8

LTC1433/LTC1434

LTC1433/LTC1434 are used at 100% duty cycle with low

input voltages.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the use

of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses.

The inductor value has a direct effect on ripple current. The

ripple current

∆I

fL

V

V

V

L

OUT

OUT

IN









1

1–

Core losses are dependent on the peak-to-peak ripple

current and core material. Hence, by choosing a larger

inductance the peak-to-peak inductor ripple current will

decrease, therefore decreasing core loss. To further re-

duce losses, low core loss material such as molypermalloy

or Kool M

An indirect way that the inductor affects efficiency is

through the usage of the big P-channel at low load

currents. Lower inductance values will result in high peak

inductor current. Because one of the conditions that

determines the turning on of the large P-channel is peak

current, this will result in the usage of the large P-channel

even though the load current is low. Hence, efficiency at

low load current will be affected. See Efficiency Consider-

ations.

Inductor Core Selection

Once the 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 inductance selected. As inductance

increases, core losses go down. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses will increase.

Kool M

µ is a registered trademark of Magnetics, Inc.

0

400

500

600

150

1433/34 F02

300

200

50

100

200

100

0

700

Another important point to note is that at a low supply

(see Typical Performance Characteristics). Therefore, the

user should calculate the power dissipation when the

As the operating frequency is increased the gate charge

losses will be higher, reducing efficiency. The maximum

recommended switching frequency is 700kHz. When us-

ing Figure 2 for synchronizable applications, the value of

your center frequency (see Phase-Locked Loop and Fre-

quency Synchronization).

Low Supply Operation

The LTC1433/LTC1434 can function down to 3V and the

maximum allowable output current is also reduced at low

input voltages. Figure 3 shows the amount of change as

the supply is reduced down to 2.5V. The minimum guar-

anteed input supply is 3V.

SUPPLY VOLTAGE (V)

4.0

70

80

1433/34 F03

60

50

3.5

3.0

2.5

100

90

Figure 3. Maximum Allowable Output Current vs Supply Voltage