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

Since the current limit comparator contains leading edge

blanking, an external RC filter is not required for proper

operation. However, an external filter can be designed by

adding a capacitor across the SENSE and ILIMT pins (CF

in Figure 7a). The filter component should be placed close

to the SENSE and ILIMT pins.

If RDS(ON) sensing is employed, the Kelvin sense con-

nection should run from the SENSE pin and the RILIMT

resistor to the source and drain terminals of the top power

MOSFET respectively, as shown in Figure 7b. The external

RC filter should not be added since the source terminal

is switching.

VIN

TO RLIMIT

TO SENSE PIN

RSENSE

3775 F06

TOP MOSFET

DRAIN

Figure 6. Kelvin SENSE Connection

for Topside Current Limiting Sensing

VIN

LTC3775

ILIMIT

SENSE

CIN

RLIMIT

CF

VIN

RSENSE

TG

3775 F07a

SW

Figure 7a. External Filter for Topside Current Sensing

VIN

LTC3775

ILIMIT

TG

CIN

RLIMIT

VIN

QT

SENSE

3775 F07b

SW

Figure 7b. Kelvin Connection for Topside RDS(ON) Sensing

The bottom side current limit threshold is set by the resis-

tor, RILIMB, from the ILIMB pin to SGND and the ILIMB pin

10μA pull-up current. The voltage at ILIMB is attenuated

5× internally before it is applied to the input of bottom

current limit comparator, CBLIM. This voltage must be

quiet. Connect RILIMB from the ILIMB pin to a quiet ground

near the LTC3775 SGND pin. The other input of CBLIM is

connected to the SW pin. The SW pin is also shared with

the bottom gate driver and should be connected near the

drain of the bottom MOSFET, QB.

Pulse-Skipping Mode

The LTC3775 can operate in one of two modes selectable

with the MODE/SYNC pin: pulse-skipping mode or forced

continuous mode. Pulse-skipping mode is selected when

increased efficiency at light loads is desired, as shown in

Figure 8. In this mode, the bottom MOSFET is turned off

when inductor current reverses in order to minimize the

efficiency loss due to reverse current flow. As the load

current decreases (see Figure 9), the duty cycle is reduced

to maintain regulation until the minimum on-time (50ns)

is reached. When the load decreases below this point,

the LTC3775 begins to skip cycles to maintain regulation.

This reduces the frequency and improves efficiency by

minimizing gate charge losses.

In forced continuous mode, the bottom MOSFET is always

on when the top MOSFET is off, allowing the inductor cur-

rent to reverse at low currents. This mode is less efficient

due to switching, but has the advantages of better transient

100

90

VIN = 12V

VOUT = 1.2V

80 SW FREQ = 500kHz

FRONT PAGE CIRCUIT

70

60 PULSE-SKIPPING

50

MODE

40

30

CONTINUOUS

MODE

20

10

0

0.01

0.1

1

10

LOAD CURRENT (A)

100

3775 F08

Figure 8. Efficiency in Pulse-Skipping/Forced Continuous Modes

3775fa

15

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