LT3758 datasheet(11/20 Pages) LINER

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LT3798

3798f

Programming Output Current

The maximum output current depends on the supply volt-

age and the output voltage in a flyback topology. With the

VIN_SENSE pin connected to 100μA current source and a DC

supply voltage, the maximum output current is determined

at the minimum supply voltage, and the maximum output

voltage using the following equation:

IOUT(MAX) = 2•(1– D)•

NPS

42 • RSENSE

where

VOUT •NPS

VOUT •NPS + VIN

The maximum control voltage to achieve this maximum

output current is 2V • (1-D).

It is suggested to operate at 95% of these values to give

margin for the part’s tolerances.

When designing for power factor correction, the output

current waveform is going to have a half sine wave squared

shape and will no longer be able to provide the above

currents. By taking the integral of a sine wave squared

over half a cycle, the average output current is found to

be half the value of the peak output current. In this case,

the recommended maximum average output current is

as follows:

IOUT(MAX) = 2•(1−D) •

NPS

42 • RSENSE

• 47.5%

where

VOUT •NPS

VOUT •NPS + VIN

The maximum control voltage to achieve this maximum

output current is (1-D) • 47.5%.

For control voltages below the maximum, the output cur-

rent is equal to the following equation:

IOUT = CTRL •

NPS

42 • RSENSE

The VREF pin supplies a 2V reference voltage to be used

with the control pins. To set an output current, a resistor

divider is used from the 2V reference to one of the control

pins. The following equation sets the output current with

a resistor divider:

= R2

2NPS

42 •IOUT •RSENSE

–1

⎛

⎝

⎜

⎞

⎠

⎟

where R1 is the resistor connected to the VREF pin and the

CTRL pin and R2 is the resistor connected to the CTRL

pin and ground.

Setting VIN_SENSE Resistor

The VIN_SENSE resistor sets the current feeding the internal

multiplier that modulates the current limit for power factor

correction. At the maximum line voltage, VMAX, the current

is set to 360μA. Under this condition, the resistor value is

equal to (VMAX/360μA).

For DC input or non-PFC AC input applications, connect

a 25k resistor from VIN_SENSE to INTVCC instead of the

AC line voltage.

Critical Conduction Mode Operation

Critical conduction mode is a variable frequency switching

scheme that always returns the secondary current to zero

with every cycle. The LT3798 relies on boundary mode and

discontinuous mode to calculate the critical current because

the sensing scheme assumes the secondary current returns

to zero with every cycle. The DCM pin uses a fast current

input comparator in combination with a small capacitor to

detect dv/dt on the third winding. To eliminate false trip-

ping due to leakage inductance ringing, a blanking time of

between 600ns and 2μs is applied after the switch turns off,

depending on the current limit shown in the Leakage In-

ductance Blanking Time vs SENSE Current Limit Threshold

curve in the Typical Performance Characteristics section.

The detector looks for 80μA of current through the DCM

pin due to falling voltage on the third winding when the

secondary diode turns off. This detection is important since

the output current is calculated using this comparator’s

output. This is not the optimal time to turn the switch on

because the switch voltage is still close to VIN + VOUT • NPS

and would waste all the energy stored in the parasitic ca-

pacitance on the switch node. Discontinuous ringing begins

when the secondary current reaches zero and the energy

in the parasitic capacitance on the switch node transfers

OPERATION

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