Datasheet

LT3751
10
3751fc
operaTion
The LT3751 can be used as either a fast, efficient high
voltage capacitor charger controller or as a high voltage,
low noise voltage regulator. The FB pin voltage determines
one of the three primary modes: charge mode, low noise
regulation, or no-load operation (see Figure 1).
Figure 1. FB Pin Modes
CHARGE MODE
When the FB pin voltage is below 1.16V, the LT3751 acts
as a rapid capacitor charger. The charging operation has
four basic states for charge mode steady-state operation
(see Figure 2).
1. Start-Up
The first switching cycle is initiated approximately 2µs
after the CHARGE pin is raised high. During this phase,
the start-up one-shot enables the master latch turning
on the external NMOS and beginning the first switching
cycle. After start-up, the master latch will remain in the
switching-enable state until the target output voltage is
reached or a fault condition occurs.
The LT3751 utilizes circuitry to protect against transformer
primary current entering a runaway condition and remains
in start-up mode until the DCM comparator has enough
headroom. Refer to the Start-Up Protection section for
more detail.
2. Primary-Side Charging
When the NMOS switch latch is set, and depending on
the
use
of LVGATE, the gate driver rapidly charges the gate
pin to V
CC
– 2V in high voltage applications or directly to
V
CC
in low voltage applications (refer to the Application
FB PIN
VOLTAGE
3751 F01
NO-LOAD
OPERATION
REGULATION
CHARGE
MODE
1.34V
1.16V
0.0V
Figure 2. Idealized Charging Waveforms
V
TRANS
– V
DS(ON)
L
PRI
V
TRANS
– V
DS(ON)
–N (V
TRANS
– V
DS(ON)
)
V
OUT
+ V
DIODE
V
OUT
+ V
DIODE
L
SEC
I
PK
I
PK
N
–(V
OUT
+ V
DIODE
)
N
I
LPRI
I
LSEC
V
PRI
V
SEC
V
DRAIN
V
OUT
+ V
DIODE
N
V
TRANS
+
V
TRANS
V
DS(ON)
3751 F02
V
DS(ON)
1.
PRIMARY-SIDE
CHARGING
2.
SECONDARY
ENERGY TRANSFER
AND OUTPUT
DETECTION
3.
DISCONTINUOUS
MODE
DETECTION