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LTM4605

4605fa

APPLICATIONS INFORMATION

output voltage falls by more than 70%, then the maximum

output current is progressively lowered to about 30% of

its full current limit value for boost mode and about 40%

for buck mode.

Standby Mode (STBYMD)

The standby mode (STBYMD) pin provides several choices

for start-up and standby operational modes. If the pin is

pulled to ground, the SS pin is internally pulled to ground,

preventing start-up and thereby providing a single control

pin for turning off the controller. If the pin is left open or

decoupled with a capacitor to ground, the SS pin is internally

provided with a starting current, permitting external control

for turning on the controller. If the pin is connected to a

voltage greater than 1.25V, the internal regulator (INTVCC)

will be on even when the controller is shut down (RUN

pin voltage <1.6V). In this mode, the onboard 6V linear

regulator can provide power to keep-alive functions such

as a keyboard controller.

INTVCC and EXTVCC

An internal P-channel low dropout regulator produces 6V

at the INTVCC pin from the VIN supply pin. INTVCC powers

the control chip and internal circuitry within the module.

The LTM4605 also provides the external supply voltage pin

EXTVCC. When the voltage applied to EXTVCC rises above

5.7V, the internal regulator is turned off and an internal

switch connects the EXTVCC pin to the INTVCC pin thereby

supplying internal power. The switch remains close as long

as the voltage applied to EXTVCC remains above 5.5V. This

allows the MOSFET driver and control power to be derived

from the output when (5.7V < VOUT < 7V) and from the

internal regulator when the output is out of regulation (start-

up, short-circuit). If more current is required through the

EXTVCC switch than is speciﬁed, an external Schottky diode

can be interposed between the EXTVCC and INTVCC pins.

Ensure that EXTVCC ≤ VIN.

The following list summarizes the three possible connec-

tions for EXTVCC:

1. EXTVCC left open (or grounded). This will cause INTVCC

to be powered from the internal 6V regulator at the cost

of a small efﬁciency penalty.

2. EXTVCC connected directly to VOUT (5.7V < VOUT <

7V). This is the normal connection for a 6V regulator and

provides the highest efﬁciency.

3. EXTVCC connected to an external supply. If an external

supply is available in the 5.5V to 7V range, it may be

used to power EXTVCC provided it is compatible with the

MOSFET gate drive requirements.

Thermal Considerations and Output Current Derating

In different applications, the LTM4605 operates in a variety

of thermal environments. The maximum output current is

limited by the environmental thermal condition. Sufﬁcient

cooling should be provided to ensure reliable operation.

Whenthecoolingislimited,properoutputcurrentde-ratingis

necessary, considering ambient temperature, airﬂow, input/

output condition, and the need for increased reliability.

The power loss curves in Figures 5 and 6 can be used

in coordination with the load current derating curves in

Figures 7 to 12 for calculating an approximate

θJA for

the module. Column designation delineates between no

heatsink, and a BGA heatsink. Each of the load current

derating curves will lower the maximum load current as

a function of the increased ambient temperature to keep

the maximum junction temperature of the power module

at 115°C maximum. This will allow a safe margin to work

at the maximum operating temperature below 125°C.

Each of the derating curves and the power loss curve that

corresponds to the correct output voltage can be used to

solve for the approximate

θJA of the condition. A complete

explanation of the thermal characteristics is provided in

the thermal application note for the LTM4605.

DESIGN EXAMPLES

Buck Mode Operation

As a design example, use input voltage VIN = 12V to 20V,

VOUT = 12V and f = 400kHz.

Set the PLLFLTR pin at 2.4V or more for 400kHz frequency

and connect FCB to ground for continuous current mode

operation. If a divider is used to set the frequency as shown

in Figure 14, the bottom resistor R3 is recommended not

to exceed 1k.

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