LTC3776EUF 查看數據表（PDF） - Linear Technology

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LTC3776EUF

Dual 2-Phase, No RSENSE™, Synchronous Controller for DDR/QDR Memory Termination

Linear Technology 

U

OPERATIO (Refer to Functional Diagram)

phase detector adjusts the voltage on the PLLLPF pin to

align the turn-on of controller 1’s external P-channel

MOSFET to the rising edge of the synchronizing signal.

Thus, the turn-on of controller 2’s external P-channel

MOSFET is 180 degrees out of phase with the rising edge

of the external clock source.

The typical capture range of the LTC3776’s phase-locked

loop is from approximately 200kHz to 1MHz, with a

guarantee over all process variations and temperature to

be between 250kHz and 850kHz. In other words, the

LTC3776’s PLL is guaranteed to lock to an external clock

source whose frequency is between 250kHz and 850kHz.

Alternatively, the SYNC/SSEN pin may be tied to VIN to

–10

RBW = 3kHz

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

0

6

12

18

24

30

FREQUENCY (MHz)

37361 F01a

Figure 1a. Output Noise Spectrum of Conventional Buck

Switching Converter (LTC3776 with Spread Spectrum

Disabled) Showing Fundamental and Harmonic Frequencies

–10

RBW = 1kHz

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

0

6

12

18

FREQUENCY (MHz)

24

30

37361 F01c

Figure 1c. Output Noise Spectrum of the LTC3776 Spread

Spectrum Buck Switching Converter. Note the Reduction in

Fundamental and Harmonic Peak Spectral Amplitude

Compared to Figure 1a.

LTC3776

enable spread spectrum operation (see Spread Spectrum

Operation section).

Spread Spectrum Operation

Switching regulators can be particularly troublesome in

applications where electromagnetic interference (EMI) is

a concern. Switching regulators operate on a cycle-by-

cycle basis to transfer power to an output. In most cases,

the frequency of operation is either fixed or is a constant

based on the output load. This method of conversion

creates large components of noise at the frequency of

operation (fundamental) and multiples of the operating

frequency (harmonics). Figures 1a and 1b depict the

–10

RBW = 30Hz

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

410

450 490 530 570 610

FREQUENCY (kHz)

37361 F01b

Figure 1b. Zoom-In of Fundamental Frequency of Conventional

Buck Switching Converter

–10

RBW = 30Hz

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

410

450 490 530 570 610

FREQUENCY (kHz)

37361 F01d

Figure 1d. Zoom-In of Fundamental Frequency of the

LTC3776 Spread Spectrum Switching Converter. Note the

>20dB Reduction in Peak Amplitude and Spreading of the

Frequency Spectrum (Between Approximately 450kHz and

580kHz) Compared to Figure 1b.

3776f

11

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