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

To prevent discharging a pre-biased VOUT, the LTC3775

always starts switching in pulse-skipping mode up to SS =

0.54V, regardless of the mode selected by the MODE/SYNC

pin. Thus if VOUT > 0V during power-up, VOUT will remain

at the pre-biased voltage (if there is no load) until the SS

voltage catches up with VOUT, after which VOUT will track

the SS ramp. The LTC3775 reverts to the selected mode

once SS > 0.54V.

Constant Switching Frequency

The internal oscillator can be programmed from 250kHz

to 1MHz with an external resistor from the FREQ pin to

ground, in order to optimize component size, efficiency

and noise for the specific application. The internal oscillator

can also be synchronized to an external clock connected

to the MODE/SYNC pin and can lock to a range of ±20%

of the programmed free-running frequency. When locked

to an external clock, pulse-skipping mode operation is

automatically disabled. Constant frequency operation of-

fers a number of benefits: inductor and capacitor values

can be chosen for a precise operating frequency and the

feedback loop can be similarly tightly specified. Noise

generated by the circuit will always be at known frequen-

cies. Subharmonic oscillation and slope compensation,

common headaches with constant frequency current

mode switchers, are absent in voltage mode designs like

the LTC3775.

is low during its on-time, the voltage drop from the drain

to source is proportional to the current flow. Alternatively,

for better accuracy, the topside current may be monitored

with a sense resistor.

The benefit of having two comparators is to allow continu-

ous monitoring and cycle-by-cycle control of the inductor

current regardless of the operating duty cycle. In high

duty cycle operation the top MOSFET, QT, is on most of

the time. Thus, a high side comparator is necessary to

limit the output current during high duty cycle operation.

Architectures that contain only one comparator to monitor

the low side MOSFET will not effectively limit the output

current during high duty cycle operation. Conversely, during

low duty cycle operation, a low side comparator is neces-

sary to limit the output current. Another common current

sensing scheme uses a sense resistor in series with the

inductor to allow continuous monitoring. However, this

scheme restricts the range of VOUT due to the common

mode range of the current limit comparator. The LTC3775

does not have this VOUT restriction.

Figure 4 shows the current limit circuitry. The top current

limit comparator, CTLIM monitors the current through the

top MOSFET, QT, when TG is high. If the inductor current

exceeds the current limit threshold when QT is on, QT turns

off immediately and the bottom MOSFET, QB, turns on. The

SENSE pin is the input for CTLIM. For applications where

Thermal Shutdown

The LTC3775 has a thermal detector that pulls the driver

outputs low if the junction temperature of the chip ex-

ceeds 165°C. The thermal shutdown circuit has 25°C of

hysteresis.

Current Limit

The LTC3775 includes an onboard cycle-by-cycle current

limit circuit that limits the maximum output current to a

user-programmed level. The current limit circuit consists

of two comparators, CTLIM and CBLIM that monitor the

voltage drop across the top and bottom MOSFETs respec-

tively. Since the MOSFET’s effective resistance, RDS(ON),

LTC3775

CTLIM

TURN OFF TG

VIN

ILIMT

100μA

SENSE

10μA

ILIMB

CBLIM 0.2 • VILIMB SW

EXTEND BG

SGND

PGND

RILIMT

VIN

RSENSE

TG

QT

(OPT)

RILIMB

BG

QB

VOUT

3775 F04

Figure 4. LTC3775 Current Limit Circuit

3775fa

13

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