LTC1702AIGN datasheet(13/36 Pages) LINER

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LTC1702A

1702afa

APPLICATIONS INFORMATION

Any time QB is on and the current flowing to the output is

reasonably large, the SW node at the drain of QB will be

somewhat negative with respect to PGND. The LTC1702A

senses this voltage and inverts it to allow it to compare the

sensed voltage with a positive voltage at the IMAX pin. The

IMAX pin includes a trimmed 10µA pull-up, enabling the

user to set the voltage at IMAX with a single resistor, RIMAX,

to ground. The LTC1702A compares the two inputs and

limits the output current when the magnitude of the

negative voltage at the SW pin is greater than the voltage

at IMAX.

The LTC1702A current limit detector connects to an inter-

nal circuit that discharges the soft-start capacitor quickly

if activated. The soft-start sink current depends on the

overdrive presented to the current limit detector. If the

regulator output is short circuited, the soft-start sink

current is typically 1mA. With a soft-start capacitor less

than 0.01

µF, the current-limit detector compensation is

slightly under damped. With an instantaneous short-

circuit, the current-limit detector fires and the soft-start

capacitor rapidly discharges to ground. Depending on the

current limit behavior of the regulator powering the

LTC1702A, current in the switch inductor of the shorted

output can exceed 100A before the soft-start capacitor is

discharged. This high input current surge also pulls down

the input voltage to the LTC1702A and all other circuits

connected to the LTC1702A input. After the soft-start

capacitor is discharged, the output is turned off and the

LTC1702A begins a new soft-start cycle. If the over current

condition persists, the current limit detector fires again

and the cycle repeats. With a soft-start capacitor greater

than 0.01

µF, the current limit detector compensation is

slightly over damped. With an instantaneous short-circuit

condition, the soft-start capacitor is again quickly dis-

charged. However, the SS pin does not pull to ground but

only discharges until the current limit loop is in regulation.

Short-circuit current is limited to the programmed current

limit level. In this scenario, the LTC1702A regulates in

current limit and does not rerun soft-start cycles. There-

fore, the user must balance the trade off between soft-start

time required for the system versus desired current limit

behavior. Consult the Current Limit Programming section

for more information.

Note that even brief overcurrent excursions will fire the

current limit circuit, quickly removing power to the load. If

the ability to withstand larger overcurrent surges without

tripping off is desired, consider using the pin-compatible

LTC1702, which provides this capability in exchange for

increased stress on the power MOSFETs.

Power MOSFET RDS(ON) varies from MOSFET to MOSFET,

limiting the accuracy obtainable from the LTC1702A cur-

rent limit loop. Additionally, ringing on the SW node due to

parasitics can add to the apparent current, causing the

loop to engage early. The LTC1702A current limit is

designed primarily as a disaster prevention, “no blow up”

circuit, and is not useful as a precision current regulator.

It should typically be set around 50% above the maximum

expected normal output current to prevent component

tolerances from encroaching on the normal current range.

See the Current Limit Programming section for advice on

choosing a valve for RIMAX.

DISCONTINUOUS/Burst Mode OPERATION

Theory of operation

The LTC1702A switching logic has three modes of opera-

tion. Under heavy loads, it operates as a fully synchro-

nous, continuous conduction switching regulator. In this

mode of operation (“continuous” mode), the current in the

inductor flows in the positive direction (toward the output)

during the entire switching cycle, constantly supplying

current to the load. In this mode, the synchronous switch

(QB) is on whenever QT is off, so the current always flows

through a low impedance switch, minimizing voltage drop

and power loss. This is the most efficient mode of opera-

tion at heavy loads, where the resistive losses in the power

devices are the dominant loss term.

Continuous mode works efficiently when the load current

is greater than half of the ripple current in the inductor. In

a buck converter like the LTC1702A, the average current in

the inductor (averaged over one switching cycle) is equal

to the load current. The ripple current is the difference

between the maximum and the minimum current during a

switching cycle (see Figure 5a). The ripple current

depends on inductor value, clock frequency and output

voltage, but is constant regardless of load as long as the

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