LT3751EUFD datasheet(14/28 Pages) LINER

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LT3751

3751f

OPERATION

Periodic Refresh

When the LT3751 enters regulation, the internal circuitry

deactivates switching when the internal one-shot clock

is high. The clock operates at a 1/20th duty cycle with a

minimum blank time of 1.5μs. This reset pulse is timed to

drastically reduce switching frequency content within the

audio spectrum and is active during all loading conditions.

Each reset pulse guarantees at least one energy cycle. A

minimum load is required to prevent the LT3751 from

entering overvoltage Burst Mode Operation.

Heavy Load Operation

The LT3751 enters peak current mode control at higher

output load conditions. The control loop maximizes the

number of switch cycles between each reset pulse. Since

the control scheme operates in boundary mode, the reso-

nant boundary-mode period changes with varying peak

primary current:

Period

=IPK •LPRI •

VTRANS

VOUT

⎡

⎣

⎢

⎤

⎦

⎥

and the power output is proportional to the peak primary

current:

POUT =

1/ 2 •IPK

VTRANS

VOUT

⎡

⎣

⎢

⎤

⎦

⎥

Noise becomes an issue at very low load currents. The

LT3751 remedies this problem by setting the lower peak

current limit to one tenth the maximum level and begins

to employ duty-cycle control.

Light Load Operation

The LT3751 uses duty cycle control to drastically reduce

audible noise in both the transformer (mechanical) and

the ceramic capacitors (piezoelectric effects). Internal

control circuitry forces a one-shot condition at a periodic

rate greater than 20kHz and out of the audio spectrum.

The regulation loop then determines the number of pulses

that are required to maintain the correct output voltage.

Figure 5 shows the use of duty-cycle control.

No Load Operation

The LT3751 can remain in low noise regulation at very low

loading conditions. Below a certain load current threshold

(Light Load Operation), the output voltage would continue

to increase and a runaway condition could occur. This is

due to the periodic one-shot forced by the periodic refresh

circuitry. By design, the LT3751 has built-in overvoltage

protection associated with the FB pin.

When the FB pin voltage exceeds 1.34V (±20mV), the

LT3751 enters overvoltage Burst Mode Operation.

Overvoltage Burst Mode Operation does not reset with

the one-shot clock. Instead, the pulse train is completely

load-dependent. These bursts are asynchronous and can

contain long periods of inactivity. This allows regulation at

a no-load condition but with the increase of audible noise

and voltage ripple. Note that when operating in overvoltage

Burst Mode Operation, the output voltage will increase

10% above the nominal output voltage.

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