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1
LTC1265/LTC1265-3.3/LTC1265-5
V
IN
5.4V TO
12V
L1*
33
H
R
SENSE
**
0.1
V
OUT
5V
1A
C
OUT
220
F
10V
1000pF
130pF
3900pF
1k
0.1
F
C
IN
68
F
20V
D1
PWR V
IN
V
IN
SW
PGND
SENSE
+
SENSE
SGND
I
TH
C
T
LTC1265-FO1
SHDN
*
**
COILTRONICS CTX33-4
IRC LRC2010-01-R100-J
MBRS130LT3
AVX TPSE227K010
AVX TPSE686K020
LTC1265-5
+
+
Figure 1. High Efficiency Step-Down Converter
1.2A, High Efficiency
Step-Down DC/DC Converter
s
High Efficiency: Up to 95%
s
Current Mode Operation for Excellent Line and Load
Transient Response
s
Internal 0.3
Power Switch (V
IN
= 10V)
s
Short-Circuit Protection
s
Low Dropout Operation: 100% Duty Cycle
s
Low-Battery Detector
s
Low 160
A Standby Current at Light Loads
s
Active-High Micropower Shutdown: I
Q
< 15
A
s
Peak Inductor Current Independent of Inductor Value
s
Available in 14-pin SO Package
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 0.3
switch
(V
IN
=10V) allowing up to 1.2A of output current.
Under no load condition, the converter draws only 160
A.
In shutdown it typically draws a mere 5
A 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.
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
LTC1265-5 Efficiency
Burst Mode is a trademark of Linear Technology Corporation.
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
LOAD CURRENT (A)
0.01
70
EFFICIENCY (%)
75
80
85
90
100
0.10
1.00
LTC1265 TA01
95
V
IN
= 6V
V
IN
= 9V
V
IN
= 12V
L = 33
H
V
OUT
= 5V
R
SENSE
= 0.1
C
T
= 130pF
, LTC and LT are registered trademarks of Linear Technology Corporation.
2
LTC1265/LTC1265-3.3/LTC1265-5
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
FB
Feedback Current into Pin 9
LTC1265
0.2
1
A
V
FB
Feedback Voltage
LTC1265C
q
1.22
1.25
1.28
V
V
IN
= 9V, LTC1265I
q
1.20
1.25
1.30
V
V
OUT
Regulator Output Voltage
LTC1265-3.3: I
LOAD
= 800mA
q
3.22
3.3
3.40
V
LTC1265-5: I
LOAD
= 800mA
q
4.9
5
5.2
V
V
OUT
Output Voltage Line Regulation
V
IN
= 6.5V to 10V, I
LOAD
= 800mA
40
0
40
mV
Output Voltage Load Regulation
LTC1265-3.3: 10mA < I
LOAD
< 800mA
40
65
mV
LTC1265-5: 10mA < I
LOAD
< 800mA
60
100
mV
Burst Mode Operation Output Ripple
I
LOAD
= 0mA
50
mV
P-P
I
Q
Input DC Supply Current (Note 3)
Active Mode: 3.5V < V
IN
< 10V
1.8
2.4
mA
Sleep Mode: 3.5V < V
IN
< 10V
160
230
A
Sleep Mode: 5V < V
IN
< 10V (LTC1265-5)
160
230
A
Shutdown: V
SHDN
= V
IN
, 3.5V < V
IN
< 10V
5
15
A
V
LBTRIP
Low-Battery Trip Point
1.15
1.25
1.35
V
I
LBIN
Current into Pin 4
0.5
A
I
LBOUT
Current Sunk by Pin 3
V
LBOUT
= 0.4V, V
LBIN
= 0V
0.5
1.0
1.5
mA
V
LBOUT
= 5V, V
LBIN
= 10V
1.0
A
V
8
V
7
Current Sense Threshold Voltage
LTC1265: V
SENSE
= 5V, V
9
= V
OUT
/4 + 25mV (Forced)
25
mV
V
SENSE
= 5V, V
9
= V
OUT
/4 25mV (Forced)
130
150
180
mV
LTC1265-3.3: V
SENSE
= V
OUT
+ 100mV (Forced)
25
mV
V
SENSE
= V
OUT
100mV (Forced)
130
150
180
mV
LTC1265-5: V
SENSE
= V
OUT
+ 100mV (Forced)
25
mV
V
SENSE
= V
OUT
100mV (Forced)
130
150
180
mV
R
ON
ON Resistance of Switch
LTC1265C
q
0.3
0.60
LTC1265I
0.3
0.70
I
5
C
T
Pin Discharge Current
V
OUT
in Regulation, V
SENSE
= V
OUT
40
60
100
A
V
OUT
= 0V
2
10
A
t
OFF
Switch Off Time (Note 4)
C
T
= 390pF, I
LOAD
= 800mA (LTC1265C)
q
4
5
6
s
C
T
= 390pF, I
LOAD
= 800mA (LTC1265I)
q
3.5
5
7
s
Input Supply Voltage (Pins 1, 2, 13) .......... 0.3V to 13V
DC Switch Current (Pin 14) .................................... 1.2A
Peak Switch Current (Pin 14) ................................. 1.6A
Switch Voltage (Pin 14) .................................. V
IN
13.0
Operating Temperature Range
LTC1265C ............................................... 0
to 70
C
LTC1265I ........................................ 40
C to 85
C
Junction Temperature (Note 2) ............................. 125
C
Storage Temperature Range .................... 65
to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
(Voltages Refer to GND Pin) (Note 1)
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ORDER
PART NUMBER
LTC1265CS
LTC1265CS-5
LTC1265CS-3.3
LTC1265IS
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 10V, V
SHDN
= 0V, unless otherwise specified.
TOP VIEW
S PACKAGE
14-LEAD PLASTIC SO
*ADJUSTABLE OUTPUT VERSION
T
JMAX
= 125
C,
JA
= 110
C/W
1
2
3
4
5
6
7
14
13
12
11
10
9
8
PWR V
IN
V
IN
LB
OUT
LB
IN
C
T
I
TH
SENSE
SW
PWR V
IN
PGND
SGND
SHDN
N/C (V
FB
*)
SENSE
+
3
LTC1265/LTC1265-3.3/LTC1265-5
Note 3: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 4: In applications where R
SENSE
is placed at ground potential, the
off time increases by approximately 40%.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formulas:
LTC1265CS, LTC1265CS-3.3, LTC1265CS-5:
T
J
= T
A
+ (P
D
110
C/W)
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
LOAD CURRENT (A)
0.01
70
EFFICIENCY (%)
75
80
85
90
100
0.10
1.00
1265 G01
95
V
IN
= 5V
V
IN
= 9V
V
IN
= 12V
LTC1265-3.3
V
OUT
= 3.3V
R
SENSE
= 0.1
C
T
= 130pF
COIL = CTX33-4
INPUT VOLTAGE (V)
4
80
EFFICIENCY (%)
82
86
88
90
100
94
6
8
9
13
LTC1265 G03
84
96
98
92
5
7
10
11
12
I
LOAD
= 250mA
I
LOAD
= 800mA
LTC1265-3.3
R
SENSE
= 0.1
C
T
= 130pF
COIL = CTX33-4
INPUT VOLTAGE (V)
4
80
EFFICIENCY (%)
82
86
88
90
100
94
6
8
9
13
1265 G02
84
96
98
92
5
7
10
11
12
I
LOAD
= 250mA
I
LOAD
= 800mA
LTC1265-5
R
SENSE
= 0.1
C
T
= 130pF
COIL = CTX33-4
Switch Resistance
INPUT VOLTAGE (V)
3
0
RDS
(ON)
(
)
0.1
0.3
0.4
0.5
7
11
13
0.9
1265 G05
0.2
5
9
6
10
12
4
8
0.6
0.7
0.8
T
J
= 70
C
T
J
= 125
C
T
J
= 25
C
T
J
= 0
C
(V
IN
V
OUT
) VOLTAGE (V)
0
NORMALIZED FREQUENCY
0.4
0.8
1.2
1.0
0.6
0.2
2
1
4
6
8
1265 G04
10
0
3
5
7
9
0
C
25
C
70
C
Operating Frequency
vs (V
IN
V
OUT
)
Efficiency vs Input Voltage
(V
OUT
= 3.3V)
Efficiency vs Input Voltage
(V
OUT
= 5V)
Efficiency vs Load Current
Switch Leakage Current
TEMPERATURE (
C)
0
LEAKAGE CURRENT (nA)
180
240
300
270
210
150
90
30
80
1265 G06
120
60
0
20
40
60
100
V
IN
= 12V
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
IH
Shutdown Pin High
Min Voltage at Pin 10 for Device to be in Shutdown
1.2
V
V
IL
Shutdown Pin Low
Max Voltage at Pin 10 for Device to be Active
0.6
V
I
10
Shutdown Pin Input Current
V
SHDN
= 8V
0.5
A
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 10V, V
SHDN
= 0V, unless otherwise specified.
4
LTC1265/LTC1265-3.3/LTC1265-5
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
PWR V
IN
(Pins 1, 13): Supply for the Power MOSFET and
its Driver. Must decouple this pin properly to ground. Must
always tie Pins 1 and 13 together.
V
IN
(Pin 2): Main Supply for All the Control Circuitry in the
LTC1265.
LB
OUT
(Pin 3): Open-Drain Output of the Low-Battery
Comparator. This pin will sink current when Pin 4 (LB
IN
)
goes below 1.25V. During shutdown, this pin is high
impedance.
LB
IN
(Pin 4): The () Input of the Low-Battery Comparator.
The (+) input is connected to a reference voltage of 1.25V.
C
T
(Pin 5): External capacitor C
T
from Pin 5 to ground sets
the switch off time. The operating frequency is dependent
on the input voltage and C
T
.
I
TH
(Pin 6): Feedback Amplifier Decoupling Point. The
current comparator threshold is proportional to Pin 6
voltage.
SENSE
(Pin 7): Connect to the () input of the current
comparator. For LTC1265-3.3 and LTC1265-5, it also
connects to an internal resistive divider which sets the
output voltage.
SENSE
+
(Pin 8): The (+) Pin to the Current Comparator. A
built-in offset between Pins 7 and 8 in conjunction with
R
SENSE
sets the current trip threshold.
N/C,V
FB
(Pin 9): For the LTC1265 adjustable version, this
pin serves as the feedback pin from an external resistive
divider used to set the output voltage. On the LTC1265-3.3
and LTC1265-5 versions, this pin is not used.
SHDN (Pin 10): Pulling this pin HIGH keeps the internal
switch off and puts the LTC1265 in micropower shut-
down. Do not float this pin.
SGND (Pin 11): Small-Signal Ground. Must be routed
separately from other grounds to the () terminal of C
OUT
.
PGND (Pin 12): Switch Driver Ground. Connects to the
() terminal of C
IN
. Anode of the Schottky diode must be
connected close to this pin.
SW (Pin 14): Drain of the P-Channel MOSFET Switch.
Cathode of the Schottky diode must be connected close to
this pin.
PI
N
FU
N
CTIO
N
S
U
U
U
DC Supply Current
Supply Current in Shutdown
Gate Charge Losses
INPUT VOLTAGE (V)
0
0
SUPPLY CURRENT (mA)
0.3
0.6
0.9
2.1
1.5
2
6
8
14
1.8
1.2
4
10
12
DOES NOT INCLUDE
GATE CHARGE
ACTIVE MODE
SLEEP MODE
1265 G07
INPUT VOLTAGE (V)
3
SUPPLY CURRENT (
A)
6
8
11
1265 G08
4
2
0
5
7
9
13
5
7
3
1
10
4
6
8
12
SHUTDOWN = 3V
T
A
= 25C
FREQUENCY (kHz)
0
0
SWITCHING CURRENT (mA)
1.0
2.0
3.0
4.0
200
400
1265 G09
600
5.0
5.5
4.5
3.5
2.5
1.5
0.5
800
1000
V
IN
= 12V
V
IN
= 9V
V
IN
= 6V
5
LTC1265/LTC1265-3.3/LTC1265-5
FU
N
CTIO
N
AL DIAGRA
U
U
W
(Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)
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 C
T
(Pin 5). The
operating frequency is then determined by the off time and
the difference between V
IN
and V
OUT
.
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 V
FB
(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
OPERATIO
N
U
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 V
TH1
, 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 V
OUT
is above the desired output voltage, shutting
off the switch and causing the capacitor to discharge. This
(Refer to Functional Diagram)
PWR V
IN
SENSE
+
SENSE
5pF
100k
LB
IN
SGND
I
TH
V
IN
SENSE
V
FB
V
TH1
V
TH2
SLEEP
PGND
SW
13k
V
OS
V
FB
ADJUSTABLE
VERSION
25mV TO 150mV
OFF-TIME
CONTROL
SHDN
LB
0UT
C
T
+
V
+
C
R
S
Q
+
T
+
S
+
A3
REFERENCE
+
G
1265 FD
5
2
3
6
11
4
10
9
7
8
14
12
1, 13
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