L2 = 2

L

100 ns

¹

·

©

§

L =

LM2705

SNVS191E – NOVEMBER 2002 – REVISED MAY 2013

www.ti.com

off the NMOS switch.The SW voltage will then rise to the output voltage plus a diode drop and the inductor

current will begin to decrease as shown in Figure 16. During this time the energy stored in the inductor is

the inductor again. This energy transfer from the inductor to the output causes a stepping effect in the output

ripple as shown in Figure 16.

This cycle is continued until the voltage at FB reaches 1.237V. When FB reaches this voltage, the enable

comparator then disables the device turning off the NMOS switch and reducing the Iq of the device to 40uA. The

in Figure 16. When the FB pin drops slightly below 1.237V, the enable comparator enables the device and

0.01µA. In shutdown mode the output voltage will be a diode drop lower than the input voltage.

APPLICATION INFORMATION

INDUCTOR SELECTION - BOOST REGULATOR

The appropriate inductor for a given application is calculated using the following equation:

(2)

input voltage for the application, such as for battery powered applications. For the LM2705 constant-off time

control scheme, the NMOS power switch is turned off when the current limit is reached. There is approximately a

100ns delay from the time the current limit is reached in the NMOS power switch and when the internal logic

actually turns off the switch. During this 100ns delay, the peak inductor current will increase. This increase in

inductor current demands a larger saturation current rating for the inductor. This saturation current can be

approximated by the following equation:

(3)

Choosing inductors with low ESR decrease power losses and increase efficiency.

Care should be taken when choosing an inductor. For applications that require an input voltage that approaches

the output voltage, such as when converting a Li-Ion battery voltage to 5V, the 400ns off time may not be enough

time to discharge the energy in the inductor and transfer the energy to the output capacitor and load. This can

cause a ramping effect in the inductor current waveform and an increased ripple on the output voltage. Using a

For typical curves and evaluation purposes the DT1608C series inductors from Coilcraft were used. Other

acceptable inductors would include, but are not limited to, the SLF6020T series from TDK, the NP05D series

from Taiyo Yuden, the CDRH4D18 series from Sumida, and the P1166 series from Pulse.

INDUCTOR SELECTION - SEPIC REGULATOR

The following equation can be used to calculate the approximate inductor value for a SEPIC regulator:

(4)

The boost inductor, L1, can be smaller or larger but is generally chosen to be the same value as L2. See

Figure 23 and Figure 24 for typical SEPIC applications.

DIODE SELECTION

To maintain high efficiency, the average current rating of the schottky diode should be larger than the peak

efficiency in portable applications. Choose a reverse breakdown of the schottky diode larger than the output

voltage.

8

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