Datasheet

LT3751
11
3751fc
Information section for proper use of LVGATE). With the
gate driver output high, the external NMOS turns on,
forcing V
TRANS
– V
DS(ON)
across the primary winding.
Consequently, current in the primary coil rises linearly at
a rate (V
TRANS
– V
DS(ON)
)/L
PRI
. The input voltage is mir-
rored on the secondary winding –N • (V
TRANS
– V
DS(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 V
OUT
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 capaci-
tor, (V
OUT
+ V
DIODE
)/N will appear across the primary
winding. 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 V
TRANS
+ (V
OUT
+ V
DIODE
)/N to V
TRANS
. When
the drain voltage falls to V
TRANS
+ 20µA • R
DCM
, the DCM
operaTion
Figure 3. Start-Up Protection Circuitry
Figure 4. DCM Comparator Thresholds
comparator sets the NMOS switch latch and a new switch
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 V
DRAIN
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 = V
TH1
)
BELOW V
TH2
(WAIT FOR TIME-OUT)
V
DRAIN
3751 F04
V
OUT
DCM
1-SHOT
V
TH1
V
TH2
V
!
BOUNDARY-MODE
(DCM THRESHOLD = V
TH2
)
t
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, V
DRAIN
,
is below the DCM comparator threshold (see Entering
Normal Boundary Mode), the DCM comparator will never
fire and the start-up circuitry is dominant.