LT3751EUFD datasheet(11/28 Pages) LINER

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LT3751

3751f

Information section for proper use of LVGATE). With the

gate driver output high, the external NMOS turns on,

forcing VTRANS – VDS(ON) across the primary winding.

Consequently, current in the primary coil rises linearly at

a rate (VTRANS – VDS(ON))/LPRI. The input voltage is mir-

rored on the secondary winding –N • (VTRANS – VDS(ON))

which reverse-biases the diode and prevents current ﬂow

in the secondary winding. Thus, energy is stored in the

core of the transformer.

3. Secondary Energy Transfer

When current limit is reached, the current limit comparator

resets the NMOS switch latch and the device enters the

third phase of operation, secondary energy transfer. The

energy stored in the transformer core forward-biases the

diode and current ﬂows into the output capacitor. During

this time, the output voltage (neglecting the diode drop)

is reﬂected back to the primary coil. If the target output

voltage is reached, the VOUT comparator resets the master

latch and the DONE pin goes low. Otherwise, the device

enters the next phase of operation.

4. Discontinuous Mode Detection

During secondary energy transfer to the output capacitor,

(VOUT + VDIODE)/N will appear across the primary wind-

ing. A transformer with no energy cannot support a DC

voltage, so the voltage across the primary will decay to

zero. In other words, the drain of the NMOS will ring

down from VTRANS + (VOUT + VDIODE)/N to VTRANS. When

the drain voltage falls to VTRANS + 20μA • RDCM, the DCM

OPERATION

Figure 3. Start-Up Protection Circuitry

Figure 4. DCM Comparator Thresholds

comparator sets the NMOS switch latch and a new charge

cycle begins. Steps 2-4 continue until the target output

voltage is reached.

Start-Up Protection

The LT3751 at start-up, when the output voltage is very low

(or shorted), usually does not have enough VDRAIN node

voltage to trip the DCM comparator. The part in start-up

mode uses the internal 26kHz clock and an auxiliary current

comparator. Figure 3 shows a simpliﬁed block diagram of

the start-up circuitry.

FROM AUXILIARY

CURRENT

COMPARATOR

3751 F03

FROM CLK

FROM GATE

DRIVER ON

FROM DCM

COMPARATOR

RESET

INCREMENT

COUNTER 2

RESET

INCREMENT

COUNTER 1

SWITCH

LATCH

START-UP

(DCM THRESHOLD = VTH1)

BELOW VTH2

(WAIT FOR TIME-OUT)

VDRAIN

3751 F04

VOUT

DCM

1-SHOT

VTH1

VTH2

BOUNDARY-MODE

(DCM THRESHOLD = VTH2)

Toggling the CHARGE pin always generates a start-up

one-shot to turn on the external switch, initiating the charg-

ing process. After the start-up one-shot, the LT3751 waits

for either the DCM comparator to generate a one-shot or

the output of the start-up protection circuitry going high,

which ever comes ﬁrst. If the switch drain node, VDRAIN,

is below the DCM comparator threshold (see Entering

Normal Boundary Mode), the DCM comparator will never

ﬁre and the start-up circuitry is dominant.

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