LTC1265/LTC1265-3.3/LTC1265-5
1.2A, High Efficiency
Step-Down DC/DC Converter
FEATURES
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DESCRIPTIO
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High Efficiency: Up to 95%
Current Mode Operation for Excellent Line and Load
Transient Response
Internal 0.3
Ω
Power Switch (V
IN
= 10V)
Short-Circuit Protection
Low Dropout Operation: 100% Duty Cycle
Low-Battery Detector
Low 160µA Standby Current at Light Loads
Active-High Micropower Shutdown: I
Q
< 15µA
Peak Inductor Current Independent of Inductor Value
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 0.3Ω switch
(V
IN
=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.
APPLICATIO S
s
s
s
s
s
s
s
s
5V to 3.3V Conversion
Distributed Power Systems
Step-Down Converters
Inverting Converters
Memory Backup Supply
Portable Instruments
Battery-Powered Equipment
Cellular Telephones
TYPICAL APPLICATIO
V
IN
5.4V TO
12V
+
C
IN
†††
68µF
20V
0.1µF
PWR V
IN
SHDN
V
IN
SW
L1*
33µH
D1
†
R
SENSE
**
0.1Ω
1k
LTC1265-5
I
TH
C
T
PGND
SENSE
+
V
OUT
5V
1A
EFFICIENCY (%)
+
C
OUT
220µF
10V
††
3900pF
130pF
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
U
LTC1265-5 Efficiency
100
95
90
V
IN
= 12V
85
80
75
70
0.01
L = 33µH
V
OUT
= 5V
R
SENSE
= 0.1Ω
C
T
= 130pF
0.10
LOAD CURRENT (A)
1.00
LTC1265 TA01
U
U
V
IN
= 6V
V
IN
= 9V
1
LTC1265/LTC1265-3.3/LTC1265-5
ABSOLUTE
MAXIMUM
RATINGS
(Voltages Refer to GND Pin) (Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
PWR V
IN
1
V
IN
2
LB
OUT
3
LB
IN
4
C
T
5
I
TH
6
SENSE
–
7
14 SW
13 PWR V
IN
12 PGND
11 SGND
10 SHDN
9
8
N/C (V
FB
*)
SENSE
+
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
ORDER
PART NUMBER
LTC1265CS
LTC1265CS-5
LTC1265CS-3.3
LTC1265IS
S PACKAGE
14-LEAD PLASTIC SO
*ADJUSTABLE OUTPUT VERSION
T
JMAX
= 125°C,
θ
JA
= 110°C/W
Consult factory for Military grade parts.
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.
SYMBOL
I
FB
V
FB
V
OUT
∆V
OUT
PARAMETER
Feedback Current into Pin 9
Feedback Voltage
Regulator Output Voltage
Output Voltage Line Regulation
Output Voltage Load Regulation
Burst Mode Operation Output Ripple
I
Q
Input DC Supply Current (Note 3)
CONDITIONS
LTC1265
LTC1265C
V
IN
= 9V, LTC1265I
LTC1265-3.3: I
LOAD
= 800mA
LTC1265-5: I
LOAD
= 800mA
V
IN
= 6.5V to 10V, I
LOAD
= 800mA
LTC1265-3.3: 10mA < I
LOAD
< 800mA
LTC1265-5: 10mA < I
LOAD
< 800mA
I
LOAD
= 0mA
Active Mode: 3.5V < V
IN
< 10V
Sleep Mode: 3.5V < V
IN
< 10V
Sleep Mode: 5V < V
IN
< 10V (LTC1265-5)
Shutdown: V
SHDN
= V
IN
, 3.5V < V
IN
< 10V
1.15
V
LBOUT
= 0.4V, V
LBIN
= 0V
V
LBOUT
= 5V, V
LBIN
= 10V
LTC1265: V
SENSE–
= 5V, V
9
= V
OUT
/4 + 25mV (Forced)
V
SENSE–
= 5V, V
9
= V
OUT
/4 – 25mV (Forced)
LTC1265-3.3: V
SENSE–
= V
OUT
+ 100mV (Forced)
V
SENSE–
= V
OUT
– 100mV (Forced)
LTC1265-5: V
SENSE–
= V
OUT
+ 100mV (Forced)
V
SENSE–
= V
OUT
– 100mV (Forced)
LTC1265C
LTC1265I
V
OUT
in Regulation, V
SENSE–
= V
OUT
V
OUT
= 0V
C
T
= 390pF, I
LOAD
= 800mA (LTC1265C)
C
T
= 390pF, I
LOAD
= 800mA (LTC1265I)
q
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
1.22
1.20
3.22
4.9
–40
TYP
0.2
1.25
1.25
3.3
5
0
40
60
50
1.8
160
160
5
1.25
1.0
25
150
25
150
25
150
0.3
0.3
MAX
1
1.28
1.30
3.40
5.2
40
65
100
2.4
230
230
15
1.35
0.5
1.5
1.0
180
180
180
0.60
0.70
100
10
6
7
UNITS
µA
V
V
V
V
mV
mV
mV
mV
P-P
mA
µA
µA
µA
V
µA
mA
µA
mV
mV
mV
mV
mV
mV
Ω
Ω
µA
µA
µs
µs
V
LBTRIP
I
LBIN
I
LBOUT
V
8
– V
7
Low-Battery Trip Point
Current into Pin 4
Current Sunk by Pin 3
Current Sense Threshold Voltage
0.5
130
130
130
R
ON
I
5
t
OFF
ON Resistance of Switch
C
T
Pin Discharge Current
Switch Off Time (Note 4)
40
4
3.5
60
2
5
5
2
U
W
U
U
W W
W
LTC1265/LTC1265-3.3/LTC1265-5
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.
SYMBOL
V
IH
V
IL
I
10
PARAMETER
Shutdown Pin High
Shutdown Pin Low
Shutdown Pin Input Current
CONDITIONS
Min Voltage at Pin 10 for Device to be in Shutdown
Max Voltage at Pin 10 for Device to be Active
V
SHDN
= 8V
MIN
1.2
0.6
0.5
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%.
TYP
MAX
UNITS
V
V
µA
ELECTRICAL CHARACTERISTICS
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 PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100
95
V
IN
= 5V
EFFICIENCY (%)
EFFICIENCY (%)
V
IN
= 9V
85
80
75
70
0.01
V
IN
= 12V
LTC1265-3.3
V
OUT
= 3.3V
R
SENSE
= 0.1Ω
C
T
= 130pF
COIL = CTX33-4
0.10
LOAD CURRENT (A)
1.00
1265 G01
92
90
88
86
84
82
80
4
EFFICIENCY (%)
90
Operating Frequency
vs (V
IN
– V
OUT
)
1.2
1.0
0°C
70°C
RDS
(ON)
(Ω)
NORMALIZED FREQUENCY
LEAKAGE CURRENT (nA)
0.8
0.6
0.4
0.2
0
0
1
2
5 6 7 8
(V
IN –
V
OUT
) VOLTAGE (V)
4
3
9
10
U W
25°C
1265 G04
Efficiency vs Input Voltage
(V
OUT
= 5V)
100
98
96
94
I
LOAD
= 250mA
I
LOAD
= 800mA
100
98
96
94
92
90
88
86
LTC1265-5
R
SENSE
= 0.1Ω
C
T
= 130pF
COIL = CTX33-4
5
6
7
9 10 11
8
INPUT VOLTAGE (V)
12
13
84
82
80
Efficiency vs Input Voltage
(V
OUT
= 3.3V)
LTC1265-3.3
R
SENSE
= 0.1Ω
C
T
= 130pF
COIL = CTX33-4
I
LOAD
= 250mA
I
LOAD
= 800mA
4
5
6
7
9 10 11
8
INPUT VOLTAGE (V)
12
13
1265 G02
LTC1265 G03
Switch Resistance
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
3
4
5
6 7 8 9 10 11 12 13
INPUT VOLTAGE (V)
1265 G05
Switch Leakage Current
300
270
240
V
IN
= 12V
T
J
= 125°C
T
J
= 70°C
T
J
= 25°C
T
J
= 0°C
210
180
150
120
90
60
30
0
0
20
60
40
TEMPERATURE (°C)
80
100
1265 G06
3
LTC1265/LTC1265-3.3/LTC1265-5
TYPICAL PERFORMANCE CHARACTERISTICS
DC Supply Current
2.1
1.8
SUPPLY CURRENT (mA)
1.5
1.2
0.9
0.6
0.3
0
0
2
10
4
8
6
INPUT VOLTAGE (V)
12
14
1265 G07
DOES NOT INCLUDE
GATE CHARGE
SUPPLY CURRENT (µA)
ACTIVE MODE
6
5
4
3
2
1
0
3
4
5
6 7 8 9 10 11 12 13
INPUT VOLTAGE (V)
1265 G08
SWITCHING CURRENT (mA)
SLEEP MODE
PIN FUNCTIONS
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.
4
U W
Supply Current in Shutdown
8
7
5.5
Gate Charge Losses
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
200
400
600
FREQUENCY (kHz)
800
1000
1265 G09
SHUTDOWN = 3V
T
A
= 25C
V
IN
= 12V
V
IN
= 9V
V
IN
= 6V
U
U
U
LTC1265/LTC1265-3.3/LTC1265-5
FUNCTIONAL DIAGRA
V
SLEEP
R
Q
S
V
TH2
V
TH1
C
25mV TO 150mV
5pF
V
OS
13k
T
+
5
C
T
OFF-TIME
CONTROL
2
V
IN
3
LB
0UT
11
SGND
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 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
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
–
SENSE
–
V
FB
A3
+
–
I
TH
6
G
100k
–
S
+
–
+
–
+
W
U
U
–
+
U
(Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)
1, 13
PWR V
IN
SENSE
+
8
14
SW
12
PGND
9
V
FB
ADJUSTABLE
VERSION
SENSE
–
7
REFERENCE
10
SHDN
4
LB
IN
1265 FD
5
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