LTC1702IGN datasheet(24/36 Pages) LINER

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LTC1702

Accuracy Trade-Offs

The VDS sensing scheme used in the LTC1702 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 LTC1702 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 LTC1702 to generate a third

regulated voltage output. This output can be directly

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

be added, either unregulated or with additional external

circuitry at the FCB pin.

The extra auxiliary output is taken from a second winding

on the core of the inductor on one channel, converting it

into a transformer (Figure 13). The auxiliary output voltage

is set by the main output voltage and the turns ratio of the

extra winding to the primary winding. Load regulation at

the auxiliary output will be relatively good as long as the

main output is running in continuous mode. As the load on

the main channel drops and the LTC1702 switches to

discontinuous or Burst Mode operation, the auxiliary

output will not be able to maintain regulation, especially if

the load at the auxiliary output remains heavy.

To avoid this, the auxiliary output voltage can be divided

down with a conventional feedback resistor string with the

divided auxiliary output voltage fed back to the FCB pin

(Figure 13). The FCB pin threshold is trimmed to 800mV

APPLICATIONS INFORMATION

CURRENT LIMIT PROGRAMMING

Programming the current limit on the LTC1702 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 LTC1702 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

ILIM

DS ON

()

()+

()

100

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

chosen properly. The 100mV term is an approximate

factor that corrects for errors caused by ringing on the

switch node (illustrated in Figure 6). This factor will

change depending on the layout and the components

used, but 100mV is usually a good starting point. VDROP is

then programmed 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 (< 20k) should

be checked carefully, since small changes in RIMAX can

cause large ILIM changes when the 100mV correction

factor makes up a large percentage of the total VPROG

value. If VPROG is set too low, the LTC1702 may fail to start

up.

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