AN2115 datasheet(21/33 Pages) STMICROELECTRONICS

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AN2115

Application information

Doc ID 11165 Rev 5

21/33

If the output capacitors are of the tantalum type, the ESR zero is within the system

bandwidth and it can be used to stabilize the system so that the zero due to the

compensation network will be rendered unnecessary.

6.2

Application losses and efficiency

There are many losses affecting the efficiency of the application. Some of these losses are

related to the device and others are related to the external components. The most important

losses are described below.

6.2.1

Conduction losses

These losses are basically due to the significant resistances of the internal switches and the

external inductor. Usually the current ripple across the inductor is negligible and so to

estimate the conduction losses of the inductor, the average output current can be

considered.

The conduction losses of the switches depend also on the duty cycle of the application.

The RMS current flowing through the high side MOSFET is (Io)

2D, while the RMS current

flowing through the low side MOSFET is (Io)

2 (1-D). So, the total conduction losses of the

application are:

Equation 18

Where RON-HS and RON-LS are the series resistances of the high side and low side

MOSFETs respectively, and RL is the series resistance of the inductor. The conduction

losses due to the ESR of the input and output capacitors are usually negligible, particularly

when using ceramic caps (very low ESR). In any case, when the ESR values for these caps

are high, their conduction losses are:

Equation 19

Where

ΔI is the current ripple flowing through the choke and D the duty cycle of the

application. The conduction losses are particularly important at high current cause they

depend on its squared value.

6.2.2

Switching losses

The switching losses are due to the turning on and off of the internal high side MOSFET.

Equation 20

where TON and TOFF are the turn on and turn off times of the internal high side switch.

These are approximately in the range of 15 ns to 20 ns.This loss is important at high

frequency.

MOS

–

() R

–

() R

•

()

•

CIN COUT

–

()

•

() ESR

CIN

ΔI

--------

•

ESR

COUT

•

SWITCHING

•

OFF

()

------------------------------------

•

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