LT3751 datasheet(6/16 Pages) AD

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LT3750A

Rev. 0

For more information www.analog.com

OPERATION

The LT3750A is designed to charge capacitors quickly and

efficiently. Operation can be best understood by referring to

Figure 1 and Figure 2. Operation proceeds in four phases:

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

energy transfer, 4. Discontinuous mode sensing.

1. Start-Up

Start-up occurs for approximately 20µs 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.

ILPRI

ILSEC

•

VTRANS

VPRI

VSEC

3750a F01a

–

1:N

VDRAIN

ILPRI

ILSEC

1:N

•

VTRANS

VPRI

VSEC

3750a F01b

–

VDRAIN

ILPRI

ILSEC

1:N

•

VTRANS

VPRI

VSEC

3750a F01c

–

VDRAIN

(1a) Equivalent Circuit During Primary-Side Charging

(1b) Equivalent Circuit During Secondary Energy

Transfer and Output Detection

(1c) Equivalent Circuit During Discontinuous Mode Detection

Figure 1. Equivalent Circuits

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

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)

3750a F02

VDS(ON)

PRIMARY-SIDE

CHARGING

SECONDARY

ENERGY TRANSFER

AND OUTPUT

DETECTION

DISCONTINUOUS

MODE

DETECTION

Figure 2. Idealized Charging Waveforms

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