LTC7803 datasheet(21/42 Pages) AD

21 / 42 page

LTC7811

Rev. 0

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APPLICATIONS INFORMATION

To ensure that the application will deliver full load cur-

rent over the full operating temperature range, choose

the minimum value for VSENSE(MAX) in the Electrical

Characteristics table.

Next, determine the DCR of the inductor. When provided,

use the manufacturer’s maximum value, usually given at

20°C. Increase this value to account for the temperature

coefficient of copper resistance, which is approximately

0.4%/°C. A conservative value for TL(MAX) is 100°C. To

scale the maximum inductor DCR to the desired sense

resistor value, use the divider ratio:

RD =

RSENSE(EQUIV)

DCRMAX atTL(MAX)

C1 is usually selected to be in the range of 0.1μF to 0.47μF.

This forces R1||R2 to around 2k, reducing error that might

have been caused by the SENSE+ pin’s ≈1μA current.

The target equivalent resistance R1||R2 is calculated from

the nominal inductance, C1 value, and DCR:

R1!R2 =

(DCR at 20°C) • C1

The sense resistor values are:

R1=

R1!R2

; R2 =

R1•RD

1−RD

The maximum power loss in R1 is related to duty cycle.

For the buck controllers, the maximum power loss occurs

in continuous mode at the maximum input voltage:

PLOSS R1=

(VIN(MAX) − VOUT)• VOUT

For the boost controller, the maximum power loss occurs

in continuous mode at VIN = VOUT/2:

PLOSS R1=

(VOUT(MAX) − VIN)• VIN

Ensure that R1 has a power rating higher than this value.

If high efficiency is necessary at light loads, consider this

power loss when deciding whether to use DCR sensing

or sense resistors. Light load power loss can be mod-

estly higher with a DCR network than with a sense resis-

tor, due to the extra switching losses incurred through

R1. However, DCR sensing eliminates a sense resistor,

reduces conduction losses and provides higher efficiency

at heavy loads. Peak efficiency is about the same with

either method.

Boost Channel Low-Side Current Sensing

The boost channel current sense resistor can alternatively

be connected in series with the source of the bottom

switch as shown in Figure 2c. In this configuration, the

boost channel input voltage and output voltage are both

limited only by external components. Be aware that the

regulator may exhibit a slower transient response since

the inductor current cannot be monitored when the bottom

switch is off. Additionally, the SENSE3+ and SENSE3- pins

must be differentially filtered to keep switching noise from

corrupting the current sense signal. Typically, resistors

in the range of 50Ω to 500Ω should be placed series with

the SENSE pins, and a 1nF capacitor should be placed

between SENSE3+ and SENSE3-, as close as possible to

the LTC7811. The IMON3 boost channel current monitor

feature is not compatible with low-side current sensing.

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