LTC4245 datasheet(18/36 Pages) LINER

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LTC4245

4245fa

When the sequence bit C6 is set, setting the On control

bit of a supply, through the I2C interface, starts the supply

turn-on sequence from that supply onwards. For example,

setting bit D1 will turn-on 5V, 3.3V, –12V supplies, in that

order. A logic one can then be written to bit D0 to ramp

the 12V supply. At the end of this ramp-up, bit D1 is set.

But since 5V is already powered-up, the sequence stops

there.

The I2C interface provides the most ﬂexibility in turning

supplies on and off. With bit C6 cleared, any supply or

supplies can be turned on by setting their On control

bits. The On control bits cannot be set when any supply

is ramping (therefore using TIMER and SS pins). The SS

busy bit, A1, indicates this blanking period. The On control

bits can be reset though, even when a supply is ramping.

Two or more On control bits may be set at the same time

to ramp multiple supplies in the same timing cycle. When

all supplies are turned on the LTC4245 goes through the

PGI timing cycle.

Supply Voltage Conﬁguration

The CFG pin enables the LTC4245 to be used in non-CPCI

applications. It is a three-state input pin. In a CPCI applica-

tion with all four supplies, the CFG pin is tied to ground.

Floating the CFG pin disables the VEE undervoltage lockout

(UVLO), start-up foldback and power bad functions. It also

makes the ±12V turn-ons coincident by using the 12V FET

On control bit, D0, to control the –12V supply MOSFET.

This allows the three positive supplies to power-up and

HEALTHY# to assert, even when a negative supply is

either unavailable or does not meet the required thresh-

olds. If unused the VEEIN, VEESENSE, VEEGATE and VEEOUT

pins should be tied to ground. Since the circuit breaker

and active current limit circuits are not disabled, a lower

negative supply could be hot plugged. It would turn on

whenever the 12V supply turns on. Care should be taken

that the supply does not collapse under overcurrent con-

ditions. At low supplies, the ECB and ACL circuits stop

functioning. With the UVLO already disabled, the LTC4245

may not detect a fault condition on the VEE supply. Large

currents, limited only by MOSFET and sense resistances,

could ﬂow, potentially damaging the board traces and

connector pins.

If the CFG pin is tied high, the 5V supply thresholds change

to 3.3V levels, while keeping the ﬂoating state functionality.

The 5V supply UVLO, power bad thresholds and foldback

proﬁle become similar to those of the 3.3V supply. The

5VIN and 5VOUT inputs to the ADC use the same LSB and

full-scale as the 3VIN and 3VOUT pins. This allows the use

of an extra 3.3V supply instead of a 5V supply as in a PCI

Express application.

Overcurrent Fault

The LTC4245 has different current limiting behavior dur-

ing start-up, when supply ramps up under TIMER and SS

control, and normal operation. As such it can generate an

overcurrent fault during both phases of operation. Both

set the faulting supply’s overcurrent fault bit (bits E4 to

E7) and shut off all external FETs.

During start-up when both TIMER and SS are ramping,

the current limit is a function of SS pin voltage and the

ramping supplies’ output voltages. A supply could power

up entirely in current limit depending on the bypass ca-

pacitor at the outputs of the ramping supplies. The TIMER

pin sets the time duration for current limit during start-up.

This time involves the TIMER charging up to 2.56V with a

100μA current source and then resetting to 0.23V with a

switch. At the end of the timing cycle if the supply is still

in current limit, i.e., the gate of it’s external MOSFET is still

being actively controlled, an overcurrent fault is declared

for that supply and all MOSFETs are shut off (Figure 6).

Therefore the maximum time a supply can stay in current

limit at start-up is given by:

START

TMCAP

⎡⎣

⎤⎦

••

23 3

(4)

APPLICATIO S I FOR ATIO

Figure 6. Start-Up Into a Short on 3.3V Output

TIME 10ms/DIV

4245 F06

3.3V SUPPLY

CURRENT 2.5A/DIV

3VGATE 2.5V/DIV

SS 2.5V/DIV

TIMER 2.5V/DIV

ON 5V/DIV

12VOUT, 5VOUT

3VOUT, VEEOUT,

10V/DIV

HEALTHY#

5V/DIV

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