LTM4609EVPBF datasheet(12/24 Pages) LINER

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LTM4609

4609fa

APPLICATIONS INFORMATION

In buck mode, the allowed maximum average load cur-

rent is:

IOUT(MAX,BUCK) =

130mV

RSENSE

ΔIL

The maximum current sensing RSENSE value for the boost

mode is:

RSENSE(MAX,BOOST) =

2 •160mV • VIN

2•IOUT(MAX,BOOST) •VOUT +ΔIL •VIN

The maximum current sensing RSENSE value for the buck

mode is:

RSENSE(MAX,BUCK) =

2 •130mV

2•IOUT(MAX,BUCK) – ΔIL

A 20% to 30% margin on the calculated sensing resistor is

usually recommended. Please refer to Table 3 for the rec-

ommended sensing resistors for different applications.

Soft-Start

The SS pin provides a means to soft-start the regulator.

A capacitor on this pin will program the ramp rate of the

output voltage. A 1.7μA current source will charge up the

external soft-start capacitor. This will control the ramp of

the internal reference and the output voltage. The total

soft-start time can be calculated as:

tSOFTSTART =

2.4V •CSS

1.7µA

When the RUN pin falls below 1.6V, then soft-start pin is

reset to allow for proper soft-start control when the regula-

tor is enabled again. Current foldback and force continuous

mode are disabled during the soft-start process. The soft-

start function can also be used to control the output ramp

up time, so that another regulator can be easily tracked.

Do not apply more than 6V to the SS pin.

Run Enable

The RUN pin is used to enable the power module. The pin

can be driven with a logic input, and not exceed 6V.

The RUN pin can also be used as an undervoltage lockout

(UVLO) function by connecting a resistor from the input

supply to the RUN pin. The equation:

V_UVLO= R1+R2

•1.6V

Power Good

The PGOOD pin is an open drain pin that can be used to

monitor valid output voltage regulation. This pin monitors

a ±7.5% window around the regulation point, and tracks

with margining.

COMP Pin

This pin is the external compensation pin. The module

has already been internally compensated for most output

voltages. A spice model will be provided for other control

loop optimization.

Fault Conditions: Current Limit and Overcurrent

Foldback

LTM4609 has a current mode controller, which inherently

limits the cycle-by-cycle inductor current not only in steady

state operation, but also in transient. Refer to Table 3.

To further limit current in the event of an overload condi-

tion, the LTM4609 provides foldback current limiting. If the

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

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