LT3751 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT3751

Capacitor Charger Controller with Regulation

Linear Technology 

LT3751

OPERATION

Information section for proper use of LVGATE). With the

gate driver output high, the external NMOS turns on,

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

Consequently, current in the primary coil rises linearly at

a rate (VTRANS – VDS(ON))/LPRI. The input voltage is mir-

rored on the secondary winding – N • (VTRANS – VDS(ON))

which reverse-biases the diode and prevents current flow

in the secondary winding. Thus, energy is stored in the

core of the transformer.

3. Secondary Energy Transfer

When current limit is reached, the current limit comparator

resets the NMOS switch latch and the device enters the

third phase of operation, secondary energy transfer. The

energy stored in the transformer core forward-biases the

diode and current flows into the output capacitor. During

this time, the output voltage (neglecting the diode drop)

is reflected back to the primary coil. If the target output

voltage is reached, the VOUT comparator resets the master

latch and the DONE pin goes low. Otherwise, the device

enters the next phase of operation.

4. Discontinuous Mode Detection

During secondary energy transfer to the output capacitor,

(VOUT + VDIODE)/N will appear across the primary wind-

ing. A transformer with no energy cannot support a DC

voltage, so the voltage across the primary will decay to

zero. In other words, the drain of the NMOS will ring

down from VTRANS + (VOUT + VDIODE)/N to VTRANS. When

the drain voltage falls to VTRANS + 20μA • RDCM, the DCM

V

VTH1 VTH2

VDRAIN

comparator sets the NMOS switch latch and a new charge

cycle begins. Steps 2-4 continue until the target output

voltage is reached.

Start-Up Protection

The LT3751 at start-up, when the output voltage is very low

(or shorted), usually does not have enough VDRAIN node

voltage to trip the DCM comparator. The part in start-up

mode uses the internal 26kHz clock and an auxiliary current

comparator. Figure 3 shows a simplified block diagram of

the start-up circuitry.

FROM AUXILIARY

CURRENT

INCREMENT

COMPARATOR

COUNTER 1

–

FROM DCM

COMPARATOR

RESET

FROM CLK

INCREMENT

+

FROM GATE

DRIVER ON

COUNTER 2

RESET

Figure 3. Start-Up Protection Circuitry

SWITCH

LATCH

3751 F03

Toggling the CHARGE pin always generates a start-up

one-shot to turn on the external switch, initiating the charg-

ing process. After the start-up one-shot, the LT3751 waits

for either the DCM comparator to generate a one-shot or

the output of the start-up protection circuitry going high,

which ever comes first. If the switch drain node, VDRAIN,

is below the DCM comparator threshold (see Entering

Normal Boundary Mode), the DCM comparator will never

fire and the start-up circuitry is dominant.

VOUT

DCM

1-SHOT

START-UP

(DCM THRESHOLD = VTH1)

BOUNDARY-MODE

(DCM THRESHOLD = VTH2)

t

BELOW VTH2

(WAIT FOR TIME-OUT)

3751 F04

Figure 4. DCM Comparator Thresholds

3751f

11

Share Link: