LTC4268IDKD-1#PBF【PDF 35/46 页】IC PD HIGH POWER W/CNTRL 32-DFN

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42681fc

LTC4268-1

Converter Start-Up

The standard topology for the LTC4268-1 utilizes a third

transformer winding on the primary side that provides

both feedback information and local V

power for the

LTC4268-1 (see Figure 16). This power bootstrapping

improves converter efficiency but is not inherently self-

starting. Start-up is affected with an external trickle charge

resistor and the LTC4268-1s internal V

undervoltage

lockout circuit. The V

undervoltage lockout has wide

hysteresis to facilitate start-up.

In operation, the trickle charge resistor R

is connected

to V

and supplies a small current, typically on the order

of 1mA to charge C

. Initially the LTC4268-1 is off and

draws only its start-up current. When C

reaches the

turn-on threshold voltage the LTC4268-1 turns on

abruptly and draws its normal supply current.

Switching action commences and the converter begins to

deliver power to the output. Initially the output voltage is

low and the flyback voltage is also low, so C

supplies

most of the LTC4268-1 current (only a fraction comes

from R

.) V

voltage continues to drop until after some

time, typically tens of milliseconds, the output voltage

approaches its desired value. The flyback winding then

provides the LTC4268-1 supply current and the V

voltage stabilizes.

applicaTions inForMaTion

If C

is undersized, V

reaches the V

turn-off threshold

before stabilization and the LTC4268-1 turns off. The V

node then begins to charge back up via R

to the turn-on

threshold, where the part again turns on. Depending upon

the circuit, this may result in either several on-off cycles

before proper operation is reached, or permanent relaxation

oscillation at the V

node.

is selected to yield a worst-case minimum charging

current greater than the maximum rated LTC4268-1 start-

up current, and a worst-case maximum charging current

less than the minimum rated LTC4268-1 supply current.

TR(MAX)

IN(MIN)

CC(ON_MAX)

CC(ST _MAX)

and

TR(MIN)

IN(MAX)

CC(ON_MIN)

CC(MIN)

Make C

large enough to avoid the relaxation oscillatory

behavior described above. This is complicated to deter-

mine theoretically as it depends on the particulars of the

secondary circuit and load behavior. Empirical testing is

recommended. Note that the use of the optional soft-start

function lengthens the power-up timing and requires a

correspondingly larger value for C

VCC

42681 F16

VCC

THRESHOLD

LTC4268-1 PG

GND

Figure 16. Typical Power Bootstrapping

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