LTC1702AIGN datasheet(24/36 Pages) LINER

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LTC1702A

1702afa

chosen properly. The CF term is an approximate factor that

corrects for errors caused by ringing on the switch node

(illustrated in Figure 6). This correction factor will change

depending on the layout and the components used, but

–10mV is usually a good starting point. However, to

provide adequate margin and to accommodate for offsets

and external variations, it is recommended that VPROG be

calculated with CF = –10

± 50mV. VPROG is then pro-

grammed at the IMAX pin using the internal 10µA pull-up

and an external resistor:

RILIM = VPROG/10µA

The resulting value of RILIM should be checked in an actual

circuit to ensure that the ILIM 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 ILIM changes. If VPROG is set too low, the

LTC1702A may fail to start up.

Accuracy Trade-Offs

The VDS sensing scheme used in the LTC1702A is not

particularly accurate, primarily due to uncertainty in the

RDS(ON) from MOSFET to MOSFET. A second error term

arises from the ringing present at the SW pin, which

causes the VDS to look larger than (ILOAD)(RDS(ON)) at the

beginning of QB’s on-time. These inaccuracies do not

prevent the LTC1702A current limit circuit from protecting

itself and the load from damaging overcurrent conditions,

but they do prevent the user from setting the current limit

to a tight tolerance if more than one copy of the circuit is

being built. The 50% factor in the current setting equation

above reflects the margin necessary to ensure that the

circuit will stay out of current limit at the maximum normal

load, even with a hot MOSFET that is running quite a bit

higher than its RDS(ON) spec.

FCB OPERATION/SECONDARY WINDINGS

The FCB pin can be used in conjunction with a secondary

winding on one side of the LTC1702A to generate a third

regulated voltage output. This output can be directly

regulated at the FCB pin. In theory, a fourth output could

APPLICATIONS INFORMATION

Type 3 Loop:

Tan

BOOST

CC K

REF

OUT

REF

=+ °

⎛

⎝⎜

⎞

⎠⎟

πƒ

()

πƒ

= ()

πƒ

()

–

CURRENT LIMIT PROGRAMMING

Programming the current limit on the LTC1702A 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 LTC1702A current limit circuit inverts

the voltage at IMAX before comparing it with the negative

voltage across QB, 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:

PROG

LIM

DS ON

()

()+

()

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

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

changes with temperature. Setting ILIM to 150% of the

maximum normal operating current is usually safe and will

adequately protect the power components if they are

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