LTC1702IGN datasheet(13/36 Pages) LINER

13 / 36 page

LTC1702

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 LTC1702

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 LTC1702 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 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 LTC1702 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 LTC1702 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 LTC1702 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 LTC1702 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 LTC1702, 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

LTC1702 remains in continuous mode. See the Inductor

Selection section for a detailed description of ripple

current.

As the output load current decreases in continuous mode,

the average current in the inductor will reach a point where

it drops below half the ripple current. At this point, the

inductor current will reverse during a portion of the

switching cycle, or begin to flow from the output back to

the input. This does not adversely affect regulation, but

does cause additional losses as a portion of the inductor

current flows back and forth through the resistive power

switches, giving away a little more power each time and

lowering the efficiency. There are some benefits to allow-

ing this reverse current flow: the circuit will maintain

regulation even if the load current drops below zero (the

load supplies current to the LTC1702) and the output

LTC1702IGN Datenblatt(PDF) 13 Page - Linear Technology

LTC1702IGN Datenblatt(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

Datenblatt Download

Link URL