LTC1872 datasheet(15/36 Pages) LINER

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LTC1871

1871fe

APPLICATIONS INFORMATION

Boost Converter: 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 Mμ® cores. Actual core loss is independent of core

size for a ﬁxed inductor value, but is very dependent on

the inductance selected. As inductance increases, core

losses go down. Unfortunately, increased inductance

requires more turns of wire and therefore, copper losses

will increase. Generally, there is a tradeoff between core

losses and copper losses that needs to be balanced.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper losses and preventing saturation.

Ferrite core material saturates “hard,” meaning that the

inductance collapses rapidly when the peak design current

is exceeded. This results in an abrupt increase in inductor

ripple current and consequently, output voltage ripple. Do

not allow the core to saturate!

Molypermalloy (from Magnetics, Inc.) is a very good,

low cost core material for toroids, but is more expensive

than ferrite. A reasonable compromise from the same

manufacturer is Kool Mμ.

Boost Converter: Power MOSFET Selection

The power MOSFET serves two purposes in the LTC1871:

it represents the main switching element in the power path,

and its RDS(ON) represents the current sensing element

for the control loop. Important parameters for the power

MOSFET include the drain-to-source breakdown voltage

(BVDSS), the threshold voltage (VGS(TH)), the on-resistance

(RDS(ON))versusgate-to-sourcevoltage,thegate-to-source

and gate-to-drain charges (QGS and QGD, respectively),

the maximum drain current (ID(MAX)) and the MOSFET’s

thermal resistances (RTH(JC) and RTH(JA)).

The gate drive voltage is set by the 5.2V INTVCC low drop

regulator. Consequently, logic-level threshold MOSFETs

should be used in most LTC1871 applications. If low input

voltage operation is expected (e.g., supplying power from

a lithium-ion battery or a 3.3V logic supply), then sublogic-

level threshold MOSFETs should be used.

Pay close attention to the BVDSS speciﬁcations for the

MOSFETs relative to the maximum actual switch voltage in

the application. Many logic-level devices are limited to 30V

or less, and the switch node can ring during the turn-off of

the MOSFET due to layout parasitics. Check the switching

waveforms of the MOSFET directly across the drain and

source terminals using the actual PC board layout (not

just on a lab breadboard!) for excessive ringing.

During the switch on-time, the control circuit limits the

maximum voltage drop across the power MOSFET to about

150mV (at low duty cycle). The peak inductor current

is therefore limited to 150mV/RDS(ON). The relationship

between the maximum load current, duty cycle and the

RDS(ON) of the power MOSFET is:

RDS(ON) VSENSE(MAX) •

1– DMAX

•IO(MAX) • T

The VSENSE(MAX) term is typically 150mV at low duty

cycle, and is reduced to about 100mV at a duty cycle of

92% due to slope compensation, as shown in Figure 10.

The ρT term accounts for the temperature coefﬁcient of

the RDS(ON) of the MOSFET, which is typically 0.4%/°C.

Figure 11 illustrates the variation of normalized RDS(ON)

over temperature for a typical power MOSFET.

DUTY CYCLE

100

150

0.8

1871 F10

0.2

0.4

0.5

1.0

200

Figure 10. Maximum SENSE Threshold Voltage vs Duty Cycle

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