LTC1159CS datasheet(8/20 Pages) LINER

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LTC1159/LTC1159-3.3/LTC1159-5

APPLICATIO S I FOR ATIO

inductance collapses abruptly when the peak design cur-

rent is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage

ripple which can cause Burst Mode operation to be falsely

triggered in the LTC1159. Do not allow the core to saturate!

Molypermalloy (from Magnetics, Inc.) is a low loss core

material for toroids, but it is more expensive than ferrite.

A reasonable compromise from the same manufacturer is

Kool M

µ. Toroids are very space efficient, especially when

you can use several layers of wire. Because they generally

lack a bobbin, mounting is more difficult. However, new

surface mount designs available from Coiltronics do not

increase the height significantly.

Power MOSFET Selection

Two external power MOSFETs must be selected for use

with the LTC1159: a P-channel MOSFET for the main

switch and an N-channel MOSFET for the synchronous

switch.

The peak-to-peak drive levels are set by the VCC voltage on

the LTC1159. This voltage is typically 4.5V during start-up

and 5V to 7V during normal operation (see EXTVCC Pin

Connection). Consequently,

logic-level threshold

MOSFETs must be used in most LTC1159 family applica-

tions. The only exception is applications in which EXTVCC

is powered from an external supply greater than 8V, in

which standard threshold MOSFETs (VGS(TH) < 4V) may be

used. Pay close attention to the BVDSS specification for the

MOSFETs as well; many of the logic-level MOSFETs are

limited to 30V.

Selection criteria for the power MOSFETs include the “ON”

resistance RDS(ON), reverse transfer capacitance CRSS,

input voltage, and maximum output current. When the

LTC1159 is operating in continuous mode, the duty cycle

for the P-channel MOSFET is given by:

P-Ch Duty Cycle =

VOUT

VIN

N-Ch Duty Cycle =

VIN – VOUT

VIN

The MOSFET dissipations at maximum output current are

given by:

f =

tOFF

)

)1– VOUT

VIN

where tOFF = 1.3 × 104 × CT

Once the frequency has been set by CT, the inductor L

must be chosen to provide no more than 0.025V/RSENSE

of peak-to-peak inductor ripple current. This results in a

minimum required inductor value of:

LMIN = 5.1 × 105 × RSENSE × CT × VREG

As the inductor value is increased from the minimum value,

the ESR requirements for the output capacitor are eased at

the expense of efficiency. If too small an inductor is used,

the LTC1159 may not enter Burst Mode operation and

efficiency will be severely degraded at low currents.

Inductor Core Selection

Once the minimum value for L is known, the type of

inductor must be selected. High efficiency converters

generally cannot afford the core loss found in low cost

powdered iron cores, forcing the use of more expensive

ferrite, molypermalloy, or Kool M

µ® cores. Actual core loss

is independent of core size for a fixed inductor value, but it

is very dependent on the inductance selected. As induc-

tance increases, core losses go down but copper (I2R)

losses will increase.

Ferrite designs have very low core loss, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” which means that

FREQUENCY (kHz)

200

400

600

1400

1000

100

1200

800

150

200

250

VOUT = 5V

VIN = 48V

VIN=12V

VIN = 24V

LTC1159 • F03

Figure 3. Timing Capacitor Selection

Kool M

µ is a registered trademark of Magnetics, Inc.

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