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PDF LTC1147-3.3 Data sheet ( Hoja de datos )
| Número de pieza | LTC1147-3.3 | |
| Descripción | High Efficiency Step-Down Switching Regulator Controllers | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1147-3.3
LTC1147-5/LTC1147L
High Efficiency Step-Down
Switching Regulator Controllers
FEATURES
s Very High Efficiency: Over 95% Possible
s Wide VIN Range: 3.5V* to 16V
s Current Mode Operation for Excellent Line and Load
Transient Response
s High Efficiency Maintained Over Three Decades of
Output Current
s Low 160µA Standby Current at Light Loads
s Logic Controlled Micropower Shutdown: IQ < 20µA
s Short-Circuit Protection
s Very Low Dropout Operation: 100% Duty Cycle
s High Efficiency in a Small Amount of Board Space
s Output Can Be Externally Held High in Shutdown
s Available in 8-Pin SO Package
U
APPLICATIO S
s Notebook and Palmtop Computers
s Portable Instruments
s Battery-Operated Digital Devices
s Cellular Telephones
s DC Power Distribution Systems
s GPS Systems
DESCRIPTIO
The LTC®1147 series are step-down switching regulator
controllers featuring automatic Burst ModeTM operation to
maintain high efficiencies at low output currents. These
devices drive an external P-channel power MOSFET at
switching frequencies exceeding 400kHz using a constant
off-time current mode architecture providing constant
ripple current in the inductor.
The operating current level is user-programmable via an
external current sense resistor. Wide input supply range
allows operation from 3.5V* to 14V (16V maximum).
Constant off-time architecture provides low dropout regu-
lation limited by only the RDS(ON) of the external MOSFET
and resistance of the inductor and current sense resistor.
The LTC1147 series incorporates automatic power saving
Burst Mode operation to reduce switching losses when
load currents drop below the level required for continuous
operation. Standby power is reduced to only 2mW at
VIN = 10V (at IOUT = 0). Load currents in Burst Mode
operation are typically 0mA to 300mA.
For applications where even higher efficiency is required,
refer to the LTC1148 data sheet and Application Note 54.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
*LTC1147L and LTC1147L-3.3 only.
TYPICAL APPLICATI
+
1µF
0V = NORMAL
>1.5V = SHUTDOWN
RC
1k
CC
3300pF
CT
470pF
VIN (5.2V TO 14V)
VIN
PDRIVE
LTC1147-5
SHDN
SENSE +
ITH
CT
SENSE –
GND
+ CIN
100µF
P-CHANNEL
Si4431DY L*
50µH
RSENSE**
0.05Ω
VOUT
5V/2A
1000pF
D1
MBRD330
+ COUT
390µF
LT1147 • F01
*COILTRONICS CTX50-2-MP
**KRL SL-1-C1-0R050J
Figure 1. High Efficiency Step-Down Converter
LTC1147-5 Efficiency
100
95
VIN = 6V
90
VIN = 10V
85
80
75
70
0.001
0.01 0.1
LOAD CURRENT (A)
1
LT1147 • TA01
1
1 page  
LTC1147-3.3
LTC1147-5/LTC1147L
PI FU CTIO S
VIN (Pin 1): Main Supply Pin. Must be closely decoupled
to ground Pin 7.
CT (Pin 2): External capacitor CT from Pin 2 to ground sets
the operating frequency. The actual frequency is also
dependent upon the input voltage.
ITH (Pin 3): Gain Amplifier Decoupling Point. The current
comparator threshold increases with the Pin 3 voltage.
SENSE – (Pin 4): Connects to internal resistive divider
which sets the output voltage. Pin 4 is also the (–) input for
the current comparator.
SENSE + (Pin 5): The (+) input to the current comparator.
A built-in offset between Pins 4 and 5 in conjunction with
RSENSE sets the current trip threshold.
SHDN/VFB (Pin 6): When grounded, the fixed output
versions of the LTC1147 family operate normally. Pulling
Pin 6 high holds the P-channel MOSFET off and puts the
LTC1147 in micropower shutdown mode. Requires CMOS
logic signal with tr, tf < 1µs. Do not leave this pin floating.
On the LTC1147L this pin serves as the feedback pin from
an external resistive divider used to set the output voltage.
GND (Pin 7): Two independent ground lines must be
routed separately to: 1) the (–) terminal of COUT, and 2) the
cathode of the Schottky diode and (–) terminal of CIN.
PDRIVE (Pin 8): High current drive for the P-channel
MOSFET. Voltage swing at this pin is from VIN to ground.
UU
W
FU CTIO AL DIAGRA Pin 6 Connection Shown For LTC1147-3.3 and LTC1147-5; Changes Create LTC1147L.
1 VIN
8 PDRIVE
7 GND
SENSE+
5
SENSE –
4
VFB
6
SLEEP
V
+
S
–
VTH2
VTH1
R
Q
S
–
T
+
OFF-TIME
VIN
2
CT
CONTROL
SENSE –
25mV TO 150mV
C
–+
ITH 3
13k
VOS
G
1.25V
SHDN 6
REFERENCE
5pF
100k
LTC1147 • FD
5
5 Page 
LTC1147-3.3
LTC1147-5/LTC1147L
APPLICATIO S I FOR ATIO
where L1, L2, etc., are the individual losses as a percent-
age of input power. (For high efficiency circuits only small
errors are incurred by expressing losses as a percentage
of output power.)
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1147 circuits: 1) LTC1147 DC bias current,
2) MOSFET gate charge current, 3) I2R losses, and 4)
voltage drop of the Schottky diode.
1. The DC supply current is the current which flows into
VIN (Pin 1) less the gate charge current. For VIN = 10V
the LTC1147 series DC supply current is 160µA for no
load, and increases proportionally with load up to a
constant 1.6mA after the LTC1147 series has entered
continuous mode. Because the DC bias current is
drawn from VIN, the resulting loss increases with
input voltage. For VIN = 10V the DC bias losses are
generally less than 1% for load currents over 30mA.
However, at very low load currents the DC bias current
accounts for nearly all of the loss.
2. MOSFET gate charge current results from switching
the gate capacitance of the power MOSFET. Each time
a MOSFET gate is switched from low to high to low
again, a packet of charge dQ moves from VIN to
ground. The resulting dQ/dt is a current out of VIN
which is typically much larger than the DC supply
current. In continuous mode, IGATECHG = f(QP). The
typical gate charge for a 0.135Ω P-channel power
MOSFET is 40nC. This results in IGATECHG = 4mA in
100kHz continuous operation for a 2% to 3% typical
midcurrent loss with VIN = 10V.
Note that the gate charge loss increases directly with
both input voltage and operating frequency. This is the
principal reason why the highest efficiency circuits
operate at moderate frequencies. Furthermore, it ar-
gues against using a larger MOSFET than necessary to
control I2R losses, since overkill can cost efficiency as
well as money!
3. I2R losses are easily predicted from the DC resis-
tances of the MOSFET, inductor and current shunt. In
continuous mode the average output current flows
through L and RSENSE, but is “chopped” between the
P-channel and Schottky diode. The MOSFET RDS(ON)
multiplied by the P-channel duty cycle can be summed
with the resistances of L and RSENSE to obtain I2R
losses. For example, if RDS(ON) = 0.1Ω, RL = 0.15Ω,
and RSENSE = 0.05Ω, then the total resistance is 0.3Ω
at VIN ≈ 2VOUT. This results in losses ranging from 3%
to 10% as the output current increases from 0.5A to
2A. I2R losses cause the efficiency to roll off at high
output currents.
4. The Schottky diode is a major source of power loss at
high currents and gets worse at high input voltages.
The diode loss is calculated by multiplying the forward
voltage drop times the Schottky diode duty cycle
multiplied by the load current. For example, assuming
a duty cycle of 50% with a Schottky diode forward
voltage drop of 0.4V, the loss increases from 0.5% to
8% as the load current increases from 0.5A to 2A.
Figure 5 shows how the efficiency losses in a typical
LTC1147 series regulator end up being apportioned.
The gate charge loss is responsible for the majority of
the efficiency lost in the midcurrent region. If Burst
Mode operation was not employed at low currents,
the gate charge loss alone would cause efficiency to
drop to unacceptable levels. With Burst Mode opera-
tion, the DC supply current represents the lone (and
unavoidable) loss component which continues to
become a higher percentage as output current is
reduced. As expected, the I2R losses and Schottky
diode loss dominate at high load currents.
100
GATE CHARGE
95
LTC1147 IQ
90
I2R
SCHOTTKY
DIODE
85
80
0.01
0.03 0.1 0.3
1
OUTPUT CURRENT (A)
3
LTC1147 • F05
Figure 5. Efficiency Loss
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC1147-3.3.PDF ] | |
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