LT3751EUFD datasheet(10/28 Pages) LINER

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LT3751

3751f

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).

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

FB PIN

VOLTAGE

3751 F01

OVERVOLTAGE

REGULATION

CHARGE

MODE

1.34V

1.16V

0.0V

Figure 2. Idealized Charging Waveforms

VTRANS – VDS(ON)

LPRI

VTRANS – VDS(ON)

–N (VTRANS – VDS(ON))

VOUT + VDIODE

LSEC

IPK

–(VOUT + VDIODE)

ILPRI

ILSEC

VPRI

VSEC

VDRAIN

VOUT + VDIODE

VTRANS +

VTRANS

VDS(ON)

3751 F02

VDS(ON)

PRIMARY-SIDE

CHARGING

SECONDARY

ENERGY TRANSFER

AND OUTPUT

DETECTION

DISCONTINUOUS

MODE

DETECTION

Burst Mode is a registered trademark of Linear Technology Corporation.

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