LTC3775 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3775

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

Linear Technology 

LTC3775

APPLICATIONS INFORMATION

MODE/SYNC Pin

The MODE/SYNC pin is a dual function pin that can be

used to program the operating mode or to synchronize

the switching frequency to an external clock. Pulse-

skipping mode is enabled when the MODE/SYNC pin is

above 1.2V. The mode is forced continuous when the pin

is below 1.2V.

If this pin is left floating, an internal 50k pull-down resistor

defaults the selection to forced continuous mode. During

power-up, the LTC3775 overrides this mode selection and

operates in pulse-skipping mode to prevent the discharge

of a pre-biased output capacitor.

The internal LTC3775 oscillator can be synchronized

to an external clock with a signal greater than 1.5V. A

low-to-high transition on the MODE/SYNC pin resets the

oscillator sawtooth waveform (high) and forces TG low

(see Figure 17).The external oscillator frequency must be

within ±20% of the frequency programmed by the RSET

resistor, or else the part will revert to free-running mode.

The internal oscillator locks to the external clock after

the second clock transition is received. When external

synchronization is detected, the LTC3775 will operate in

forced continuous mode.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3775. Check the following in your layout:

1. Keep the signal and power grounds separate. The signal

ground consists of the LTC3775 SGND pin and the (–)

terminal of VOUT. The power ground consists of the

optional Schottky diode anode, the source of the bottom

side MOSFET, and the (–) terminal of the input capacitor.

Connect the signal ground to the (–) terminal of the

output capacitor. Also, try to connect the (–) terminal

of the output capacitor as close as possible to the (–)

terminals of the input capacitor.

2. The high di/dt loop formed by the top N-channel MOSFET,

the bottom MOSFET and the CIN capacitor should have

short leads and PC trace lengths to minimize high

frequency noise and voltage stress from inductive

ringing.

3. Connect the drain of the topside MOSFET directly to the

(+) plate of CIN, and connect the source of the bottom

side MOSFET directly to the (–) terminal of CIN. This

capacitor provides the AC current to the MOSFETs.

4. Place the ceramic CINTVCC decoupling capacitor im-

mediately next to the IC, between INTVCC and SGND.

Likewise, the CB capacitor should also be next to the

IC between BOOST and SW.

5. Place the small-signal components away from high

frequency switching nodes (BOOST, SW, TG and BG).

6. For optimum load regulation and true remote sensing,

the top of the output resistor divider should connect

independently to the top of the output capacitor (Kelvin

connection), staying away from any high dV/dt traces.

Place the divider resistors near the LTC3775 in order

to keep the high impedance FB node short.

7. For applications with multiple switching power convert-

ers connected to the same input supply, make sure

that the input filter capacitor for the LTC3775 is not

shared with other converters. AC input current from

another converter could cause substantial input voltage

EXTERNAL CLOCK

AT MODE/SYNC PIN

PWM RAMP

TG

3775 F17

Figure 17. External Synchronization

3775fa

24

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