LT3750 datasheet(6/16 Pages) LINER

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LT3750

3750fa

OPERATIO

The LT3750 is designed to charge capacitors quickly and

efficiently. Operation can be best understood by referring

to Figures 1 and 2. Operation proceeds in four phases:

1. Start-up, 2. Primary-side charging, 3. Secondary en-

ergy transfer, 4. Discontinuous mode sensing.

1. Start-Up

Start-up occurs for approximately 20

µs after the charge

pin is raised high. During this phase, a one-shot enables

the master latch and turns on the NMOS. The master latch

will remain in the set state until the target output voltage

is reached or a fault condition resets it.

2. Primary Side Charging

When the NMOS on latch is set, the gate driver rapidly

charges the gate pin to VCC – 2V. The external NMOS turns

on forcing VTRANS – VDS(ON) across the primary winding.

Consequently, current in the primary coil rises linearly at

ILPRI

ILSEC

•

VTRANS

VPRI

VSEC

3750 F01a

–

1:N

VDRAIN

(1a) Equivalent Circuit During Primary-Side Charging

ILPRI

ILSEC

1:N

•

VTRANS

VPRI

VSEC

3750 F01b

–

VDRAIN

(1b) Equivalent Circuit During Secondary Energy

Transfer and Output Detection

ILPRI

ILSEC

1:N

•

VTRANS

VPRI

VSEC

3750 F01c

–

VDRAIN

(1c) Equivalent Circuit During Discontinuous Mode Detection

Figure 1. Equivalent Circuits

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)

3750 F02

VDS(ON)

PRIMARY-SIDE

CHARGING

SECONDARY

ENERGY TRANSFER

AND OUTPUT

DETECTION

DISCONTINUOUS

MODE

DETECTION

Figure 2. Idealized Charging Waveforms

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