LTC4280CUFD datasheet(13/28 Pages) LINER

13 / 28 page

LTC4280

4280f

APPLICATIONS INFORMATION

A typical LTC4280 application is in a high availability system

in which a positive voltage supply is distributed to power

individual cards. The device measures card voltages and

currents and records past and present fault conditions.

The system queries each LTC4280 over the I2C periodically

and reads status and measurement information.

A basic LTC4280 application circuit is shown in Figure 1.

The following sections cover turn-on, turn-off and various

faults that the LTC4280 detects and acts upon. External

component selection is discussed in detail in the Design

Example section.

Turn-On Sequence

The power supply on a board is controlled by using

an external N-channel pass transistor (Q1) placed in

the power path. Note that resistor RS provides current

detection. Resistors R1, R2 and R3 deﬁne undervoltage and

overvoltage levels. R5 prevents high frequency oscillations

in Q1 and R6 and C1 form an optional network that may

be used to provide an output dV/dt limited start-up.

Several conditions must be present before the external

MOSFET turns on. First the external supply, VDD, must

exceed its 2.84V undervoltage lockout level. Next the

internally generated supply, INTVCC, must cross its 2.64V

undervoltage threshold. This generates a 60μs to 120μs

power-on-reset pulse. During reset the fault registers are

cleared and the control registers are set or cleared as

described in the register section.

After a power-on-reset pulse, the LTC4280 goes through

the following turn-on sequence. First the UV and OV pins

indicate that input power is within the acceptable range,

which is indicated by bits C0-C1 in Table 4. Second, the EN

pin is externally pulled low. Finally, all of these conditions

must be satisﬁed for the duration of 100ms to ensure that

any contact bounce during insertion has ended.

When these initial conditions are satisﬁed, the ON pin is

checked and it’s state written to bit A3 in Table 2. If it is

high, the external MOSFET is turned on. If the ON pin is

low, the external MOSFET is turned on when the ON pin is

brought high or if a serial bus turn-on command is sent

by setting bit A3.

The MOSFET is turned on by charging up the GATE with

a 20μA current source. When the GATE voltage reaches

the MOSFET threshold voltage, the MOSFET begins to

turn on and the SOURCE voltage then follows the GATE

voltage as it increases.

Figure 1. Typical Application

3.4K

PLUG-IN

CARD

1.18k

10Ω

0.005Ω

FDC653N

30.1k

VOUT

12V

3.57k

4280 F01

330μF

0.1μF

34.8k

BACKPLANE

0.1μF

CTIMER

0.68μF

GND

SCL

ALERT

SDA

12V

100k

P6KE16A

15k

6.8nF

UV VDD SENSE

SENSE–

LTC4280

GATE

ADR1 ADR2

ADR0

TIMER INTVCC

GND

SOURCE

SDAI

SDA0

SCL

ALERT

ADIN

GPIO

FILTER

47nF

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