LT1507CS8 datasheet(6/20 Pages) LINER

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LT1507

PIN FUNCTIONS

saturation voltage and maximum duty cycle. A typical

value for minimum input voltage is 1V above output

voltage. Start-up conditions may require more voltage at

light loads. See Minimum Input Voltage for details.

VSW (Pin 3): The switch pin is driven up to the input voltage

in the ON state and is an open circuit in the OFF state. At

higher load currents, pin voltage during the off condition

will be one diode drop below ground as set by the external

catch diode. At lighter loads the pin will assume an

intermediate state equal to output voltage during part of

the switch OFF time. Maximum

negative voltage on the

switch pin is 1V with respect to the GND pin, so it must

always be clamped with a catch diode to the GND pin.

SHDN (Pin 4): The shutdown pin is used to turn off the

regulator and to reduce input drain current to a few

microamperes. Actually this pin has two separate thresh-

olds, 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). This is sometimes used to prevent the regulator

from delivering power until the input voltage has reached

a predetermined level.

BLOCK DIAGRAM

The LT1507 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 will have 90

° phase shift at a much

lower frequency, but will not have the additional 90

° shift

until well beyond the LC resonant frequency. This makes

it much easier to frequency compensate the feedback loop

and also gives much quicker transient response.

High switch efficiency is attained by using the BOOST pin

to provide a voltage to the switch driver which is higher

than the input voltage, allowing the switch to be saturated.

This boosted voltage is generated with an external capaci-

tor and diode.

Two comparators are connected to the shutdown pin. One

has a 2.38V threshold for undervoltage lockout and the

second has a 0.4V threshold for complete shutdown.

SYNC (Pin 5): 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 1MHz. See

Sychronizing section for details.

FB/SENSE (Pin 7): The feedback pin is used to set output

voltage using an external voltage divider that generates

2.42V at the pin with the desired output voltage. The fixed

voltage (– 3 .3V) parts have the divider included on the chip

and the feedback pin is used as a sense pin connected

directly to the 5V output. Two additional functions are

performed by the feedback pin. When the pin voltage

drops below 1.7V, switch current limit is reduced. Below

1V, switching frequency is also reduced. See More Than

Just Voltage Feedback.

VC (Pin 8): 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 do

double duty as a current clamp or control loop override.

This pin sets at about 1V for very light loads and 2V at

maximum load. It can be driven to ground to shut off the

regulator, but if driven high, current must be limited to 4mA.

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