LT3845 데이터 시트보기 (PDF) - Linear Technology

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LT3845

High Voltage Synchronous Current Mode Step-Down Controller with Adjustable Operating Frequency

Linear Technology 

LT3845

APPLICATIONS INFORMATION

where QG(TOTAL) is the total MOSFET gate charge of the

TG and BG.

In applications where these conditions are exceeded, VCC

must be derived from an external source after start-up.

Maximum continuous regulator power dissipation may be

exceeded for short duration VIN transients.

In LT3845 converter applications with output voltages in

the 9V to 20V range, back-feeding VCC and VBOOST from

the converter output is trivial, accomplished by connect-

ing diodes from the output to these supply pins. Deriving

these supplies from output voltages greater than 20V will

require additional regulation to reduce the feedback voltage.

Outputs lower than 9V will require step-up techniques to

increase the feedback voltage to something greater than

the 8V VCC regulated output. Low power boost switchers

are sometimes used to provide the step-up function, but

a simple charge-pump can perform this function in many

instances.

Charge Pump Doubler

VCC

LT3845

B0520

1μF

VOUT

B0520

1μF

BG

Si1555DL

Charge Pump Tripler

B0520

B0520 1μF

VOUT

VCC

LT3845

BG

B0520 1μF

1μF

Si1555DL

Si1555DL

3845 AI01

Inductor Auxiliary Winding

TG

SW

LT3845

VCC

BG

•N

VOUT

•

3845 AI04

Burst Mode

The LT3845 employs low current Burst Mode functional-

ity to maximize efficiency during no load and low load

conditions. Burst Mode operation is enabled by shorting

the BURST_EN pin to SGND. Burst Mode operation can be

disabled by shorting BURST_EN to either VFB or VCC.

When the required switch current, sensed via the VC

pin voltage, is below 15% of maximum, the Burst Mode

operation is employed and that level of sense current is

latched onto the IC control path. If the output load requires

less than this latched current level, the converter will

overdrive the output slightly during each switch cycle.

This overdrive condition is sensed internally and forces

the voltage on the VC pin to continue to drop. When the

voltage on VC drops 150mV below the 15% load level,

switching is disabled and the LT3845 shuts down most

of its internal circuitry, reducing total quiescent current

to 120μA. When the converter output begins to fall, the

VC pin voltage begins to climb. When the voltage on the

VC pin climbs back to the 15% load level, the IC returns

to normal operation and switching resumes. An internal

clamp on the VC pin is set at 100mV below the switch

disable threshold, which limits the negative excursion of

the pin voltage, minimizing the converter output ripple

during Burst Mode operation.

During Burst Mode operation, VIN pin current is 20μA and

VCC current is reduced to 100μA. If no external drive is

provided for VCC, all VCC bias currents originate from the

3845fd

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