LT3751
12
3751fd
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OPERATION
At very low output voltages, the boundary-mode switch-
ing 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 continuous-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 nomi-
nal 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 oper-
ation 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 cur-
rent 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 sig-
nal is negative edge triggered by the switch node, V
DRAIN
,
and indicates that the energy in the secondary winding
has depleted. For this to happen, V
DRAIN
must exceed
VTRANS + ΔVDRAIN prior to its negative edge; otherwise,
the DCM comparator will not generate a one-shot to initi-
ate 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. V
TH1
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 V
TH2
(I
OFFSET
= 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 tech-
niques. Figure 6 shows how both techniques are com-
bined 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 maxi-
mum 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.