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

Output Overvoltage Protection

An overvoltage comparator, MAX, guards against transient

overshoots (>10%) as well as other more serious condi-

tions that may overvoltage the output. In such cases, the

top MOSFET is turned off and the bottom MOSFET is turned

on until the overvoltage condition is cleared.

Run/Shutdown

The LTC3775 can be put into a low power shutdown mode

with quiescent current <14μA by pulling the RUN/SHDN

pin below 0.74V. The RUN/SHDN pin can also be used as

an accurate external UVLO (undervoltage lockout) input

with a threshold of 1.22V. The driver outputs stay low if

this pin is <1.22V. The external resistive divider R4 and R5

shown in the Block Diagram can be used to set the UVLO

level based on VIN. The VIN voltage at which the switching

starts is given by the following formula:

UVLO (Upper) = 1.22V • (1 + R4/R5) – (1μA • R4)

The RUN/SHDN pin has an internal 1μA pull-up for default

turn-on if this pin is left floating. This 1μA pull-up current is

included in the above UVLO calculation. When RUN/SHDN

goes above 1.22V, this pull-up current is increased to 5μA.

This provides some amount of hysteresis to the UVLO

threshold. The lower UVLO level becomes:

UVLO (Lower) = 1.22V • (1 + R4/R5) – (5μA • R4)

So the amount of hysteresis is given by:

UVLO (Hysteresis) = 4μA • R4

LTC3775

VIN

R4

R5

IRUN

1μA

RUN

1.22V

0.74V

4μA

–

+

TURN OFF TG

EN

–

+

CHIP

SHUTDOWN

SHDN

3775 F02

Figure 2. RUN Pin Control

12

Soft-Start

The LTC3775 includes a soft-start circuit that provides a

smooth output voltage ramp during start-up. The SS pin

requires an external capacitor, CSS, to GND with the value

determined by the required soft-start time. An internal 1μA

current source charges CSS. When the voltage on the SS

pin is less than the 0.6V internal reference, the LTC3775

regulates the VFB voltage to the SS pin voltage instead of

the 0.6V reference. As the SS voltage rises linearly from

0V to 0.6V and beyond, the output voltage, VOUT, rises

smoothly from zero to its final value. The total soft-start

time can be calculated as:

tSOFTSTART

=

0.9 • CSS

1μA

The SS pin is pulled low in the following conditions: during

an LDO undervoltage condition (INTVCC < 3.6V), during

shutdown (RUN pin < 1.22V), during an overtemperature

condition (TJ > 165°C) and during current limit.

If either the top or bottom current limit comparator trips,

the SS pin is pulled low until the inductor current regu-

lates at around the current limit setting. Once the fault is

cleared, SS will start charging up allowing the duty cycle

and output voltage to increase gradually. Due to the cur-

rent limit action on the SS pin, it is important to avoid

an overcurrent condition during start-up of the power

supply, or VOUT will fail to start up properly.

VOUT

0.5V/DIV

IL

5A/DIV

SWITCHOVER

FROM PULSE-

SKIPPING TO

CONTINUOUS

MODE

VSS

1V/DIV

VIN = 12V

VOUT = 1.2V

CSS = 0.01μF

MODE = 0V

2ms/DIV

SW FREQ = 500kHz

3775 F03

Figure 3. Typical Start-Up Waveform

for a Buck Converter Using the LTC3775

3775fa

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