LTC3418 Ver la hoja de datos (PDF) - Linear Technology

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LTC3418

8A, 4MHz, Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC3418

APPLICATIONS INFORMATION

VOUT

VFB

LTC3418

SGND

R2

R1

3418 F01

Figure 1. Setting the Output Voltage

Burst Clamp Programming

If the voltage on the SYNC/MODE pin is less than VIN by

1V, Burst Mode operation is enabled. During Burst Mode

operation, the voltage on the SYNC/MODE pin determines

the burst clamp level, which sets the minimum peak induc-

tor current, IBURST, for each switching cycle. A graph show-

ing the relationship between the minimum peak inductor

current and the voltage on the SYNC/MODE pin can be

found in the Typical Performance Characteristics section.

In the graph, VBURST is the voltage on the SYNC/MODE

pin. IBURST can only be programmed in the range of 0A

to 10A. For values of VBURST less than 0.4V, IBURST is set

at 0A. As the output load current drops, the peak inductor

currents decrease to keep the output voltage in regulation.

When the output load current demands a peak inductor

current that is less than IBURST, the burst clamp will force

the peak inductor current to remain equal to IBURST regard-

less of further reductions in the load current. Since the

average inductor current is greater than the output load

current, the voltage on the ITH pin will decrease. When

the ITH voltage drops to 350mV, sleep mode is enabled

in which both power MOSFETs are shut off and switching

action is discontinued to minimize power consumption.

All circuitry is turned back on and the power MOSFETs

begin switching again when the output voltage drops out

of regulation. The value for IBURST is determined by the

desired amount of output voltage ripple. As the value of

IBURST increases, the sleep period between pulses and the

output voltage ripple increase. The burst clamp voltage,

VBURST, can be set by a resistor divider from the VFB pin

to the SGND pin as shown in the Typical Application on

the front page of this data sheet.

Pulse skipping, which is a compromise between low output

voltage ripple and efficiency during low load current opera-

tion, can be implemented by connecting the SYNC/MODE

pin to ground. This sets IBURST to 0A. In this condition, the

peak inductor current is limited by the minimum on-time

of the current comparator; and the lowest output voltage

ripple is achieved while still operating discontinuously.

During very light output loads, pulse skipping allows only

a few switching cycles to be skipped while maintaining

the output voltage in regulation.

Voltage Tracking

The LTC3418 allows the user to program how its output

voltage ramps during start-up by means of the TRACK

pin. Through this pin, the output voltage can be set up to

either track coincidentally or ratiometrically follow another

output voltage as shown in Figure 2. If the voltage on the

TRACK pin is less than 0.8V, voltage tracking is enabled.

During voltage tracking, the output voltage regulates to

the tracking voltage through a resistor divider network.

VOUT2

VOUT1

TIME

3418 F02a

Figure 2a. Coincident Tracking

VOUT2

VOUT1

TIME

3418 F02a

Figure 2b. Ratiometric Sequencing

3418fb

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