LTC1702ACGN#PBF Datasheet LTC1702ACGN#TRPBF, LTC1702AIGN#TRPBF, LTC1702AIGN#PBF | P13-P15

LTC1702A

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APPLICATIONS INFORMATION

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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 I

MAX

pin. The

MAX

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

user to set the voltage at I

MAX

with a single resistor, R

IMAX

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 I

MAX

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 0.01µF, 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 0.01µF, 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 R

DS(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 R

IMAX

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

LTC1702A

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LTC1702A 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 LTC1702A) and the output

ripple voltage and frequency remain constant at all loads,

easing filtering requirements. Circuits that take advantage

of this behavior can force the LTC1702A to operate in

continuous mode at all loads by tying the FCB (Force

Continuous Bar) pin to ground.

Discontinuous Mode

To minimize the efficiency loss due to reverse current flow

at light loads, the LTC1702A switches to a second mode of

APPLICATIONS INFORMATION

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Figure 6. Ringing at SW Causes Discontinuous

Comparator to Trip Early

Figure 5a. Continuous Mode

Figure 5b. Discontinuous Mode

TIME

50ns

BLANK

TIME

DISCONTINUOUS

COMPARATOR

TURNS OFF BG

1702A F06

TIME

operation: discontinuous mode (Figure 5b). In discontinu-

ous mode, the LTC1702A detects when the inductor

current approaches zero and turns off QB for the remain-

der of the switch cycle. During this time, the voltage at the

SW pin will float about V

OUT

, the voltage across the

inductor will be zero, and the inductor current remains

zero until the next switching cycle begins and QT turns on

again. This prevents current from flowing backwards in

QB, eliminating that power loss term. It also reduces the

ripple current in the inductor as the output current ap-

proaches zero.

The LTC1702A detects that the inductor current has reached

zero by monitoring the voltage at the SW pin while QB is

on. Since QB acts like a resistor, SW should ideally be right

at 0V when the inductor current reaches zero. In reality, the

SW node will ring to some degree immediately after it is

switched to ground by QB, causing some uncertainty as to

the actual moment the average current in QB goes to zero.

The LTC1702A minimizes this effect by ignoring the SW

node for a fixed 50ns after QB turns on when the ringing

is most severe, and by including a few millivolts offset in

the comparator that monitors the SW node. Despite these

precautions, some combinations of inductor and layout

parasitics can cause the LTC1702A to enter discontinuous

mode erratically. In many cases, the time that QB turns off

will correspond to a peak in the ringing waveform at the

SW pin (Figure 6). This erratic operation isn’t pretty, but

retains much of the efficiency benefit of discontinuous

mode and maintains regulation at all times.

TIME

RIPPLE

AVERAGE

INDUCTOR CURRENT

1702A F05a

TIME

RIPPLE

AVERAGE

INDUCTOR CURRENT

1702A F05b

LTC1702A

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Burst Mode Operation

Discontinuous mode removes the resistive loss drop term

in QB, but the LTC1702A is still switching QT and QB on

and off once a cycle. Each time an external MOSFET is

turned on, the internal driver must charge its gate to V

Each time it is turned off, that charge is lost to ground. At

the high switching frequencies that the LTC1702A oper-

ates at, the charge lost to the gates can add up to tens of

milliamps from V

. As the load current continues to drop,

this quickly become the dominant power loss term, reduc-

ing efficiency once again.

Once again, the LTC1702A switches to a new mode to

minimize efficiency loss: Burst Mode operation. As the

circuit goes deeper and deeper into discontinuous mode,

the QB on-time reduces. When the load drops to the point

that the output begins to rise, the LTC1702A senses this

rise and shuts both QT and QB off completely, skipping

several switching cycles until the output falls back into

range. It then resumes switching in discontinuous mode

and the burst sequence repeats. The total deviation from

the regulated output is within the 1% regulation tolerance

of the LTC1702A.

In Burst Mode operation, both resistive loss and switching

loss are minimized while keeping the output in regulation.

As the load current falls to zero in Burst Mode operation,

the most significant loss term becomes the 3mA quiescent

current drawn by each side of the LTC1702A—usually

much less than the minimum load current in a typical low

voltage logic system. Burst Mode operation maximizes

efficiency at low load currents, but can cause low fre-

quency ripple in the output voltage as the cycle-skipping

circuitry switches on and off.

FCB Pin

In some circumstances, it is desirable to control or disable

discontinuous and Burst Mode operations. The FCB (Force

Continuous Bar) pin allows the user to do this. When the

FCB pin is high, the LTC1702A is allowed to enter discon-

tinuous and Burst Mode operations at either side as

required. If FCB is taken low, discontinuous and Burst

Mode operations are disabled and both sides of the

LTC1702A run in continuous mode regardless of load.

APPLICATIONS INFORMATION

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This does not affect output regulation but does reduce

efficiency at low output currents. The FCB pin threshold is

specified at 0.8V ±50mV, and includes 20mV of hyster-

esis, allowing it to be used as a precision small-signal

comparator.

Paralleling Outputs

Synchronous regulators (like the LTC1702A) are known

for their bullheadedness when their outputs are paralleled

with other regulators. In particular, a synchronous regu-

lator paralleled with another regulator whose output is

slightly higher (perhaps just by millivolts) will happily sink

amps of current attempting to pull its own output back

down to what it thinks is the right value.

The LTC1702A discontinuous mode allows it to be paral-

leled with another regulator without fighting. A typical

system might use the LTC1702A as a primary regulator

and a small LDO as a backup regulator to keep SRAM alive

when the main power is off. When the LTC1702A is shut

down (by pulling RUN/SS to ground), both QT and QB turn

off and the output goes into a high impedance state,

allowing the smaller regulator to support the output volt-

age. However, if the LTC1702A is powered back up in

continuous mode, it will begin a soft-start cycle with a low

duty cycle, pulling the output down and corrupting the

data stored in SRAM. The solution is to tie FCB high,

allowing the device to start in discontinuous mode. Any

reverse current flow in QB will trip the discontinuous mode

circuitry, preventing the LTC1702A from pulling down the

output. The Typical Applications section shows an ex-

ample of such a circuit.

OVERVOLTAGE FAULT

The LTC1702A includes a single overvoltage fault flag for

both channels: FAULT. FAULT is an open-drain output

with an internal 10µA pull-up. If either FB pin rises more

than 15% above the nominal 800mV value for more than

25µs, the overvoltage comparator will trip, setting an

internal latch. This latch releases the pull-down at FAULT,

allowing the 10µA pull-up to take it high. When FAULT

goes high, the LTC1702A stops all switching, turns both

QB (bottom synchronous) MOSFETs on continuously and

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LTC1702ACGN#PBF

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Manufacturer:

Analog Devices / Linear Technology

Description:

Switching Voltage Regulators 2x 550kHz Sync 2-PhSw Reg Cntr

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LTC1702ACGN#PBF

LTC1702ACGN#PBF

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