Datasheet LTC1265, LTC1265IS, LTC1265CS, LTC1265-5, LTC1265CS-5 Datasheet (Linear Technology)

FEATURES

LTC1265/LTC1265-3.3/LTC1265-5

1.2A, High Efficiency

Step-Down DC/DC Converter

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DESCRIPTIO

■

High Efficiency: Up to 95%

■

Current Mode Operation for Excellent Line and Load
Transient Response

■

Internal 0.3Ω Power Switch (VIN = 10V)

■

Short-Circuit Protection

■

Low Dropout Operation: 100% Duty Cycle

■

Low-Battery Detector

■

Low 160µA Standby Current at Light Loads

■

Active-High Micropower Shutdown: IQ < 15µA

■

Peak Inductor Current Independent of Inductor Value

■

Available in 14-pin SO Package

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APPLICATIO S

■

5V to 3.3V Conversion

■

Distributed Power Systems

■

Step-Down Converters

■

Inverting Converters

■

Memory Backup Supply

■

Portable Instruments

■

Battery-Powered Equipment

■

Cellular Telephones

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 drop­out 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

V

IN

5.4V TO
12V

†††

+

C

IN

68µF
20V

3900pF

1k

130pF

0.1µF

PWR V

SHDN

I

TH

C

T

IN

LTC1265-5

SGND

Figure 1. High Efficiency Step-Down Converter

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V

PGND

SENSE

SENSE

IN

SW

LTC1265-5 Efficiency

L1*
33µH

†

D1

+

1000pF

–

COILTRONICS CTX33-4

*

IRC LRC2010-01-R100-J

**

†

MBRS130LT3

††

AVX TPSE227K010

†††

AVX TPSE686K020

R

SENSE

0.1Ω

**

V

OUT

5V
1A

††

C

+

OUT

220µF
10V

LTC1265-FO1

100

95

90

85

EFFICIENCY (%)

80

75

70

0.01

VIN = 6V

VIN = 9V

VIN = 12V

L = 33µH
V

OUT

R

SENSE

C

T

0.10 1.00

LOAD CURRENT (A)

= 5V

= 0.1Ω

= 130pF

LTC1265 TA01

1

LTC1265/LTC1265-3.3/LTC1265-5

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ABSOLUTE MAXIMUM RATINGS

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PACKAGE/ORDER INFORMATION

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(Voltages Refer to GND Pin) (Note 1)

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

– 13.0

IN

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

PWR V

LB

SENSE

OUT

LB

TOP VIEW

1

IN

2

V

IN

3
4

IN

5

C

T

6

I

TH

–

7

S PACKAGE

14-LEAD PLASTIC SO

*ADJUSTABLE OUTPUT VERSION

T

= 125°C, θJA = 110°C/W

JMAX

14

SW

13

PWR V

12

PGND

11

SGND

10

SHDN

9

N/C (V

8

SENSE

IN

*)

FB

+

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, V

The ● denotes the specifications which apply over the full operating

= 0V, unless otherwise specified.

SHDN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

FB

V

FB

V

OUT

∆V

OUT

I

Q

V

LBTRIP

I

LBIN

I

LBOUT

V8 – V

R

ON

7

Feedback Current into Pin 9 LTC1265 0.2 1 µA
Feedback Voltage LTC1265C ● 1.22 1.25 1.28 V

= 9V, LTC1265I ● 1.20 1.25 1.30 V

V

IN

Regulator Output Voltage LTC1265-3.3: I

LTC1265-5: I
Output Voltage Line Regulation VIN = 6.5V to 10V, I
Output Voltage Load Regulation LTC1265-3.3: 10mA < I

LTC1265-5: 10mA < I
Burst Mode Operation Output Ripple I

= 0mA 50 mV

LOAD

= 800mA ● 3.22 3.3 3.40 V

LOAD

= 800mA ● 4.9 5 5.2 V

LOAD

= 800mA –40 0 40 mV

LOAD

< 800mA 40 65 mV

LOAD

< 800mA 60 100 mV

LOAD

Input DC Supply Current (Note 3) Active Mode: 3.5V < VIN < 10V 1.8 2.4 mA

Sleep Mode: 3.5V < V

Sleep Mode: 5V < V

Shutdown: V

SHDN

< 10V 160 230 µA

IN

< 10V (LTC1265-5) 160 230 µA

IN

= VIN, 3.5V < VIN < 10V 5 15 µA

Low-Battery Trip Point 1.15 1.25 1.35 V
Current into Pin 4 0.5 µA
Current Sunk by Pin 3 V

Current Sense Threshold Voltage LTC1265: V

= 0.4V, V

LBOUT

= 5V, V

V

LBOUT

V

SENSE
SENSE

LTC1265-3.3: V

V

LTC1265-5: V

V

SENSE
SENSE

= 0V 0.5 1.0 1.5 mA

LBIN

= 10V 1.0 µA

LBIN

–

= 5V, V9 = V

–

= 5V, V9 = V

–

= V

SENSE

–

= V

SENSE

–

= V

–

= V

OUT

OUT

/4 + 25mV (Forced) 25 mV

OUT

/4 – 25mV (Forced) 130 150 180 mV

OUT

+ 100mV (Forced) 25 mV

OUT

– 100mV (Forced) 130 150 180 mV

OUT

+ 100mV (Forced) 25 mV

– 100mV (Forced) 130 150 180 mV

ON Resistance of Switch LTC1265C ● 0.3 0.60 Ω

LTC1265I 0.3 0.70 Ω

I

t

5

OFF

CT Pin Discharge Current V

in Regulation, V

OUT

= 0V 2 10 µA

V

OUT

Switch Off Time (Note 4) CT = 390pF, I

C

= 390pF, I

T

= 800mA (LTC1265C) ● 456 µs

LOAD

= 800mA (LTC1265I) ● 3.5 5 7 µs

LOAD

SENSE

–

= V

OUT

40 60 100 µA

ORDER

PART NUMBER

LTC1265CS
LTC1265CS-5
LTC1265CS-3.3
LTC1265IS

P-P

2

LTC1265/LTC1265-3.3/LTC1265-5

INPUT VOLTAGE (V)

4

80

EFFICIENCY (%)

82

86

88

90

100

94

6

8

913

LTC1265 G03

84

96

98

92

57

10

11

12

I

LOAD

= 250mA

I

LOAD

= 800mA

LTC1265-3.3
R

SENSE

= 0.1Ω

C

T

= 130pF

COIL = CTX33-4

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, V

The ● denotes the specifications which apply over the full operating

= 0V, unless otherwise specified.

SHDN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

IH

V

IL

I

10

Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.

Note 2: T
dissipation P

Shutdown Pin High Min Voltage at Pin 10 for Device to be in Shutdown 1.2 V
Shutdown Pin Low Max Voltage at Pin 10 for Device to be Active 0.6 V
Shutdown Pin Input Current V

= 8V 0.5 µA

SHDN

Note 3: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.

is calculated from the ambient temperature TA and power

J

according to the following formulas:

D

Note 4: In applications where R
off time increases by approximately 40%.

is placed at ground potential, the

SENSE

LTC1265CS, LTC1265CS-3.3, LTC1265CS-5:
TJ = TA + (PD • 110°C/W)

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TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency vs Load Current

100

95

90

85

EFFICIENCY (%)

80

75

70

0.01

VIN = 5V

VIN = 9V

VIN = 12V

0.10 1.00

LOAD CURRENT (A)

LTC1265-3.3

= 3.3V

V

OUT

= 0.1Ω

R

SENSE

= 130pF

C

T

COIL = CTX33-4

1265 G01

Efficiency vs Input Voltage
(V

= 5V)

OUT

100

98
96
94
92
90
88

EFFICIENCY (%)

86

LTC1265-5

84

R

SENSE

C

T

82

COIL = CTX33-4

80

4

I

= 250mA

LOAD

I

= 800mA

LOAD

= 0.1Ω

= 130pF

57

6

8

INPUT VOLTAGE (V)

11

913

12

10

1265 G02

Efficiency vs Input Voltage
(V

= 3.3V)

OUT

Operating Frequency
vs (V

– V

IN

1.2

1.0

0.8

0.6

0.4

NORMALIZED FREQUENCY

0.2

0

OUT

21468

(V

IN – VOUT

)

0°C

70°C

) VOLTAGE (V)

25°C

1265 G04

1003579

Switch Resistance

0.9

0.8

0.7

0.6

(Ω)

0.5

(ON)

0.4

RDS

0.3

0.2

0.1

0

3

48

59

= 125°C

T

J

T

= 70°C

J

T

= 25°C

J

T

= 0°C

J

6

7

INPUT VOLTAGE (V)

Switch Leakage Current

300

VIN = 12V

270

240
210

180

150

120

90

LEAKAGE CURRENT (nA)

60
30

12

10

13

11

1265 G05

0

20

0

TEMPERATURE (°C)

60

80

40

100

1265 G06

3

LTC1265/LTC1265-3.3/LTC1265-5

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TYPICAL PERFORMANCE CHARACTERISTICS

DC Supply Current

2.1
DOES NOT INCLUDE

GATE CHARGE

1.8

1.5

1.2

0.9

0.6

SUPPLY CURRENT (mA)

0.3

0

0

4

2

INPUT VOLTAGE (V)

ACTIVE MODE

SLEEP MODE

10

814

6

12

1265 G07

Supply Current in Shutdown Gate Charge Losses

8

SHUTDOWN = 3V
T

= 25C

A

7

6

5

4

3

SUPPLY CURRENT (µA)

2

1

0

4

5

3

6

7

INPUT VOLTAGE (V)

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PIN FUNCTIONS

PWR V

its Driver. Must decouple this pin properly to ground. Must
always tie Pins 1 and 13 together.

V

IN

LTC1265.

LB

Comparator. This pin will sink current when Pin 4 (LBIN)
goes below 1.25V. During shutdown, this pin is high
impedance.

LB

The (+) input is connected to a reference voltage of 1.25V.
CT (Pin 5): External capacitor CT from Pin 5 to ground sets

the switch off time. The operating frequency is dependent
on the input voltage and CT.

I

TH

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.

(Pins 1, 13): Supply for the Power MOSFET and

IN

(Pin 2): Main Supply for All the Control Circuitry in the

(Pin 3): Open-Drain Output of the Low-Battery

OUT

(Pin 4): The (–) Input of the Low-Battery Comparator.

IN

(Pin 6): Feedback Amplifier Decoupling Point. The

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

SWITCHING CURRENT (mA)

1.0

0.5
0

200 400

8

9

11

10

12

13

1265 G08

0

FREQUENCY (kHz)

600

VIN = 12V

VIN = 9V

VIN = 6V

800 1000

1265 G09

SENSE+ (Pin 8): The (+) Pin to the Current Comparator. A
built-in offset between Pins 7 and 8 in conjunction with
R

N/C,V

sets the current trip threshold.

SENSE

(Pin 9): For the LTC1265 adjustable version, this

FB

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 shut­down. 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 CIN. 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

LTC1265/LTC1265-3.3/LTC1265-5

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FUNCTIONAL DIAGRA

SLEEP

+

S

–

V

TH2

5

C

T

Q

V

–

TH1

+

(Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)

PWR V

1, 13

PGND

12

IN

+

SENSE

SW

14

–

V

+

SENSE

78

–

9

V

FB

ADJUSTABLE
VERSION

–

R

S

T

OFF-TIME
CONTROL

V

2 3

IN

–

SENSE
V

FB

C

+

I

TH

LB

11

SGND

25mV TO 150mV

13k

6

0UT

A3

V

OS

–

G

+

+

–

REFERENCE

4

10

SHDN

LB

IN

5pF

100k

1265 FD

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OPERATION

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 V

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 volt­age 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

(Refer to Functional Diagram)

.

OUT

–

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

, comparator T trips, sets the flip flop and causes

TH1

the switch to turn on. Also, the timing capacitor is re­charged. 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 de­creases 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 automati­cally 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

is above the desired output voltage, shutting

OUT

off the switch and causing the capacitor to discharge. This

5

LTC1265/LTC1265-3.3/LTC1265-5

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OPERATION

(Refer to Functional Diagram)

capacitor discharges past V
below V

. Comparator S then trips and a sleep signal is

TH2

until its voltage drops

TH1

generated. The circuit now enters into sleep mode with the
power MOSFET turned off. In sleep mode, the LTC1265 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies­cent current from 2mA to 160µA. When the output capaci-
tor discharges by the amount of the hysteresis of the
comparator V, the P-channel switch turns on again and the
process repeats itself. During Burst Mode operation the
peak inductor current is set at 25mV/R

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.

SENSE

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APPLICATIONS INFORMATION

The basic LTC1265 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement, and begins with the selection of R
Once R

is known, CT and L can be chosen. Next, the

SENSE

Schottky diode D1 is selected followed by CIN and C

SENSE

OUT

.

.

To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset VOS is incorporated
in the gain stage. This prevents the current from increas­ing until the output voltage has dropped below a minimum
threshold.

Using constant off-time architecture, the operating fre­quency is a function of the voltage. To minimize the
frequency variation as dropout is approached, the off-time
controller increases the discharge current as VIN drops
below V

+ 2V. In dropout the P-channel MOSFET is

OUT

turned on continuously (100% duty cycle) providing low
dropout operation with V

I

OUT(MAX)

137.5mV

=

R

SENSE

150mV

=

R

(Amps)

SENSE

OUT

–

≅ VIN.

25mV

2 • R

(Amps)

SENSE

R

R

Selection for Output Current

SENSE

is chosen based on the required output current.

SENSE

With the current comparator monitoring the voltage devel­oped across R

, the threshold of the comparator

SENSE

determines the peak inductor current. Depending on the
load current condition, the threshold of the comparator
lies between 25mV/R

and 150mV/R

SENSE

SENSE

. The maxi-

mum output current of the LTC1265 is:

I

OUT(MAX)

where I

150mV

=

R

SENSE

is the peak-to-peak inductor ripple current.

RIPPLE

–

I

RIPPLE

2

(Amps)

At a relatively light load, the LTC1265 is in Burst Mode
operation. In this mode the peak inductor current is set at
25mV/R

. To fully benefit from Burst Mode operation,

SENSE

the inductor current should be continuous during burst
periods. Hence, the peak-to-peak inductor ripple current
must not exceed 25mV/R

To account for light and heavy load conditions, the I

SENSE

.

OUT(MAX)

is then given by:

Solving for R

and allowing a margin of variations in

SENSE

the LTC1265 and extended component values yields:

R

SENSE

=

I

OUT(MAX)

100mV

(Ω)

The LTC1265 is rated with a capability to supply a maximum
of 1.2A of output current.

R

R

that can be used is 0.083Ω.

SENSE

versus maximum output is given in Figure 2.

SENSE

0.5

0.4

0.3

(Ω)

SENSE

R

0.2

0.1

0

0

MAXIMUM OUTPUT CURRENT (A)

Figure 2. Selecting R

Therefore, the minimum value of

A graph for selecting

0.2

0.4

0.6

0.8

SENSE

1

1265 G10

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LTC1265/LTC1265-3.3/LTC1265-5

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APPLICATIONS INFORMATION

Under short-circuit condition, the peak inductor current is
determined by:

I

SC(PK)

In this condition, the LTC1265 automatically extends the
off time of the P-channel MOSFET to allow the inductor
current to decay far enough to prevent any current build­up. The resulting ripple current causes the average short­circuit current to be approximately I

CT and L Selection for Operating Frequency

The LTC1265 uses a constant off-time architecture with
t

determined by an external capacitor CT. Each time the

OFF

P-channel MOSFET turns on, the voltage on CT is reset to
approximately 3.3V. During the off time, CT is discharged
by a current that is proportional to V
is analogous to the current in inductor L, which likewise,
decays at a rate proportional to V
value must track the timing capacitor value.

The value of CT is calculated from the desired continuous
mode operating frequency:

CT =

where VD is the drop across the Schottky diode.
As the operating frequency is increased, the gate charge

losses will reduce efficiency. The complete expression for
operating frequency is given by:

f ≈

where:

=

1.3(10

1

t

OFF

150mV
R

SENSE

1

4

)f

VIN – V

)

V

IN

VIN – V

)

OUT

+ V

(Amps)

V

+ V

IN

(Hz)

)

D

OUT

D

OUT(MAX)

OUT

. Thus the inductor

OUT

(Farads)

)

.

. The voltage on C

T

2V, the LTC1265 reduces t
current in CT.
dropout. (See shelving effect shown in the Operating
Frequency curve under Typical Performance Character­istics.)

To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
R
following expression for L:

Using an inductance smaller than the above value will
result in the inductor current being discontinuous. A
consequence of this is that the LTC1265 will delay entering
Burst Mode operation and efficiency will be degraded at
low currents.

Inductor Core Selection

With the value of L selected, the type of inductor must be
chosen. Basically, there are two kinds of losses in an
inductor; core and copper losses.

Core losses are dependent on the peak-to-peak ripple
current and core material. However it is independent of
the physical size of the core. By increasing the induc­tance, the peak-to-peak inductor ripple current will de­crease, therefore reducing core loss. Utilizing low core
loss material, such as molypermalloy or Kool Mµ® will
allow user to concentrate on reducing copper loss and
preventing saturation.

Although higher inductance reduces core loss, it in­creases copper loss as it requires more windings. When
space is not at a premium, larger wire can be used to
reduce the wire resistance. This also prevents excessive
heat dissipation.

of peak-to-peak ripple current. This results in the

SENSE

L ≥ 5.2(105)R

This prevents audible operation prior to

SENSE(CT)VREG

by increasing the discharge

OFF

V

REG

t

= 1.3(104)C

OFF

V

is the desired output voltage (i.e. 5V, 3.3V). V

REG

the measured output voltage. Thus V
in regulation.

Note that as VIN decreases, the frequency decreases.
When the input-to-output voltage differential drops below

T

)

V

OUT

(sec)

)

REG/VOUT

OUT

= 1

is

CATCH DIODE SELECTION

Losses in the catch diode depend on forward drop and
switching times. Therefore Schottky diodes are a good
choice for low drop and fast switching times.

The catch diode carries load current during the off time.
The average diode current is therefore dependent on the

Kool Mµ is a registered trademark of Magnetics, Inc.

7

LTC1265/LTC1265-3.3/LTC1265-5

V

LB_TRIP

= 1.25

1 +

R4
R3

)

)

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APPLICATIONS INFORMATION

P-channel switch duty cycle. At high input voltages, the
diode conducts most of the time. As VIN approaches V
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 I
LTC1265 circuits will be well served by either a 1N5818 or
a MBRS130LT3 Schottky diode. An MBRS0520 is a good
choice for I

C

IN

In continuous mode, the input current of the converter is
a square wave of duty cycle V
voltage transients, a low ESR input capacitor must be
used. In addition, the capacitor must handle a high RMS
current. The CIN RMS current is given by:

I

RMS

This formula has a maximum at VIN = 2V
= I

OUT

design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life­time. This makes it advisable to further derate the capaci­tor, or to choose a capacitor rated at a higher temperature
than required. Do not underspecify this component. An
additional 0.1µF ceramic capacitor is also required on
PWR VIN for high frequency decoupling.

C

OUT

The selection of C
resistance (ESR) for proper operation of the LTC1265. The
required ESR of C

ESR

where I
case where the I
of C

OUT

ESR

To avoid overheating, the output capacitor must be sized
to handle the ripple current generated by the inductor. The

OUT(MAX)

I

OUT [VOUT (VIN – VOUT

≈

/2. This simple worst case is commonly used for

< 50mV/I

COUT

is the ripple current of the inductor. For the

RIPPLE

is:

< 2(R

COUT

at close to 100% duty cycle. Most

SC(PK)

≤ 500mA.

OUT/VIN

V

IN

is based upon the effective series

OUT

is:

OUT

RIPPLE

is 25mV/R

RIPPLE

)

SENSE

. To prevent large

1

/

2

)]

(A

RMS

, where I

OUT

, the required ESR

SENSE

)

OUT

RMS

,

worst-case RMS ripple current in the output capacitor is
given by:

≈

150mV

2(R

SENSE

(A

)

)

RMS

OUT

is made too small, the

OUT

and can be improved at

SENSE

has been

RIPPLE(P-P)

I

RMS

Generally, once the ESR requirement for C
met, the RMS current rating far exceeds the I
requirement.

ESR is a direct function of the volume of the capacitor.
Manufacturers such as Nichicon, AVX and Sprague should
be considered for high performance capacitors. The
OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR for its size at a somewhat
higher price.

In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Alumi­num electrolyte and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are both available
in surface mount configuration and are surge tested for
use in switching power supplies. An excellent choice is the
AVX TPS series of surface mount tantalums, available in
case heights ranging from 2mm to 4mm. Consult the
manufacturer for other specific recommendations.

When the capacitance of C
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes Burst Mode opera­tion to be activated when the LTC1265 would normally be
in continuous operation. The effect will be most pro­nounced with low value of R
higher frequencies with lower values of L.

Low-Battery Detection

The low-battery comparator senses the input voltage
through an external resistive divider. This divided voltage
connects to the (–) input of a voltage comparator (Pin 4)
which is compared with a 1.25V reference voltage. Ne­glecting Pin 4 bias current, the following expression is
used for setting the trip limit:

8

LTC1265/LTC1265-3.3/LTC1265-5

U

WUU

APPLICATIONS INFORMATION

The output, Pin 3, is an N-channel open drain that goes low
when the battery voltage is below the threshold set by R3
and R4. In shutdown, the comparator is disabled and Pin
3 is in a high impedance state.

V

IN

R4

4

R3

Figure 3. Low-Battery Comparator

–

+

1.25V REFERENCE

LTC1265

LTC1265 F03

3

THERMAL CONSIDERATIONS

In a majority of applications, the LTC1265 does not
dissipate much heat due to its high efficiency. However, in
applications where the switching regulator is running at
high duty cycles or the part is in dropout with the switch
turned on continuously (DC), the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated by the regulator
exceeds the maximum junction temperature of the part.
The temperature rise is given by:

TR = P(θJA)

where P is the power dissipated by the regulator and θ

JA

is the thermal resistance from the junction of the die to the
ambient temperature.

The junction temperature is simply given by:

TJ = TR + T

A

LTC1265 ADJUSTABLE APPLICATIONS

The LTC1265 develops a 1.25V reference voltage between
the feedback (Pin 9) terminal and signal ground (see
Figure 4). By selecting resistor R1, a constant current is
caused to flow through R1 and R2 to set overall output
voltage. The regulated output voltage is determined by:

R2

V

OUT

= 1.25

1 +

)

R1

)

For most applications a 30k resistor is suggested for R1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to the LTC1265.

V

OUT

R2

LTC1265

SGND

11

Figure 4. LTC1265 Adjustable Configuration

9

V

FB

100pF

R1

LTC1265 F04

As an example, consider the LTC1265 is in dropout at an
input voltage of 4V with a load current of 0.5A. From the
Typical Performance Characteristics graph of Switch Re­sistance, the ON resistance of the P-channel is 0.55Ω.
Therefore power dissipated by the part is:

P = I2(R

For the SO package, the θ

) = 0.1375W

DSON

is 110°C/W.

JA

Therefore the junction temperature of the regulator when
it is operating in ambient temperature of 25°C is:

TJ = 0.1375(110) + 25 = 40.1°C

Remembering that the above junction temperature is
obtained from a R
junction temperature based on a higher R

at 25°C, we need to recalculate the

DSON

DSON

since it
increases with temperature. However, we can safely as­sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.

Now consider the case of a 1A regulator with VIN = 4V and
TA = 65°C. Starting with the same 0.55Ω assumption for
R

, the TJ calculation will yield 125°C. But from the

DSON

graph, this will increase the R

to 0.76Ω, which when

DSON

used in the above calculation yields an actual TJ > 148°C.
Therefore the LTC1265 would be unsuitable for a 4V input,
1A output regulator operating at TA = 65°C.

9

LTC1265/LTC1265-3.3/LTC1265-5

U

WUU

APPLICATIONS INFORMATION

Board Layout Checklist

When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1265. These items are also illustrated graphically in
the layout diagram of Figure 5. Check the following in your
layout:

1. Are the signal and power grounds segregated? The
LTC1265 signal ground (Pin 11) must return to the (–)
plate of C
anode of the Schottky diode, and the (–) plate of CIN,
whose leads should be as short as possible.

2. Does the (+) plate of the CIN connect to the power V
(Pins 1,13) as close as possible? This capacitor pro­vides the AC current to the internal P-channel MOSFET
and its driver.

3. Is the input decoupling capacitor (0.1µF) connected
closely between power VIN (Pins 1,13) and power
ground (Pin 12)? This capacitor carries the high fre­quency peak currents.

. The power ground (Pin 12) returns to the

OUT

IN

4. Is the Schottky diode closely connected between the
power ground (Pin 12) and switch (Pin 14)?

5. Does the LTC1265 SENSE– (Pin 7) connect to a point
close to R

and the (+) plate of C

SENSE

? In adjustable

OUT

applications, the resistive divider, R1 and R2, must be
connected between the (+) plate of C

and signal

OUT

ground.

6. Are the SENSE– and SENSE+ leads routed together with
minimum PC trace spacing? The 1000pF capacitor
between Pins 7 and 8 should be as close as possible to
the LTC1265.

7. Is SHDN (Pin 10) actively pulled to ground during
normal operation? The SHDN pin is high impedance
and must not be allowed to float.

1k

3900pF

PWR V

1000pF

1

2

3

4

5

6

7

V

IN

LB

OUT

LB

IN

C

T

I

TH

SENSE

IN

LTC1265

–

1000pF

SW

PWR V

PGND

SGND

SHDN

N/C (VFB)

SENSE

V

IN

14

13

IN

D1

+

12

11

10

SHDN

9

8

+

OUTPUT DIVIDER REQUIRED

WITH ADJUSTABLE VERSION ONLY

C

IN

R1

R2

0.1µF

C

+

OUT

LTC1265 F05

Figure 5. LTC1265 Layout Diagram (See Board Layout Checklist)

L

R

SENSE

V

OUT

BOLD LINES INDICATE
HIGH PATH CURRENTS

10

LTC1265/LTC1265-3.3/LTC1265-5

U

WUU

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 0.9V
never dip below 2V as shown in Figure 6a.

3.3V

(PIN 5)

T

2.4V

swing. This voltage should

P-P

When the load currents are low (I

LOAD

< I

BURST

) 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

(PIN 5)

T

2.4V

VOLTAGE AT C

0V

TIME

(a) CONTINUOUS MODE OPERATION

(a) POORLY DECOUPLED LTC1265

VOLTAGE AT C

0V

LTC1265 F06a

Figure 6. CT Waveforms

Figure 7. Inductor Waveforms

TIME

(b) Burst Mode OPERATION

(b) WELL DECOUPLED LTC1265

LTC1265 F06b

11

LTC1265/LTC1265-3.3/LTC1265-5

LOAD CURRENT (mA)

0.01

70

EFFICIENCY (% )

75

80

85

90

100

0.1 1.0

1265 G11

95

L = DALE LPT4545-220 (22µH)
V

OUT

= 3.3V

C

T

= 100pF

U

WUU

APPLICATIONS INFORMATION

Design Example

As a design example, assume VIN = 5V, V
= 0.8A and f = 250kHz. With this information we can easily
calculate all the important components.

From (1),

R

= 100mV/0.8 = 0.125Ω

SENSE

From (2) and assuming VD = 0.4V,

CT ≅ 100pF

Using (3), the value of the inductor is:

L ≥ 5.2(105)(0.125)(100pF)3.3V = 22µH

For the catch diode, a MBRS130LT3 or 1N5818 will be
sufficient in this application.

CIN will require an RMS current rating of at least 0.4A at
temperature, and C

will require an ESR of (from 5):

OUT

OUT

= 3.3V, I

MAX

V

IN

5V

+

C

IN

V

IN

LTC1265-3.3

PGND

SENSE

SENSE

SGND

IN

SW

22µH

0.125Ω

D1

+

1000pF

–

+

LTC1265 F08

V

3.3V

0.8A

C

OUT

OUT

3900pF

0.1µF

1k

100pF

PWR V

SHDN

I

TH

C

T

Figure 8. Design Example Circuit

ESR

The inductor ripple current is given by:

I

RIPPLE

At light loads the peak inductor current is at:
I

PEAK

Therefore, at load current less than 0.1A the LTC1265 will
be in Burst Mode operation. Figure 8 shows the complete
circuit and Figure 9 shows the efficiency curve with the
above calculated component values.

< 0.25Ω

COUT

V

+ V

=

OUT

)

D

t

OFF

)

L

= 25mV/0.125 = 0.2A

= 0.22A

Figure 9. Design Example Efficiency Curve

12

U

TYPICAL APPLICATIONS

V

IN

5V

4

3

270pF

3900pF

AVX TPSD107K010

*

AVX TPSE227K010

**

†

COILCRAFT D03316-473

††

DALE WSL2010-0.1-1%

5

1k

6

7

LTC1265/LTC1265-3.3/LTC1265-5

High Efficiency 5V to 3.3V Converter

2 1, 13

V

IN

LB

IN

LTC1265-3.3

LB

OUT

C

T

I

THR

–

SENSE

1000pF

PWR V

IN

14

SW

12

PGND

11

SGND

10

SHDN SHDN

9

NC

8

+

SENSE

+

0.1µF

C

*

IN

100µF
10V

†

L1

47µH

MBRS130LT1

R

SENSE

0.1Ω

††

LTC1265 TA02

V

OUT

C

OUT

220µF
10V

3.3V

**

1A

+

AVX TPSD226K025

*

AVX TPSD107K010

**

†

L1 SELECTION

MANUFACTURER

COILCRAFT
COILTRONICS
DALE
SUMIDA

††

IRC LRC2010-01-R100-J

D1

= MBRS130LT3

VIN (V)

I

3.5

4.0

5.0

6.0

7.0

7.5

OUT(MAX)

360
430
540
630
720
740

PART NO.

DO3316-473
CTX50-4
LPT4545-500LA
CD74-470

(mA)

Positive-to-Negative (–5V) Converter

V

220pF

2200pF

IN

2 1, 13

V

PWR V

IN

4

LB

IN

LTC1265-5

3

LB

OUT

5

C

T

1k

6

I

THR

7

SENSE

1000pF

IN

SW

PGND

SGND

SHDN

SENSE

+

–

3.5V TO 7.5V

TP0610L

14

12

11

10

8

100k

SHDN

D1

L1
50µH

C

*

IN

+

†

R

SENSE

0.1Ω

0.1µF

††

22µF
25V
× 2

V

OUT

–5V

C

OUT

100µF

+

10V

LTC1265 TA03

**

13

LTC1265/LTC1265-3.3/LTC1265-5

U

TYPICAL APPLICATIONS

5V Buck-Boost Converter

V

75pF

3300pF

IN

1k

4

3

5

6

7

LB

LB

C

I

THR

SENSE

2 1, 13

V

PWR V

IN

IN

LTC1265

OUT

T

–

0.01µF

IN

SW

PGND

SGND

SHDN SHDN

V

FB

+

SENSE

(V)

V

IN

3.5

4.0

5.0

6.0

7.0

7.5

L1B

3

TOP VIEW

4

L1B

SANYO OS-CON CAPACITOR

*

IRC LRC2010-01-R162-J

**

†

L1A, L2A SELECTION

MANUFACTURER

COILTRONICS
DALE

I

OUT(MAX)

240
275
365
490
610
665

(mA)

L1A

2

•

1

L1A

PART NO.

CTX33-4
LPT4545-330LA

3.5V TO 7.5V

*

C

+

IN

4

3

•

33µF
10V*

††

L1B
33µH

R

SENSE

0.162Ω

+

**

0.1µF

14

12

11

10

9

8

100µF
16V

L1A

33µH

1

1N5818

•

100pF

††

V

OUT

2

75k

25k

5V

+

C

*

OUT

100µF
10V

LTC1265 F09

V

(V)

I

IN

OUT(MAX)

4.0

40

5.0

60

6.0

80

7.0

100

8.0

115

9.0

130

10.0

150

11.0

165

12.0

180

L1B

3

TOP VIEW

4

L1B

AVX TPSE686K020

*

AVX TPSE336K025

**

†

IRC LRC2010-01-R162-J

††

L1A,L2A SELECTION

MANUFACTURER

COILTRONICS
DALE

(mA)

L1A

2

•

1

L1A

PART NO.

CTX50-4
LPT4545-500LA

4V TO 12V

75pF

3300pF

9V to 12V and – 12V Outputs

MBRS130LT3

V

IN

+

C

*

2 1, 13

V

PWR V

IN

4

LB

IN

LTC1265

3

LB

OUT

5

C

T

1k

6

I

THR

7

SENSE

0.01µF

SW

PGND

SGND

SHDN SHDN

V

–

SENSE

IN

FB

+

0.1µF

1N914

14

12

11

10

9

8

IN

68µF
20V

SI19430DY

33µF**

4

•

3

25V

††

L1B
50µH

R

SENSE

0.162Ω

+

*

††

L1A

50µH

•

1

MBRS130LT3

100pF

2

301k

+

34k

LTC1265 TA05

+

C
68µF
20V

OUT

*

C

OUT

68µF
20V

V
12V

OUT

*

V

OUT

–12V

14

U

TYPICAL APPLICATIONS

V

51pF

3300pF

IN

4

3

5

1k

6

7

3.5V TO 12.5V

LTC1265/LTC1265-3.3/LTC1265-5

2.5mm Max Height 5V-to-3.3V (500mA)

2 1, 13

V

IN

LB

IN

LTC1265-3.3

LB

OUT

C

T

I

THR

–

SENSE

1000pF

PWR V

IN

14

SW

12

PGND

11

SGND

10

SHDN SHDN

9

N/C

8

+

SENSE

0.1µF

*

C

+

IN

15µF
10V × 2

**

MBRS0520LT1

AVX TAJB156K010

*

AVX TAJB226K06

†

IRC LRC2010-01-R200-J

††

SUMIDA CLS62-180

††

L1
18µH

†

R

SENSE

0.20Ω

LTC1265 TA06

C

OUT

22µF

+

6.3V × 2

**

V

OUT

3.3V
500mA

0V: V
5V: V

75pF

3300pF

OUT
OUT

V

= 5V
= 3.3V

3.5V TO 12.5V

Logic Selectable 0V/3.3V/5V 700mA Regulator

DALE 593D68X0020E2W

*

DALE 593D107X0010D2W

**

†

IRC LRC2010-01-R15-J

††

L1 SELECTION

56.2k

PART NO.

DO3316-333
CTX33-4
LPT4545-330LA
CD74-330

= 3.3V/5V

OUT

= 0V

OUT

45.3k

75k

LTC1265 TA07

**

C

OUT

100µF

+

10V

V

OUT

0V/3.3V/5V
700mA

MANUFACTURER

COILCRAFT

IN

1k

21, 13

V

PWR V

IN

4

LB

IN

LTC1265

3

LB

OUT

5

C

T

6

I

THR

7

SENSE

–

1000pF

IN

SW

PGND

SGND

SHDN V

V

FB

+

SENSE

0.1µF

14

12

11

10

†††

SHDN

9

8

C

*

+

IN

68µF
20V

MBRS130LT3

100pF

COILTRONICS
DALE
SUMIDA

†††

V

= 0V: V

SHDN

= 5V: V

††

L1
33µH

†

R

SENSE

0.15Ω

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

15

LTC1265/LTC1265-3.3/LTC1265-5

U

TYPICAL APPLICATIONS

4-NiCad Battery Charger

V

IN

FAST CHARGE: = 0V

TRICKLE CHARGE: > 2V

VN2222L

8V TO 12.5V

51Ω

270pF

1k

3300pF

4

3

5

6

7

LB

LB

C

I

THR

SENSE

2 1, 13

V

PWR V

IN

IN

LTC1265

OUT

T

–

SENSE

1000pF

SW

PGND

SGND

SHDN

V

IN

FB

+

0.1µF

14

12

11

10

CHARGER
ON/OFF

9

8

+

C

*

IN

22µF, 25V

MBRS130LT3

100pF

R

SENSE

0.10Ω

DALE 593D226X0025D2W

*

DALE 593D107X0016E2W

**

†

DALE WSL2010-0.10-1%

††

L1 SELECTION

MANUFACTURER

COILCRAFT
COILTRONICS
SUMIDA

††

L1
100µH

†

30k

138k

PART NO.

DO3316-104
CTX100-4P
CD105-101

**

C

OUT

100µF

+

10V

MBRS130LT3

LTC1265 TA08

V

OUT

4 NICAD
1A FAST CHARGE

0.1A TRICKLE CHARGE

U

PACKAGE DESCRIPTION

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

*

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

**

× 45°

0.016 – 0.050

(0.406 – 1.270)

0° – 8° TYP

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

TYP

Dimension in inches (millimeters) unless otherwise noted.

0.337 – 0.344*
(8.560 – 8.738)

13

12

11

3

4

0.050

(1.270)

BSC

0.004 – 0.010

(0.101 – 0.254)

(5.791 – 6.197)

0.228 – 0.244

14

1

2

10

5

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LTC1174 Step-Down Switching Regulator with Internal 0.5A Switch VIN ≤ 18.5V, Comparator/Low Battery Detector
LTC1474/LTC1475 Low Quiescent Current Step-Down Regulators Monolithic, IQ = 40µA, 400mA, MS8
LTC1574 Step-Down Switching Regulator with Internal 0.5A Switch VIN ≤ 18.5V, Comparator

and Schottky Diode
LTC1622 Low Input Voltage Step-Down DC/DC Controller Constant Frequency, 2V to 10V VIN, MS8
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LTC1772 Constant Frequency Step-Down DC/DC Controller SOT-23, 2.2V to 9.8V V

to 500mA, 2.65V to 8.5V V

OUT

IN

9

6

IN

8

0.150 – 0.157**
(3.810 – 3.988)

7

S14 1298

16

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

126535fa LT/TP 1299 2K REV A • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1995