DC is the maximum operating duty cycle of the
LTC3772B.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor ripple
current. However, this is at the expense of efficiency due
wwwt.DoaatanSihnecert4eUa.sceomin MOSFET gate charge losses.
The inductance value also has a direct effect on ripple
current. In normal operation, the ripple current, IRIPPLE, de-
creases with higher inductance or frequency and increases
with higher VIN or as VOUT approaches 1/2 VIN. The
inductor’s peak-to-peak ripple current is given by:
IRIPPLE
=
VIN
− VOUT
f(L)
⎛
⎝⎜
VOUT + VD
VIN + VD
⎞
⎠⎟
where f is the operating frequency. VD is the forward volt-
age drop of the catch diode, 0.5V typical. Accepting larger
values of IRIPPLE allows the use of low inductances, but re-
sults in higher output voltage ripple and greater core losses.
A reasonable starting point for setting ripple current is
IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE
occurs at the maximum input voltage.
Inductor Core Selection
Once the inductance value is determined, the type of induc-
tor must be selected. 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 increase.
Ferrite designs have very low core loss 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 in-
ductor ripple current and consequent output voltage ripple.
Do not allow the core to saturate!
Different core materials and shapes will change the size/
current 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 vs size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
Output Diode Selection
The catch diode carries load current during the off-time. The
average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the diode
conducts most of the time. As VIN approaches VOUT the
diode conducts only a small fraction of the time. The most
stressful condition for the diode is when the output is short-
circuited. Under this condition the diode must safely handle
IPEAK at close to 100% duty cycle. Therefore, it is impor-
tant to adequately specify the diode peak current and av-
erage power dissipation so as not to exceed the diode
ratings.
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11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC3772B.PDF ] | |
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