LTC3621 datasheet(10/20 Pages) AD

10 / 20 page

LTC3644/LTC3644-2

Rev. 0

For more information www.analog.com

OPERATION

VIN Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient voltage spikes, the LTC3644 constantly

monitors the VINX pins for an overvoltage condition.

When VINX rises above 19V, the corresponding regulator

suspends operation by shutting off both power MOSFETs.

Once VINX drops below 18.6V, the regulator immediately

resumes normal operation. The regulators execute soft-

start function when exiting an overvoltage condition.

Low Supply Operation

To ensure that the regulators will operate properly, the

LTC3644incorporatesanundervoltagelockoutcircuitthat

shuts down all channels if SVIN drops below 2.25V. Once

SVIN rises above this lower limit, all switchers will resume

normal operation if their respective RUN pins are enabled.

However, the RDS(ON) of the top and bottom switch of each

channels will be slightly higher than that specified in the

electrical characteristics due to lack of gate drive. Refer

to graph of RDS(ON) versus VIN for more details.

Phase Selection

Channels1,2andchannels3,4oftheLTC3644canoperate

in phase or 180° out-of-phase (anti-phase) depending on

whether the PHASE pin is low or high, respectively. Anti-

phase generally reduces input voltage and current ripple.

Crosstalk between switch nodes SWx and components

or sensitive lines connected to FBx can sometimes cause

unstable switching waveforms and unexpectedly large

input and output voltage ripple.

Crosstalk can generally be avoided by carefully choosing

the phase shift such that the SW edges do not coincide.

Depending on the duty cycle of the two channels, choose

the phase option that keeps the SWx edges as far away

from each other as possible. However, there are often

situations where this is unavoidable, such as when all

channels are operating at near 50% duty cycle. In such

cases, the optimized phase shift can be set by modulating

the duty cycle of an external clock on the MODE/SYNC pin

(channel 1,2 edge synced to the rising edge of the external

clock, channel 3,4 edge synced to the falling edge of the

external clock), while keeping the PHASE pin voltage tied

to INTVCC. Figure 2 shows a 90° phase shift between

channels 1, 2 and channels 3,4. Table 1 shows the phase

options set by the PHASE and MODE/SYNC pins.

Figure 2. 90° Phase Shift Set by External Clock

Table 1. Phase Selection

No External CLK

External CLK

PHASE = 0

0 degrees phase shift

PHASE = INTVCC 180 degrees phase shift

Phase shift determined by

clock edges

Soft-Start

The LTC3644 has an internal 1.1ms soft-start ramp for

each channel. During soft-start operation, the switcher

operates in pulse-skipping mode regardless of the mode

programmed on the MODE/SYNC pin. Once the soft start

period is complete, the part will transition into the desired

mode of operation.

Regulators with Combined Power Stages

The LTC3644 can be configured as quad 1.25A outputs,

triple 2.5A/1.25A/1.25A outputs, dual 2.5A outputs using

onlyoneinductorperoutput,ordual3.75A/1.25Aoutputs.

ByconnectingVIN1,2andVIN3,4together,SW1,2andSW3,4

together, and connecting FB2 and FB3 to INTVCC,LTC3644

supports dual 2.5A outputs using only one inductor per

output.Evenmore,byconnectingVIN1,2,4together,SW1,2,4

together, and FB2, FB4 to INTVCC, LTC3644 supports

3.75A/1.25A outputs.

500ns/DIV

SW4

10V/DIV

SW1

10V/DIV

EXT CLK

5V/DIV

3644 F02

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