LTC3619BEDD(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC3619BEDD
(Rev.:RevA)

400mA/800mA Synchronous Step-Down DC/DC with Average Input Current Limit

Linear Technology 

LTC3619B

Operation

The LTC3619B uses a constant-frequency, current mode

architecture. The operating frequency is set at 2.25MHz.

Both channels share the same clock and run in-phase.

The output voltage is set by an external resistor divider

returned to the VFB pins. An error amplifier compares the

divided output voltage with a reference voltage of 0.6V and

regulates the peak inductor current accordingly.

The LTC3619B continuously monitors the input current

of both channels. When the sum of the currents of both

channels exceeds the programmed input current limit set

by an external resistor, RLIM, channel 2 is current limited

while channel 1 remains regulated.

Main Control Loop

During normal operation, the top power switch (P-channel

MOSFET) is turned on at the beginning of a clock cycle

when the VFB voltage is below the reference voltage. The

current into the inductor and the load increases until the

peak inductor current (controlled by ITH) is reached. The

RS latch turns off the synchronous switch and energy

stored in the inductor is discharged through the bottom

switch (N-channel MOSFET) into the load until the next

clock cycle begins, or until the inductor current begins to

reverse (sensed by the IRCMP comparator).

The peak inductor current is controlled by the internally

compensated ITH voltage, which is the output of the er-

ror amplifier. This amplifier regulates the VFB pin to the

internal 0.6V reference by adjusting the peak inductor

current accordingly.

When the input current limit is engaged, the peak inductor

current will be lowered, which then reduces the switching

duty cycle and VOUT. This allows the input voltage to stay

regulated when its programmed current output capability

is met.

Light Load Operation

The LTC3619B will automatically transition from continuous

operation to the pulse-skipping operation when the load

current is low. The inductor current is not fixed during the

pulse-skipping mode which allows the LTC3619B to switch

at constant-frequency down to very low currents, where it

will begin skipping pulses to maintain output regulation.

This mode of operation exhibits low output ripple as well

as low audio noise and reduced RF interference while

providing reasonable low current efficiency.

Dropout Operation

When the input supply voltage decreases toward the

output voltage the duty cycle increases to 100%, which

is the dropout condition. In dropout, the PMOS switch is

turned on continuously with the output voltage being equal

to the input voltage minus the voltage drops across the

internal P-channel MOSFET and the inductor.

An important design consideration is that the RDS(ON) of the

P-channel switch increases with decreasing input supply

voltage (see the Typical Performance Characteristics sec-

tion). Therefore, the user should calculate the worst-case

power dissipation when the LTC3619B is used at 100% duty

cycle with low input voltage (see Thermal Considerations

in the Applications Information section).

Soft-Start

In order to minimize the inrush current on the input bypass

capacitor, the LTC3619B slowly ramps up the output volt-

age during start-up. Whenever the RUN1 or RUN2 pin is

pulled high, the corresponding output will ramp from zero

to full-scale over a time period of approximately 750µs. This

prevents the LTC3619B from having to quickly charge the

output capacitor and thus supplying an excessive amount

of instantaneous current.

When the output is loaded heavily, for example, with mil-

lifarad of capacitance, it may take longer than 750µs to

charge the output to regulation. If the output is still low

after the soft-start time, the LTC3619B will try to quickly

charge the output capacitor. In this case, the input current

limit (after it engages) can prevent excessive amount of

instantaneous current that is required to quickly charge

the output. See the Channel 2 Start-Up from Shutdown

curve in the Typical Performance Characteristics section.

After input current limit is engaged, the output slowly

ramps up to regulation while limited by its 500mA of

input current.

3619bfa



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