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Fabricante

LTC1265

1.2A, High Efficiency Step-Down DC/DC Converter

Linear Technology 

LTC1265/LTC1265-3.3/LTC1265-5

APPLICATIONS INFORMATION

Under short-circuit condition, the peak inductor current is

determined by:

ISC(PK) =

150mV

RSENSE

(Amps)

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 build-

up. The resulting ripple current causes the average short-

circuit current to be approximately IOUT(MAX).

CT and L Selection for Operating Frequency

The LTC1265 uses a constant off-time architecture with

tOFF determined by an external capacitor CT. Each time the

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 VOUT. The voltage on CT

is analogous to the current in inductor L, which likewise,

decays at a rate proportional to VOUT. Thus the inductor

value must track the timing capacitor value.

The value of CT is calculated from the desired continuous

mode operating frequency:

) CT

=

1

1.3(104)f

VIN – VOUT

VIN + VD

(Farads)

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

≈

1

tOFF

VIN – VOUT (Hz)

VIN + VD

where:

) tOFF = 1.3(104)CT

VREG

VOUT

(sec)

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

the measured output voltage. Thus VREG/VOUT = 1

in regulation.

Note that as VIN decreases, the frequency decreases.

When the input-to-output voltage differential drops below

2V, the LTC1265 reduces tOFF by increasing the discharge

current in CT. This prevents audible operation prior to

dropout. (See shelving effect shown in the Operating

Frequency curve under Typical Performance Character-

istics.)

To maintain continuous inductor current at light load, the

inductor must be chosen to provide no more than 25mV/

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

following expression for L:

L ≥ 5.2(105)RSENSE(CT)VREG

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 induc-

tance, the peak-to-peak inductor ripple current will de-

crease, 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 in-

creases 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.

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

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