LTC1703 datasheet(13/36 Pages) LINER

13 / 36 page

LTC1703

is limited to 10%. The maximum duty cycle limit increases

linearly between 1V and 2.5V, reaching its final value of

90% when RUN/SS is above 2.5V. Somewhere before this

point, the feedback amplifier will assume control of the

loop and the output will come into regulation. When RUN/

SS rises to 0.5V below VCC, the MIN feedback comparator

is enabled, and the LTC1703 is in full operation.

CURRENT LIMIT

The LTC1703 includes an onboard current limit circuit that

limits the maximum output current to a user-programmed

level. It works by sensing the voltage drop across QB

during the time that QB is on and comparing that voltage

to a user-programmed voltage at IMAX. Since QB looks like

a low value resistor during its on-time, the voltage drop

across it is proportional to the current flowing in it. In a

buck converter, the average current in the inductor is equal

to the output current. This current also flows through QB

during its on-time. Thus, by watching the voltage across

QB, the LTC1703 can monitor the output current.

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 LTC1703

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 LTC1703 compares the two inputs and

begins limiting the output current when the magnitude of

the negative voltage at the SW pin is greater than the

voltage at IMAX.

The current limit detector is connected to an internal gm

amplifier that pulls a current from the RUN/SS pin propor-

tional to the difference in voltage magnitudes between the

SW and IMAX pins. This current begins to discharge the

soft-start capacitor at RUN/SS, reducing the duty cycle

and controlling the output voltage until the output current

drops below the limit. The soft-start capacitor needs to

move a fair amount before it has any effect on the duty

cycle, adding a delay until the current limit takes effect

(Figure 4). This allows the LTC1703 to experience brief

overload conditions without affecting the output voltage

regulation. The delay also acts as a pole in the current limit

loop to enhance loop stability. Larger overloads cause the

soft-start capacitor to pull down quickly, protecting the

output components from damage. The current limit gm

amplifier includes a clamp to prevent it from pulling RUN/

SS below 0.5V and shutting off the device.

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

limiting the accuracy obtainable from the LTC1703 current

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

parasitics can add to the apparent current, causing the

loop to engage early. The LTC1703 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 value for RIMAX.

DISCONTINUOUS/Burst Mode OPERATION

Theory of operation

The LTC1703 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 LTC1703, 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

APPLICATIO S I FOR ATIO

LTC1703 Datasheet(PDF) 13 Page - Linear Technology

LTC1703 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL