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LTM4602

4602fa

Enable

The RUN/SS pin can be driven from logic as shown in

Figure 5. This function allows the LTM4602 to be turned

on or off remotely. The ON signal can also control the

sequence of the output voltage.

Figure 5. Enable Circuit with External Logic

Figure 6. Output Voltage Tracking with the LTC2923 Controller

Output Voltage Tracking

For the applications that require output voltage tracking,

several LTM4602 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 VOUT rising and

falling can be implemented with different sets of resistor

values. See the LTC2923 data sheet for more details.

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 LTM4602 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 LTM4602 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 inter-

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

switch connects the EXTVCC to internal 5V. Internal 5V is

supplied 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

connections 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

off when inductor current reverses. Therefore, the conduc-

RUN/SS

4602 F05

LTM4602

PGND

2N7002

SGND

VCC

VIN

RONB

VIN

RTB1

RTB2

RSET

49.9k

1.8V

3.3V

RTA2

RTA1

RONA

RAMPBUF

TRACK1

TRACK2

FB1

GATE

LTC2923

GND

4602 F06

RAMP

RSET

66.5k

1.5V

LTM4602

VIN

VOUT

LTM4602

DC/DC

VIN

VOUT

VOSET

FB2

SDO

STATUS

APPLICATIONS INFORMATION

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