LT1680ISW datasheet(8/16 Pages) LINER

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LT1680

OPERATION

Basic Control Loop

The LT1680 uses a constant frequency, current mode

architecture. The timing of the IC is provided through an

internal oscillator circuit that can be synchronized to an

external clock and is programmable to operate at frequen-

cies up to 200kHz. The oscillator creates a modified

sawtooth wave at its timing node (CT) with a slow charge,

rapid discharge characteristic.

During typical boost converter operation, the MOSFET

switch is enabled at the start of each oscillator cycle. The

switch stays enabled until the current through the switched

inductor, sensed via the voltage across a series sense

resistor (RSENSE), is sufficient to trip the current com-

parator (IC1) and reset the RS latch. When the switch is

disabled, the inductor current is redirected to the supply

output. If the current comparator threshold is not reached

throughout the entire oscillator charge period, the RS

latch is bypassed and the main switch is disabled during

the oscillator discharge time. This “minimum off time”

protects the switch, and is typically about 1

µs.

The current comparator trip threshold is set on the VC pin,

which is the output of a transconductance amplifier, or

error amplifier (EA). The error amplifier integrates the

difference between a feedback voltage (on the VFB pin) and

an internal bandgap generated reference voltage of 1.25V,

forming a signal that represents required load current. If

the supplied current is insufficient for a given load, the

output will droop, thus reducing the feedback voltage. The

error amplifier responds by forcing current out of the VC

pin, increasing the current comparator threshold. Thus,

the circuit will servo until the provided current is equal to

the required load and the average output voltage is at the

value programmed by the feedback resistors.

Input Average Current Limit

The output of the sense amplifier is monitored by a single

pole integrator comprised of an external capacitor on the

IAVG pin and an output impedance of approximately 50kΩ.

If this averaged value signal exceeds a level corresponding

to 120mV across the external sense resistor, the current

comparator threshold is clamped and cannot continue to

rise in response to the error amplifier. Thus, if average

input current requirements exceed 120mV/RSENSE, the

supply will current limit and the output voltage will fall out

of regulation. The average current limit circuit monitors

the sense amplifier output without slope compensation or

ripple current contributions. Therefore, the average input

current limit threshold is unaffected by duty cycle.

Undervoltage Lockout

The LT1680 employs an undervoltage lockout circuit

(UVLO) that monitors the 12VIN supply rail. This circuit

disables the output drive capability of the LT1680 if the

12V supply drops below 9V. Unstable mode switching is

prevented through 350mV of UVLO threshold hysteresis.

Shutdown

The LT1680 can be put into low current shutdown by

pulling the RUN/SHDN pin low, disabling all circuit func-

tions. The shutdown threshold is a bandgap referred

voltage of 1.25V typical. Use of a precision threshold on

the shutdown circuit enables use of this pin for under-

voltage protection of the VIN supply and/or power supply

sequencing.

Soft Start

The LT1680 incorporates a soft start function that oper-

ates by slowly increasing current limit. This limit is

controlled by internally clamping the VC pin to a low

voltage that climbs with time as an external capacitor on

the SS pin is charged with about 10

µA. This forces a

graceful climb of output current source capability, and

thus a graceful increase in output voltage until steady-

state regulation is achieved. The soft start timing capaci-

tor is clamped to ground during shutdown and during

undervoltage lockout, yielding a graceful output recovery

from either condition.

5V Internal Reference

Power for the oscillator timing elements and most other

internal LT1680 circuits is derived from an internal 5V

reference, accessible at the 5VREF pin. This supply pin

can be loaded with up to 10mA DC (20mA pulsed) for

convenient biasing of local elements such as control

logic, etc.

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