LTC3775 查看數據表（PDF） - Linear Technology

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LTC3775

High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller

Linear Technology 

LTC3775

APPLICATIONS INFORMATION

After the calculations have been completed, it is impor-

tant to measure the gate drive waveforms and the gate

driver supply voltage (INTVCC to PGND) over all operating

conditions (low VIN, nominal VIN and high VIN, as well

as from light load to full load) to ensure adequate power

MOSFET enhancement. Consult the power MOSFET data

sheet to determine the actual RDS(ON) for the measured

VGS, and verify your thermal calculations by measuring

the component temperatures using an infrared camera

or thermal probe.

Operation at High Supply Voltage

At high input voltages, the LTC3775’s internal LDO can

dissipate a significant amount of power, which could

cause the maximum junction temperature to be exceeded.

Conditions such as a high operating frequency, or the use

of more than one power MOSFET in parallel, could push

the junction temperature rise to high levels. To prevent

the maximum junction temperature from being exceeded,

the input supply current must be checked while operating

in continuous conduction mode at maximum VIN. See

the Thermal Considerations section for calculation of the

maximum junction temperature.

TG

10V/DIV

VSW

10V/DIV

VIN = 28V

VOUT = 0.6V

LOAD = 1A

20ns/DIV

MODE/SYNC = 0V

SW FREQ = 1MHz

3775 F11

Figure 11. Minimum On-Time Waveforms

in Forced Continuous Mode

TG

10V/DIV

VSW

10V/DIV

Low Duty Cycle Operation

The LTC3775 uses a leading edge modulation architec-

ture. Because the top MOSFET turns on when the PWM

comparator trips, the top MOSFET minimum on-time

is not dependent on the propagation delay of the PWM

comparator; it is only limited by the internal delays of the

gate drivers and the rise/fall time of the power MOSFET

gate. This allows the LTC3775 to operate in very low duty

cycle applications with a large step-down ratio. Figure 11

shows minimum on-time waveforms for forced continuous

mode operation.

If pulse-skipping mode is selected, the LTC3775 allows

the controller to skip pulses at light load, thereby reducing

switching losses and improving the efficiency. Figure 12

shows waveforms of the minimum on-time in pulse-skip-

ping mode.

If the TG on-time is less than the blanking time of the topside

current limit comparator, CTLIM, the topside comparator

never trips during normal operation. The blanking time

18

VIN = 28V

VOUT = 0.6V

LOAD = 1A

20ns/DIV

MODE/SYNC = INTVCC

SW FREQ = 1MHz

3775 F12

Figure 12. Minimum On-Time Waveforms

in Pulse-Skipping Mode

is 200ns for RDS(ON) sensing and 100ns when a sense

resistor is used. For TG on-times smaller than the topside

blanking times, the LTC3775 relies on the bottom current

limit comparator, CBLIM, to monitor the inductor current.

If CBLIM trips, the LTC3775 starts to skip pulses and at

the same time pulls down the soft-start capacitor to limit

the duty cycle. If VOUT drops sufficiently, the TG on-time

can increase enough to turn on CTLIM and limit the peak

inductor current. The minimum on-time of the application

circuit can be calculated at maximum VIN:

tON(MIN)

=

fSW

VOUT

• VIN(MAX)

3775fa

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