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

High Duty Cycle Operation

The maximum duty cycle is limited by the LTC3775 internal

oscillator reset time, the propagation delay of the PWM

comparator and the BOOST pin supply refresh rate. The

minimum off-time is typically 300ns.

The top MOSFET driver is biased from the floating bootstrap

capacitor, CB, which normally recharges during each off

cycle through an external diode when the top MOSFET turns

off. If the input voltage, VIN, decreases to a voltage close to

VOUT, the controller will enter dropout and attempt to turn

on the top MOSFET continuously. To avoid depleting the

charge on the bootstrap capacitor, CB, the LTC3775 has an

internal counter that turns on the bottom MOSFET every

eight cycles for 200ns to refresh the bootstrap capacitor.

Figure 13 shows maximum duty cycle operation with the

200ns BOOST pin supply refresh.

step-down VIN to VOUT ratios, another consideration is

the minimum on-time of the LTC3775 (see the Minimum

On-Time Considerations section). A final consideration for

operating frequency is that in noise-sensitive communica-

tions systems, it is often desirable to keep the switching

noise out of a sensitive frequency band.

The LTC3775 uses a constant frequency architecture that

can be programmed over a 250kHz to 1MHz range with a

single resistor from the FREQ pin to ground, as shown in

Figure 14. The nominal voltage on the FREQ pin is 1.22V,

and the current that flows from this pin is used to charge

and discharge an internal oscillator capacitor. The value of

RSET for a given operating frequency can be chosen from

Figure 14 or from the following equation:

RSET(k
)

=

19500

f(kHz)

TG

5V/DIV

BG

5V/DIV

OSCILLATOR

RESET

BOOST PIN

SUPPLY REFRESH

VIN = 4.5V

VOUT = 4.2V

LOAD = 0A

2μs/DIV

SW FREQ = 500kHz

3775 F13

Figure 13. Maximum Duty Cycle Waveforms

EXTERNAL COMPONENTS SELECTION

Operating Frequency

The choice of operating frequency and inductor value is

a trade-off between efficiency and component size. Low

frequency operation improves efficiency by reducing

MOSFET switching losses and gate charge losses. However,

lower frequency operation requires more inductance for a

given amount of ripple current, resulting in a larger induc-

tor size and higher cost. If the ripple current is allowed

to increase, larger output capacitors may be required to

maintain the same output ripple. For converters with high

The oscillator can also be synchronized to an external clock

applied to the MODE/SYNC pin with a frequency in the

range of ±20% of the programmed free-running frequency

set by the FREQ pin. In this synchronized mode, pulse-

skipping mode operation is disabled. The clock high level

must exceed 1.5V for a minimum of approximately 25ns

to engage the feature. The bottom MOSFET will turn-on

following the rising edge of the external clock.

1000

900

800

700

600

500

400

300

200

0 10 20 30 40 50 60 70 80 90 100

RSET VALUE (kΩ)

3775 F14

Figure 14. Frequency Set Resistor (RSET) Value

Top MOSFET Driver Supply

An external bootstrap capacitor, CB, connected to the

BOOST pin supplies the gate drive voltage for the topside

MOSFET. This capacitor is charged through diode DB from

3775fa

19

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