LT3751 查看數據表（PDF） - Linear Technology
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LT3751 Datasheet PDF : 28 Pages
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LT3751
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 overvoltage Burst Mode® Operation (see Figure 1).
FB PIN
VOLTAGE
1.34V
OVERVOLTAGE
REGULATION
1.16V
0.0V
CHARGE
MODE
3751 F01
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 VCC – 2V in high voltage applications or directly to
VCC in low voltage applications (refer to the Application
Burst Mode is a registered trademark of Linear Technology Corporation.
10
ILPRI
IPK
VTRANS – VDS(ON)
LPRI
ILSEC
IPK
N
VOUT + VDIODE
LSEC
VPRI VTRANS – VDS(ON)
VSEC
–(VOUT + VDIODE)
N
VOUT + VDIODE
–N (VTRANS – VDS(ON))
VDRAIN
VTRANS
+
VOUT
+ VDIODE
N
VTRANS
VDS(ON)
1.
PRIMARY-SIDE
CHARGING
VDS(ON)
2.
3.
SECONDARY DISCONTINUOUS
ENERGY TRANSFER
MODE
AND OUTPUT
DETECTION
DETECTION
3751 F02
Figure 2. Idealized Charging Waveforms
3751f
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