MAX4272ESA datasheet(17/24 Pages) MAXIM

17 / 24 page

3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

______________________________________________________________________________________

CTON determines the maximum time allowed to com-

plete turn-on for the MAX4273. The default values for

turn-on time (tON) and tRETRY are chosen by leaving

these pins floating; they are 10µs and 320µs, respec-

tively. These are also the minimum values (not con-

trolled and dependent on stray capacitance). Longer

timings are determined by the size of the capacitor

according to Figure 8, and can be determined in Table

5, which lists the startup and retry timing parameters.

Set the tON timer long enough to allow for the MOSFET to

be enhanced and the load capacitor to be charged com-

pletely .

There are two ways of completing the startup sequence.

Case A describes a startup sequence that does not use

the current-limiting feature and slowly turns on the MOS-

FET by limiting the gate dv/dt. Case B uses the current-

limiting feature and turns on the MOSFET as fast as

possible while still preventing a high inrush current.

Case A. Startup Without Current Regulation

There are three ways to turn on the MOSFET without

reaching the fast comparator current limit:

1) If the board capacitance is small, the inrush current

is low.

2) If the gate capacitance is high, the MOSFET turns on

slowly.

3) The fast comparator can be disabled (MAX4273

only).

In all three cases, tON is determined only by the charge

required to enhance the MOSFET. Effectively, the small

gate-charging current limits the output voltage dv/dt.

This time can be extended by connecting an external

capacitor between GATE and GND (MAX4271/

MAX4272) (Figure 9) or between GATE and CEXT

(MAX4273). The turn-on time is dominated by the exter-

nal gate capacitance if this value is considerably higher

than the MOSFET gate capacitance. Table 6 shows the

timing required to enhance the recommended MOSFET

with or without the external capacitor; Figure 3 shows

the related waveforms and timing diagrams. (See Time

to Charge Gate vs. CGATE and Startup Time with

CBOARD = 0 in the Typical Operating Characteristics.)

Remember that a high gate capacitance also increases

the turn-off time (tOFF), except in the case of a

MAX4273 fast fault.

If an external gate capacitor is not used, RS is not nec-

essary. RS prevents MOSFET self-oscillations that can

occur when CGATE is high while CBOARD is low.

Electrical characteristics as specified by the manufac-

turer’s data sheet are:

FDS6670A: CISS = 3200pF, QT(MAX) = 50nC, RDS(ON) =

8.2m

Ω

IRF7401: CISS = 1600pF, QT(MAX) = 48nC, RDS(ON) =

22m

Ω

MMSF5N03HD: CISS = 1200pF, QT(MAX) = 21nC,

RDS(ON) = 40m

Ω

Figure 7. Fast Comparator Threshold vs. RTH

200

100

400

300

500

600

700

800

020

FAST COMPARATOR THRESHOLD

VOLTAGE vs. RTH

RTH (k

Ω)

RTH (k

Ω) = VTH,FC (mV)/10

INDETERMINATE,

NOT RECOMMENDED FOR

RTH < 5k

Ω

DISABLED FOR

RTH < 200

Ω

*MAX4273 retry feature disabled by connecting CTIM to VIN.

Table 5. Startup and Retry Timing Parameters

tON

tRETRY

PART

DEFAULT (µs)

EXTERNALLY SET

DEFAULT (µs)

EXTERNALLY SET

MAX4271

tON (ms) = 0.31 x CTIM (nF)

No retry available

MAX4272

tON (ms) = 0.31 x CTIM (nF)

320

tRETRY (ms) = 32 x tON = 10 x CTIM (nF)

MAX4273*

tON (ms) = 0.31 x CTON (nF)

320

tRETRY (ms) =10 x CTIM (nF)

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