LTC1439CG datasheet(11/32 Pages) LINER

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LTC1438/LTC1439

OPERATION

the AUXDR pin is above 9.5V to allow regulated 12V

VPP supplies to be easily implemented. When AUXDR is

below 8.5V an external feedback divider may be used to set

other output voltages. Taking the AUXON pin low shuts

down the auxiliary regulator providing a convenient logic-

controlled power supply.

The AUX block can be used as a comparator having its

inverting input tied to the internal 1.19V reference. The

AUXDR pin is used as the output and requires an external

pull-up to a supply of less than 8.5V in order to inhibit the

invoking of the internal resistive divider.

INTVCC/ EXTVCC Power

Power for the top and bottom MOSFET drivers and most

of the other LTC1438/LTC1439 circuitry is derived from

the INTVCC pin. The bottom MOSFET driver supply is also

connected to INTVCC. When the EXTVCC pin is left open, an

internal 5V low dropout regulator supplies INTVCC power.

If EXTVCC is taken above 4.8V, the 5V regulator is turned

off and an internal switch is turned on to connect EXTVCC

to INTVCC. This allows the INTVCC power to be derived

from a high efficiency external source such as the output

of the regulator itself or a secondary winding, as described

in the Applications Information section.

(Refer to Functional Diagram)

Frequency Synchronization

A Phase-Locked Loop (PLL) is available on the LTC1439

to allow the oscillator to be synchronized to an external

source connected to the PLLIN pin. The output of the

phase detector at the PLL LPF pin is also the control input

of the oscillator, which operates over a 0V to 2.4V range

corresponding to – 30% to 30% in frequency. When

locked, the PLL aligns the turn-on of the top MOSFET to

the rising edge of the synchronizing signal. When PLLIN

is left open, PLL LPF goes low, forcing the oscillator to

minimum frequency.

Power-On Reset

The POR2 pin is an open drain output which pulls low

when the main regulator output voltage of the second

controller is out of regulation. When the output voltage

rises to within 7.5% of regulation, a timer is started which

releases POR2 after 216 (65536) oscillator cycles. This

function is not available on the LTC1438X.

Auxiliary Linear Regulator

The auxiliary linear regulator in the LTC1439 controls an

external PNP transistor for operation up to 500mA. A

precise internal AUXFB resistive divider is invoked when

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