LTM8049 datasheet(10/20 Pages) LINER

10 / 20 page

LTM4661

4661f

For more information www.linear.com/LTM4661

Burst Mode operation, energy is delivered to the output

until the nominal regulation value is reached, then the

LTM4661 transitions into a very low quiescent current

sleep state. In sleep, the output switches are turned off

and the LTM4661 consumes only 25µA of quiescent

current. When the output voltage droops approximately

1%, switching resumes. This maximizes efficiency at

very light loads by minimizing switching and quiescent

losses. Output voltage ripple in Burst Mode operation

is typically 1% to 2% peak-to-peak. Additional output

capacitance (22µF or greater), or the addition of a small

feedforward capacitor (10pF to 50pF) connected between

VOUT and FB, can help further reduce the output ripple.

Operation Frequency

The operating frequency of the LTM4661 is optimized to

achievethecompactpackagesizeandtheminimumoutput

ripplevoltagewhilestillkeepinghighefficiency.Thedefault

operating frequency is internally set to 1MHz. In most ap-

plications, no additional frequency adjusting is required.

If any operating frequency other than 1MHz is required by

application, the operating frequency can be increased by

adding a resistor, RFSET, between the FREQ pin and GND,

as shown in Figure 18. The operating frequency can be

calculated as:

fs MHz

(

) =

28 + RFSET kΩ

( )

RFSET kΩ

( )

Frequency Synchronization

The switching frequency of the LTM4661 can be synchro-

nized to a desired frequency by applying a clock of twice

the desired frequency to the SYNC/MODE pin. Also, the

freerunningfrequencyneedstobeadjustedtoafrequency

approximately80%ofthedesiredfrequency.Pleaseusethe

equation in the Operation Frequency section to calculate

the external RFSET resistor value.

For example, if the LTM4661 needs to be synchronized to

1.5MHz switching frequency, an external clock of 3MHz

needs to supply to SYNC/MODE pin while adding a 140kΩ

RFSET resistor between FREQ pin and GND to program the

free run frequency to 1.2MHz.

Shutdown

The boost converter is disabled by pulling SDB below

0.25V and enabled by pulling SDB above 1.6V. Note that

SDB pin can be driven above VIN or VOUT, as long as it is

limited to less than its absolute maximum rating.

Thermal Shutdown

Ifthedietemperatureexceeds170°Ctypical,theLTM4661

will go into thermal shutdown (TSD). All switches will be

shut off until the die temperature drops by approximately

7°C, when the device reinitiates a soft-start and switching

is re-enabled.

Output Disconnect

TheLTM4661’soutputdisconnectfeatureeliminatesbody

diode conduction of the internal P-channel MOSFET recti-

fiers.ThisfeatureallowsforVOUTtodischargeto0Vduring

shutdownanddrawnocurrentfromtheinputsource.Inrush

current will also be limited at turn-on, minimizing surge

currents seen by the input supply. The output disconnect

feature also allows VOUT to be pulled high, without back-

feeding the power source connected to VIN.

Short-Circuit Protection

The LTM4661 output disconnect feature allows output

short-circuit protection while maintaining a maximum set

current limit. To reduce power dissipation under overload

andshort-circuitconditions,thepeakswitchcurrentlimits

are reduced to approximately 2A. Once VOUT exceeds

approximately 1.5V, the current limits are reset to their

nominal values of 3.5A peak switching current per phase.

Output Overvoltage Protection

An overvoltage condition occurs when VOUT exceeds

approximately 16.5V. Switching is disabled and the in-

ternal soft-start ramp is reset. Once VOUT drops below

approximately 16V, a soft-start is initiated and switching

is allowed to resume. If the boost converter output is

lightly loaded such that the time constant of the output

capacitance, COUT and the output load resistance, ROUT is

near or greater than the soft-start time of approximately

10ms, the soft-start ramp may end before or soon after

APPLICATIONS INFORMATION

LTM8049 Datasheet(PDF) 10 Page - Linear Technology

LTM8049 Datasheet(HTML) 10 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL