8

LT1316

Discontinuous Mode Operation

a high duty cycle in continuous mode. For duty cycles

exceeding the LT1316’s guaranteed minimum specifica-

tion of 0.73, the circuit will need to be designed for

discontinuous operation. Additionally, very low peak cur-

rent limiting below 50mA may necessitate operating in this

mode unless high inductance values are acceptable. When

operating in discontinuous mode, a different equation

governs available output power. For each switch cycle, the

inductor current ramps down to zero, completely releas-

ing the stored energy. Energy stored in the inductor at any

each cycle, the equation for maximum power out is:

Where f =

1

)

)

When designing for very low peak currents (< 50mA), the

inductor size needs to be large enough so that on-time is

a least 1

µs. On-time can be calculated by the equation:

On-Time =

)

)

Also, at these low current levels, current overshoot due to

power transistor turn-off delay will be a significant portion

of peak current. Increasing inductor size will keep this to

a minimum.

Design Example 1

1. Find duty cycle

DC =

)

)

=

= 0.654

5 – 2 + 0.4

5 – 0.2 + 0.4

)

)

Because duty cycle is less than the LT1316 minimum

specification (0.73), the circuit can be designed for

continuous operation.

1 – DC

=

= 58mA

2(10mA)

1 – 0.654

3. Find L

L =

)

)

=

= 293

µH

5 – 2 + 0.4

0.4(58mA)

)

)

2

µs

Overshoot =

300ns

L

)

)

=

= 1.8mA

2

330

µH

)

)

Design Example 2

1. Find duty cycle:

DC =

)

)

=

= 0.89

28 – 3.3 + 0.4

28 – 0.2 + 0.4

)

)

Because duty cycle exceeds LT1316 minimum specifi-

cation of 73%, the circuit must be designed for discon-

tinuous operation.

3. Set the on-time to the data sheet minimum of 3.4

µs and

find L

L =

=

= 52

µH

(3.4

2(0.196W)(3.4

µs + 2µs)