LT3757 datasheet(8/24 Pages) AD

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LT8300

Rev. A

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OPERATION

reducing the effective quiescent current to improve light

load efficiency. In this condition, the LT8300 operates

in low ripple Burst Mode. The typical 7.5kHz minimum

Output Voltage

The RFB resistor as depicted in the Block Diagram is the

only external resistor used to program the output voltage.

The LT8300 operates similar to traditional current mode

switchers, except in the use of a unique flyback pulse

sense circuit and a sample-and-hold error amplifier, which

sample and therefore regulate the isolated output voltage

from the flyback pulse.

Operation is as follows: when the power switch M1 turns

off, the SW pin voltage rises above the VIN supply. The

amplitude of the flyback pulse, i.e., the difference between

the SW pin voltage and VIN supply, is given as:

VFLBK = (VOUT + VF + ISEC • ESR) • NPS

VF = Output diode forward voltage

ISEC = Transformer secondary current

ESR = Total impedance of secondary circuit

NPS = Transformer effective primary-to-secondary

turns ratio

The flyback voltage is then converted to a current IRFB by

the flyback pulse sense circuit (M2 and M3). This current

IRFB also flows through the internal trimmed 12.23k RREF

resistor to generate a ground-referred voltage. The result-

ing voltage feeds to the inverting input of the sample-

and-hold error amplifier. Since the sample-and-hold error

amplifier samples the voltage when the secondary current

is zero, the (ISEC • ESR) term in the VFLBK equation can be

assumed to be zero.

The bandgap reference voltage VBG, 1.223V, feeds to the

non-inverting input of the sample-and-hold error ampli-

fier. The relatively high gain in the overall loop causes

the voltage across RREF resistor to be nearly equal to the

APPLICATIONS INFORMATION

switching frequency determines how often the output volt-

age is sampled and also the minimum load requirement.

bandgap reference voltage VBG. The resulting relationship

between VFLBK and VBG can be expressed as:

VFLBK

RFB

⎛

⎝

⎜

⎞

⎠

⎟ •RREF = VBG

VFLBK =

VBG

RREF

⎛

⎝

⎜

⎞

⎠

⎟ •RFB =IRFB •RFB

VBG = Bandgap reference voltage

IRFB = RFB regulation current = 100µA

Combination with the previous VFLBK equation yields an

equation for VOUT, in terms of the RFB resistor, trans-

former turns ratio, and diode forward voltage:

VOUT = 100µA •

RFB

NPS

⎛

⎝

⎜

⎞

⎠

⎟− VF

Output Temperature Coefficient

The first term in the VOUT equation does not have tem-

perature dependence, but the output diode forward volt-

age VF has a significant negative temperature coefficient

(–1mV/°C to –2mV/°C). Such a negative temperature coef-

ficient produces approximately 200mV to 300mV voltage

variation on the output voltage across temperature.

For higher voltage outputs, such as 12V and 24V, the output

diode temperature coefficient has a negligible effect on the

output voltage regulation. For lower voltage outputs, such

as 3.3V and 5V, however, the output diode temperature

coefficient does count for an extra 2% to 5% output voltage

regulation. For customers requiring tight output voltage

regulation across temperature, please refer to other LTC

parts with integrated temperature compensation features.

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