LTC4270 datasheet(26/34 Pages) AD

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Data Sheet

LTC9101-2A/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 26 of 34

take as long as 1μs for the LTC9102 to begin regulating the current.

During this time, the current is limited only by the small impedances

in the circuit; a high current spike typically occurs, causing a voltage

transient on the VEE supply and possibly causing the LTC9101-2A/

LTC9102 to reset due to a UVLO fault. A 1μF, 100V X7R capacitor

and a SMAJ58A near each LTC9102 are recommended to minimize

spurious resets. An electrolytic bulk capacitor of at least 47μF,

100V, and a bulk TVS are also recommended per system.

LTC9102 Low Voltage Power Supplies

The LTC9102 includes internal voltage regulators that generate

low voltage supplies directly from the main PoE power supply. At

start-up, an internal regulator generates 6V at PWRIN, drawing

power from AGND. Internal 4.3V and 3.3V rails are sub-regulated

from PWRIN. The PWRIN pin requires a local 1μF, 100V bypass

capacitor.

Pull-up resistors can be connected from PWRIN to AGND to dissi-

pate heat outside the LTC9102 package. Optionally, an external

power supply can be connected to PWRIN to override the start-up

regulator and reduce power dissipation.

Figure 41 shows a pull-up resistor configuration with the internal

3.3V regulator. Bypass resistors R1, R2, R3, and R4 draw heat

away from the LTC9102s. Note that the voltage of the PWRIN

pin changes based on the LTC9102 operating mode and its corre-

sponding current consumption. If more current is consumed than

the bypass resistors provide, the start-up regulator maintains the

voltage at 6V typical. The LTC9102 can operate without the pull-up

resistors in space-constrained applications.

In applications with an external PWRIN supply, a 6.5V regulator

provides an optimum voltage to override the internal 6V start-up

regulator, while minimizing the LTC9102 device heating. The exter-

nal supply may be shared across multiple LTC9102s.

A 3.3V power supply can be connected directly to the CAP3 pin,

as shown in Figure 42. This provides the most power efficient

sleep mode. When supplying external 3.3V power, tie the EXT3

pin to CAP3. This disables the internal 3.3V regulator and prevents

power back-feed. The 3.3V regulator must power up within tCAP3EXT

specified in the Electrical Characteristics table.

If using the direct connection scheme, the 3.3V regulator that

supplies the LTC9101-2A can also supply the LTC9102s. This is the

recommended option when the LTC9101-2A and LTC9102 are on

the same side of the system isolation barrier.

Figure 41. Power Supply Configuration with Internal 3.3V Supply

Figure 42. Power Supply Configuration with External 3.3V Regulator

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