1766 데이터 시트보기 (PDF) - Linear Technology

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1766

5.5V to 60V 1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1766/LT1766-5

PIN FUNCTIONS

VIN (Pin 4): This is the collector of the on-chip power NPN

switch. VIN powers the internal control circuitry when a

voltage on the BIAS pin is not present. High dI/dt edges

occur on this pin during switch turn on and off. Keep

the path short from the VIN pin through the input bypass

capacitor, through the catch diode back to SW. All trace

inductance on this path will create a voltage spike at switch

off, adding to the VCE voltage across the internal NPN.

BOOST (Pin 6): The BOOST pin is used to provide a drive

voltage, higher than the input voltage, to the internal bipolar

NPN power switch. Without this added voltage, the typical

switch voltage loss would be about 1.5V. The additional

BOOST voltage allows the switch to saturate and voltage

loss approximates that of a 0.2Ω FET structure, but with

much smaller die area.

BIAS (Pin 10): The BIAS pin is used to improve efficiency

when operating at higher input voltages and light load cur-

rent. Connecting this pin to the regulated output voltage

forces most of the internal circuitry to draw its operating

current from the output voltage rather than the input supply.

This architecture increases efficiency especially when the

input voltage is much higher than the output. Minimum

output voltage setting for this mode of operation is 3V.

VC (Pin 11) The VC pin is the output of the error amplifier

and the input of the peak switch current comparator. It is

normally used for frequency compensation, but can also

serve as a current clamp or control loop override. VC sits

at about 0.9V for light loads and 2.1V at maximum load.

It can be driven to ground to shut off the regulator, but if

driven high, current must be limited to 4mA.

FB/SENSE (Pin 12): The feedback pin is used to set the

output voltage using an external voltage divider that gener-

ates 1.22V at the pin for the desired output voltage. The

5V fixed output voltage parts have the divider included on

the chip and the FB pin is used as a SENSE pin, connected

directly to the 5V output. Three additional functions are

performed by the FB pin. When the pin voltage drops below

0.6V, switch current limit is reduced and the external SYNC

function is disabled. Below 0.8V, switching frequency is

also reduced. See Feedback Pin Functions in Applications

Information for details.

SYNC (Pin 14): The SYNC pin is used to synchronize the

internal oscillator to an external signal. It is directly logic

compatible and can be driven with any signal between 10%

and 90% duty cycle. The synchronizing range is equal to

initial operating frequency up to 700kHz. See Synchroniz-

ing in Applications Information for details.

SHDN (Pin 15): The SHDN pin is used to turn off the

regulator and to reduce input drain current to a few mi-

croamperes. This pin has two thresholds: one at 2.38V to

disable switching and a second at 0.4V to force complete

micropower shutdown. The 2.38V threshold functions

as an accurate undervoltage lockout (UVLO); sometimes

used to prevent the regulator from delivering power until

the input voltage has reached a predetermined level.

If the SHDN pin functions are not required, the pin can

either be left open (to allow an internal bias current to lift

the pin to a default high state) or be forced high to a level

not to exceed 6V.

BLOCK DIAGRAM

The LT1766 is a constant frequency, current mode buck

converter. This means that there is an internal clock and

two feedback loops that control the duty cycle of the power

switch. In addition to the normal error amplifier, there is a

current sense amplifier that monitors switch current on a

cycle-by-cycle basis. A switch cycle starts with an oscillator

pulse which sets the RS flip-flop to turn the switch on. When

switch current reaches a level set by the inverting input of

the comparator, the flip-flop is reset and the switch turns

off. Output voltage control is obtained by using the output

of the error amplifier to set the switch current trip point.

This technique means that the error amplifier commands

current to be delivered to the output rather than voltage.

A voltage fed system will have low phase shift up to the

resonant frequency of the inductor and output capacitor,

then an abrupt 180° shift will occur. The current fed system

1766fc

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