Data Sheet

LTC9105

PIN CONFIGURATION AND FUNCTION

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Rev. A | 7 of 22

Figure 2. Pin Configuration

Table 5. Pin Function

Pin No.

Mnemonic

Description

1

VIN

PD Interface Supply Voltage. Positive input voltage for IEEE state machine and PD input voltage are referenced to this pin.

2

RSIG

External Signature Resistor Connection. Connect a 24.9kΩ resistor between RSIG and VIN.

4

RCLSA

Class Select Input. Connect a resistor between RCLSA and RTN per Table 6.

5

RCLSB

Class Select Input. Connect a resistor between RCLSB and RTN per Table 6.

6

AUX

Auxiliary Power Supply Input Sensing. This pin senses the presence of another power supply input. The AUX pin status is

reflected in the aux_status bit. The AUX pin threshold can be configured by the aux_thresh bit. See Dual-Input Prioritization

for a description of AUX pin behavior with respect to port or auxiliary priority.

8

SNSN

Current Sense Negative Input. System current is monitored using a 50mΩ sense resistor between SNSP and SNSN. See Sense

Resistor for kelvin sense requirements.

9

SNSP

Current Sense Positive Input. System current is monitored using a 50mΩ sense resistor between SNSP and SNSN. See Sense

Resistor for kelvin sense requirements.

10

GATE

External Hot Swap MOSFET Gate Control. Connect to gate of the external MOSFET.

11

SDAOUT

Serial Data Output. Open Drain Data Output for the I2C Serial Interface Bus. The LTC9105 uses two pins to implement the

bidirectional SDA function to simplify isolation of the I2C bus. To implement a standard bidirectional SDA pin, tie SDAOUT and

SDAIN together. See I2C Interface for more information.

13

PWRGD

Power Good Open Drain Output. During inrush this pin sinks 250μA (typ.) to prevent the power supply from starting up. Current

draw begins when VIN-RTN exceeds the VON threshold and continues for tPWRGD (93.75ms typ.). The PWRGD pin is high

impedance before and after inrush.

14

SCL

Serial Clock Input. High impedance clock input for the I2C serial interface bus. Internally pulled up to BYP through a 50kΩ (typ.)

resistor. See I2C Interface for more information.

15

SDAIN

Serial Data Input. High impedance data input for the I2C serial interface bus. The LTC9105 uses two pins to implement the

bidirectional SDA function to simplify isolation of the I2C bus. To implement a standard bidirectional SDA pin, tie SDAOUT and

SDAIN together. Internally pulled up to BYP through a 50kΩ (typ.) resistor.

See I2C Interface for more information.

16

AD0

I2C Chip Address Bit 0 Select. Use this pin to change the I2C chip address. Tie this pin to RTN to set I2C address bit AD0 to 0.

Tie this pin to BYP to set bit AD0 of the I2C chip address to 1. Internally pulled down to RTN through a 50kΩ (typ.) resistor.

See I2C Interface for more information.

17, 20

M[1:0]

PoE Minimum Power Select. Use these pins to set the minimum number of Mark events required to enable the external MOSFET

and power up the load. Tie high by connecting to BYP. Tie low by connecting to RTN. Internally pulled down to RTN through a

50kΩ (typ.) resistor.

18

BYP

Internal Power Supply Bypass. Connect a 1μF ceramic capacitor between this pin and RTN.

7, 19, 21 (Exposed Pad)

RTN

PD Interface Return Rail. Pin 21 (Exposed Pad) must be connected to pin 7, pin 19, and PCB heat sink.

3, 12

NC

No Connection. Not internally connected.