LT3751 LINER [Linear Technology], LT3751 Datasheet - Page 10

• Manufacturer Part Number: LT3751
• Description: Capacitor Charger Controller with Regulation
• Manufacturer: LINER [Linear Technology]

operaTion
LT3751
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).
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
V
10
CC
in low voltage applications (refer to the Application
CC
– 2V in high voltage applications or directly to
Figure 1. FB Pin Modes
1.16V
1.34V
0.0V
VOLTAGE
FB PIN
REGULATION
NO-LOAD
OPERATION
CHARGE
MODE
3751 F01
V
DRAIN
I
I
V
LSEC
I
V
LPRI
I
PK
PK
N
SEC
PRI
V
–N (V
V
TRANS
PRIMARY-SIDE
TRANS
CHARGING
V
Figure 2. Idealized Charging Waveforms
TRANS
DS(ON)
L
– V
PRI
1.
– V
DS(ON)
DS(ON)
– V
DS(ON)
)
ENERGY TRANSFER
AND OUTPUT
–(V
SECONDARY
V
DETECTION
V
V
TRANS
OUT
OUT
OUT +
2.
L
+ V
+ V
N
SEC
+
V
V
DIODE
DIODE
DIODE
OUT
+ V
)
N
DISCONTINUOUS
DIODE
DETECTION
MODE
3.
V
TRANS
V
DS(ON)
3751 F02
3751fc