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PDF LTC1265 Data sheet ( Hoja de datos )
| Número de pieza | LTC1265 | |
| Descripción | 1.2A/ High Efficiency Step-Down DC/DC Converter | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1265/LTC1265-3.3/LTC1265-5
1.2A, High Efficiency
Step-Down DC/DC Converter
FEATURES
s High Efficiency: Up to 95%
s Current Mode Operation for Excellent Line and Load
Transient Response
s Internal 0.3Ω Power Switch (VIN = 10V)
s Short-Circuit Protection
s Low Dropout Operation: 100% Duty Cycle
s Low-Battery Detector
s Low 160µA Standby Current at Light Loads
s Active-High Micropower Shutdown: IQ < 15µA
s Peak Inductor Current Independent of Inductor Value
s Available in 14-pin SO Package
U
APPLICATIO S
s 5V to 3.3V Conversion
s Distributed Power Systems
s Step-Down Converters
s Inverting Converters
s Memory Backup Supply
s Portable Instruments
s Battery-Powered Equipment
s Cellular Telephones
DESCRIPTIO
The LTC®1265 is a monolithic step-down current mode
DC/DC converter featuring Burst Mode TM operation at low
output current. The LTC1265 incorporates a 0.3Ω switch
(VIN =10V) allowing up to 1.2A of output current.
Under no load condition, the converter draws only 160µA.
In shutdown it typically draws a mere 5µA making this
converter ideal for current sensitive applications. In drop-
out the internal P-channel MOSFET switch is turned on
continuously maximizing the life of the battery source. The
LTC1265 incorporates automatic power saving Burst Mode
operation to reduce gate charge losses when the load
currents drop below the level required for continuous
operation.
The inductor current is user-programmable via an external
current sense resistor. Operation up to 700kHz permits
the use of small surface mount inductors and capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN
5.4V TO
12V
+ CIN†††
68µF
20V
0.1µF PWR VIN
SHDN
VIN
SW
1k LTC1265-5
ITH PGND
3900pF
130pF
CT
SENSE+
L1*
33µH
D1†
RSENSE**
0.1Ω
+
VOUT
5V
1A
COUT††
220µF
10V
1000pF
SENSE–
SGND
* COILTRONICS CTX33-4
** IRC LRC2010-01-R100-J
† MBRS130LT3
†† AVX TPSE227K010
††† AVX TPSE686K020
LTC1265-FO1
Figure 1. High Efficiency Step-Down Converter
100
95
90
85
80
75
70
0.01
LTC1265-5 Efficiency
VIN = 6V
VIN = 9V
VIN = 12V
L = 33µH
VOUT = 5V
RSENSE = 0.1Ω
CT = 130pF
0.10
LOAD CURRENT (A)
1.00
LTC1265 TA01
1
1 page  
LTC1265/LTC1265-3.3/LTC1265-5
W
FUNCTIONAL DIAGRA (Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)
1, 13 PWR VIN
12 PGND
14 SW
V
SENSE+
8
SENSE–
7
VFB
9 ADJUSTABLE
VERSION
SLEEP
S
VTH2
5 CT
R
Q
S
VTH1 –
T
+
OFF-TIME
CONTROL
C 25mV TO 150mV
ITH 6
13k
2 VIN
SENSE–
VFB
3 LB0UT
A3
11 SGND
5pF
VOS
G
100k
REFERENCE
10
SHDN
4 LBIN
1265 FD
U
OPERATION (Refer to Functional Diagram)
The LTC1265 uses a constant off-time architecture to
switch its internal P-channel power MOSFET. The off time
is set by an external timing capacitor at CT (Pin 5). The
operating frequency is then determined by the off time and
the difference between VIN and VOUT.
The output voltage is set by an internal resistive divider
(LTC1265-3.3 and LTC1265-5) connected to SENSE–
(Pin 7) or an external divider returned to VFB (Pin 9 for
LTC1265). A voltage comparator V, and a gain block G,
compare the divided output voltage with a reference
voltage of 1.25V.
To optimize efficiency, the LTC1265 automatically switches
between continuous and Burst Mode operation. The volt-
age comparator is the primary control element when the
device is in Burst Mode operation, while the gain block
controls the output voltage in continuous mode.
When the load is heavy, the LTC1265 is in continuous
operation. During the switch ON time, current comparator
C monitors the voltage between Pins 7 and 8 connected
across an external shunt in series with the inductor. When
the voltage across the shunt reaches the comparator’s
threshold value, its output signal will change state, setting
the flip flop and turning the internal P-channel MOSFET off.
The timing capacitor connected to Pin 5 is now allowed to
discharge at a rate determined by the off-time controller.
When the voltage on the timing capacitor has discharged
past VTH1, comparator T trips, sets the flip flop and causes
the switch to turn on. Also, the timing capacitor is re-
charged. The inductor current will again ramp up until the
current comparator C trips. The cycle then repeats.
When the load current increases, the output voltage de-
creases slightly. This causes the output of the gain stage
(Pin 6) to increase the current comparator threshold, thus
tracking the load current.
When the load is relatively light, the LTC1265 automati-
cally goes into Burst Mode operation. The current loop is
interrupted when the output voltage exceeds the desired
regulated value. The hysteretic voltage comparator V trips
when VOUT is above the desired output voltage, shutting
off the switch and causing the capacitor to discharge. This
5
5 Page 
LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
Troubleshooting Hints
Since efficiency is critical to LTC1265 applications, it is
very important to verify that the circuit is functioning
correctly in both continuous and Burst Mode operation. As
the LTC1265 is highly tolerant of poor layout, the output
voltage will still be regulated. Therefore, monitoring the
output voltage will not tell you whether you have a good or
bad layout. The waveform to monitor is the voltage on the
timing capacitor Pin 5.
In continuous mode the voltage on the CT pin is a sawtooth
with approximately 0.9VP-P swing. This voltage should
never dip below 2V as shown in Figure 6a.
When the load currents are low (ILOAD < IBURST) Burst
Mode operation occurs. The voltage on CT pin now falls to
ground for periods of time as shown in Figure 6b. During
this time the LTC1265 is in sleep mode with quiescent
current reduced to 160µA.
The inductor current should also be monitored. If the
circuit is poorly decoupled, the peak inductor current will
be haphazard as in Figure 7a. A well decoupled LTC1265
has a clean inductor current as in Figure 7b.
SLEEP MODE
3.3V 3.3V
2.4V 2.4V
0V
TIME
(a) CONTINUOUS MODE OPERATION
LTC1265 F06a
0V
Figure 6. CT Waveforms
TIME
(b) Burst Mode OPERATION
LTC1265 F06b
(a) POORLY DECOUPLED LTC1265
Figure 7. Inductor Waveforms
(b) WELL DECOUPLED LTC1265
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC1265.PDF ] | |
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