LM5026SD datasheet(16/23 Pages) NSC

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Oscillator and Sync Capability

(Continued)

Multiple LM5026 devices can be synchronized together sim-

ply by connecting the devices SYNC pins together as shown

in Figure 9. Care should be taken to ensure the ground

potential differences between devices are minimized. In this

configuration all of the devices will be synchronized to the

highest frequency device. The internal block diagram of the

oscillator and synchronization circuit is shown in Figure 10.

The SYNC I/O pin is a CMOS buffer with pull-up current

limited to 200 µA. If an external device forces the SYNC pin

low before the internal oscillator ramp completes its charging

cycle, the ramp will be reset and another cycle begins. If the

SYNC pins of multiple LM5026 devices are connected to-

gether, the first SYNC pin that pulls low will reset the oscil-

lator RAMP of all other devices. All controllers will operate in

phase when synchronized using the SYNC I/O feature. Up to

five LM5026 devices can be synchronized using this tech-

nique.

Thermal Protection

Internal Thermal Shutdown circuitry is provided to protect the

integrated circuit in the event the maximum junction tem-

perature is exceeded. When activated, typically at 165˚C,

the controller is forced into a low power standby state with

the output drivers and the bias regulator disabled. The de-

vice will restart after the thermal hysteresis (typically 25˚C).

During thermal shutdown, the soft-start capacitor is fully

discharged and the controller follows a normal start-up se-

quence after the junction temperature falls to the operating

level.

Applications Information

LINE INPUT (VIN)

The LM5026 contains an internal high voltage start-up regu-

lator that allows the input pin (VIN) to be connected directly

to a nominal 48V line voltage. The voltage applied to the VIN

pin can vary in the range of 13 to 100V with transient

capability to 105V. When power is applied and the UVLO pin

potential is greater than 0.4V, the VCC regulator is enabled

and sources current into an external capacitor connected to

the VCC pin. When the voltage on the VCC pin reaches the

regulation point of 7.7V, the internal voltage reference (REF)

is enabled. The reference regulation set point is 5V. The

controller outputs are enabled when the UVLO pin potential

is greater than 1.25V. In typical applications, an auxiliary

transformer winding is connected through a diode to the

VCC pin. This winding must raise the VCC voltage above 8V

to shut off the internal start-up regulator. It is recommended

a filtering circuit shown in Figure 11 be used to suppress

transients, which may occur at the input supply, in particular

when VIN is operated close to the maximum operating rat-

ing.

FOR APPLICATION > 100V

For applications where the system input voltage exceed

100V or IC power dissipation is a concern, the LM5026 can

be powered from an external start-up regulator as shown in

Figure 12. In this configuration, the VIN and the VCC pins

should be connected together, which allows the LM5026 to

be operated below 13V. The voltage at the VCC pin must be

greater than 8V yet not exceed 15V. An auxiliary winding can

be used to reduce the dissipation in the external regulator

once the power converter is active.

20147920

FIGURE 9. Sync from Multiple Devices

20147921

FIGURE 10. Oscillator Sync I/O Block Diagram

20147922

FIGURE 11. Input Transient Protection

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