LTC1705 datasheet(21/28 Pages) LINER

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LTC1705

APPLICATIO S I FOR ATIO

Finally, choose a convenient resistor value for R1 (10k is

usually a good value). Now calculate the remaining values:

(K is a constant used in the calculations)

f = chosen crossover frequency

G = 10(GAIN/20) (this converts GAIN in dB to G in

absolute gain)

TYPE 2 Loop:

Tan

BOOST

fGKR

CC K

REF

OUT

REF

=+ °









=−

()

−

()

TYPE 3 Loop:

Tan

BOOST

fGR

CC K

fKR

REF

OUT

REF

=+ °









=−

()

−

()

CURRENT LIMIT PROGRAMMING

Programming current limit on the LTC1705 is straight-

forward. The IMAX pin sets the current limit by setting the

maximum allowable voltage drop across QB (the bottom

MOSFET) before the current limit circuit engages. The

voltage across QB is set by its on-resistance and the

current flowing in the inductor, which is the same as the

output current. The LTC1705 current limit circuit inverts

the negative voltage across QB before comparing it with

the voltage at IMAX, allowing the current limit to be set with

a positive voltage.

To set the current limit, calculate the expected voltage

drop across QB at the maximum desired current:

VPROG = (ILIMIT) (RDS(ON))

ILIMIT should be chosen to be quite a bit higher than the

expected operating current, to allow for MOSFET RDS(ON)

changes with temperature. Setting ILIMIT to 150% of the

maximum normal operating current is usually safe and will

adequately protect the power components if they are

chosen properly. Note that the ringing on the switch node

can cause error for the current limit threshold. This factor

will change depending on the layout and the components

used. VPROG is then programmed at the IMAX pin using the

internal 10

µA pull-up and an external resistor:

RIMAX = VPROG/10µA

The resulting value of RIMAX should be checked in an

actual circuit to ensure that the current circuit kicks in as

expected. MOSFET RDS(ON) specs are like horsepower

ratings in automobiles and should be taken with a grain of

salt. Circuits that use very low values for RIMAX (<10k)

should be checked carefully, since small changes in RIMAX

can cause large ILIMIT changes when the switch node

ringing makes up a large percentage of the total VPROG

value. If VPROG is set too low, the LTC1705 may fail to start

up.

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