8

LTC1159

LTC1159-3.3/LTC1159-5

MAXIMUM OUTPUT CURRENT (A)

0

0.12

0.16

0.20

4

LTC1159 • F02

0.08

0.04

0

1

2

3

5

0.10

0.14

0.18

0.06

0.02

APPLICATIO S I FOR ATIO

15mV

150mV

circuit to allow sufficient time for the inductor current to

decay between switch cycles. The resulting ripple current

The LTC1159 uses a constant off-time architecture with

operating frequency, f:

f

)

effects of input voltage is given in Figure 3.

As the operating frequency is increased the gate charge

losses will be higher, reducing efficiency (see Efficiency

Considerations). The complete expression for operating

frequency is given by:

f =

1

)

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

minimum required inductor value of:

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

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

it is very dependent on the inductance selected. As induc-

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

inductance collapses abruptly when the peak design cur-

rent is exceeded. This results in an abrupt increase in

FREQUENCY (kHz)

0

0

200

400

600

1400

1000

50

100

1200

800

150

200

250

LTC1159 • F03

Figure 3. Timing Capacitor Selection

Kool M

µ is a registered trademark of Magnetics, Inc.