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LTM4600

4600fd

For more information www.linear.com/LTM4600

Output Voltage Tracking

For the applications that require output voltage tracking,

several LTM4600 modules can be programmed by the

power supply tracking controller such as the LTC2923.

Figure 6 shows a typical schematic with LTC2923. Coin-

cident, ratiometric and offset tracking for VO rising and

falling can be implemented with different sets of resistor

values. See the LTC2923 data sheet for more details.

Figure 6. Output Voltage Tracking with the LTC2923 Controller

VCC

VIN

RONB

VIN

RTB1

RTB2

49.9k

1.8V

3.3V

RTA2

RTA1

RONA

RAMPBUF

TRACK1

TRACK2

FB1

GATE

LTC2923

GND

4600 F06

RAMP

66.5k

1.5V

LTM4600

VIN

VOUT

LTM4600

DC/DC

VIN

VOUT

VOSET

FB2

SDO

STATUS

EXTVCC Connection

An internal low dropout regulator produces an internal 5V

supply that powers the control circuitry and FET drivers.

Therefore, if the system does not have a 5V power rail,

the LTM4600 can be directly powered by VIN. The gate

driver current through LDO is about 18mA. The internal

LDO power dissipation can be calculated as:

PLDO_LOSS = 18mA•(VIN – 5V)

The LTM4600 also provides an external gate driver volt-

age pin EXTVCC. If there is a 5V rail in the system, it is

recommended to connect EXTVCC pin to the external 5V

rail. Whenever the EXTVCC pin is above 4.7V, the internal

5V LDO is shut off and an internal 50mA P-channel switch

connects the EXTVCCtointernal5V.Internal5Vissupplied

from EXTVCC until this pin drops below 4.5V. Do not apply

more than 6V to the EXTVCC pin and ensure that EXTVCC

< VIN. The following list summaries the possible connec-

tions for EXTVCC:

1. EXTVCC grounded. Internal 5V LDO is always powered

from the internal 5V regulator.

2. EXTVCC connected to an external supply. Internal LDO

is shut off. A high efficiency supply compatible with the

MOSFET gate drive requirements (typically 5V) can im-

prove overall efficiency. With this connection, it is always

required that the EXTVCC voltage can not be higher than

VIN pin voltage.

Discontinuous Operation and FCB Pin

The FCB pin determines whether the internal bottom

MOSFET remains on when the inductor current reverses.

There is an internal 4.75k pull-down resistor connecting

this pin to ground. The default light load operation mode

is forced continuous (PWM) current mode. This mode

provides minimum output voltage ripple.

In the application where the light load efficiency is im-

portant, tying the FCB pin above 0.6V threshold enables

discontinuous operation where the bottom MOSFET turns

offwheninductorcurrentreverses.Therefore,theconduc-

tionlossisminimizedandlightloadefficiencyisimproved.

The penalty is that the controller may skip cycle and the

output voltage ripple increases at light load.

Paralleling Operation with Load Sharing

TwoormoreLTM4600modulescanbeparalleledtoprovide

higher than 10A output current. Figure 7 shows the neces-

saryinterconnectionbetweentwoparalleledmodules.The

OPTI-LOOP® current mode control ensures good current

sharing among modules to balance the thermal stress.

The new feedback equation for two or more LTM4600s

in parallel is:

VOUT = 0.6V •

100k

+RSET

RSET

where N is the number of LTM4600s in parallel.

applicaTions inForMaTion

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