LT3751EUFD datasheet(12/28 Pages) LINER

12 / 28 page

LT3751

3751f

OPERATION

At very low output voltages, the boundary-mode switching

cycle period increases signiﬁcantly such that the energy

stored in the transformer core is not depleted before the

next clock cycle. In this situation, the clock may initiate

another switching cycle before the secondary winding

current reaches zero and cause the LT3751 to enter con-

tinuous-mode conduction. Normally, this is not a problem;

however, if the secondary energy transfer time is much

longer than the CLK period, signiﬁcant primary current

overshoot can occur. This is due to the non-zero starting

point of the primary current when the switch turns on and

the ﬁnite speed of the current comparator.

The LT3751 startup circuitry adds an auxiliary current

comparator with a trip level 50% higher than the nominal

trip level. Every time the auxiliary current comparator

trips, the required clock count between switching cycles is

incremented by one. This allows more time for secondary

energy transfer.

Counter 1 in Figure 3 is set to its maximum count when

the ﬁrst DCM comparator one-shot is generated. If no

DCM one-shot is initiated in normal boundary-mode

operation during a maximum count of approximately

500μs, the LT3751 re-enters start-up mode and the count

is returned to zero.

Note that Counter 1 is initialized to zero at start-up. Thus,

the output of the startup circuitry will go high after one clock

cycle. Counter 2 is reset when the gate driver goes high.

This repeats until either the auxiliary current comparator

increments the required clock count or until VDRAIN is high

enough to sustain normal operation described in steps 2

through 4 in the previous section.

Entering Normal Boundary Mode

The LT3751 has two DCM comparator thresholds that are

dependent on what mode the part is in, either start-up

mode or normal boundary-mode, and the state of the mode

latch. For boundary-mode switching, the LT3751 requires

the DCM sense voltage (VDRAIN) to exceed VTRANS by the

ΔDCM comparator threshold, ΔVDRAIN:

ΔVDRAIN = (40μA + IOFFSET) • RDCM – 40μA • RVTRANS

where IOFFSET is mode dependent. The DCM one-shot

signal is negative edge triggered by the switch node,

VDRAIN, and indicates that the energy in the secondary

winding has depleted. For this to happen, VDRAIN must

exceed VTRANS + ΔVDRAIN prior to its negative edge; oth-

erwise, the DCM comparator will not generate a one-shot

to initiate the next switching cycle. The part would remain

stuck in this state indeﬁnitely; however, the LT3751 uses

the start-up protection circuitry to jumpstart switching if

the DCM comparator does not generate a one-shot after

a maximum time-out of 500μs.

Figure 4 shows a typical VDRAIN node waveform with

a test circuit voltage clamp applied to the output. VTH1

is the start-up threshold and is set internally by forcing

IOFFSET to 40μA. Once the ﬁrst DCM one-shot is initiated,

the mode latch is set to boundary-mode. The mode latch

then sets the clock count to maximum (500μs) and lowers

the DCM comparator threshold to VTH2 (IOFFSET = 20μA).

This provides needed hysteresis between start-up mode

and boundary-mode operation.

LOW NOISE REGULATION

Low noise voltage regulation can be achieved by adding

a resistive divider from the output node to the LT3751 FB

pin. At start-up (FB pin below 1.16V), the LT3751 enters

the charge mode to rapidly charge the output capacitor.

Once the FB pin is within the threshold range of 1.16V

to 1.34V, the part enters into low noise regulation. The

switching methodology in regulation mimics that used

in the capacitor charging mode, but with the addition of

peak current and duty cycle control techniques. Figure 5

shows the steady state operation for both regulation

techniques. Figure 6 shows how both techniques are

combined to provide stable, low noise operation over a

wide load and supply range.

During heavy load conditions, the LT3751 sets the peak

primary current to its maximum value, 106mV/RSENSE and

sets the maximum duty cycle to approximately 95%. This

allows for maximum power delivery. At very light loads,

the opposite occurs, and the LT3751 reduces the peak

primary current to approximately one tenth its maximum

value while modulating the duty cycle below 10%. The

LT3751 controls moderate loads with a combination of

peak current mode control and duty cycle control.

LT3751EUFD 数据表(PDF) 12 Page - Linear Technology

LT3751EUFD 数据表(HTML) 12 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

数据表下载