MAX4272ESA datasheet(18/24 Pages) MAXIM

18 / 24 page

3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

______________________________________________________________________________________

Case B. Startup With Current Regulation

In applications where the board capacitance (CBOARD)

at VOUT is high, the inrush current causes a voltage

drop across RSENSE that exceeds the fast comparator

threshold (VFC,TH). In this case, the current charging

CBOARD can be considered constant and the turn-on

time is determined by:

tON = CBOARD ✕ VIN / IFAST,SET

where the maximum load current IFAST,SET = VFC,TH /

RSENSE. Figure 3 shows the waveforms and timing dia-

grams for a startup transient with current regulation. (See

Startup Time (CBOARD = 470µF) in the Typical Operating

Characteristics.) When operating under this condition, an

external gate capacitor is not required. Adding an exter-

nal gate capacitor at GATE to GND reduces the regulat-

ed current ripple but increases tOFF by increasing the

gate delay (td) (Figure 4).

The actual startup time is determined by the longer of

the two timings of Case A and Case B. Set the startup

timer tSTART at 2 ✕ tON to guarantee enough time for

the output voltage to settle; also take into consideration

device parameter variation.

Retry

The retry timer defines the dead time before the IC tries

to restart a startup sequence following a fault detection.

This feature is available on the MAX4272/MAX4273.

Before selecting the retry timer value, determine how

long a temporary high-current fault condition may be

present. In the event of a permanent fault, the automat-

ic retry will effectively force current pulses through the

Figure 8. Startup and Retry Timeout

tON AND tRETRY vs.

CTON AND CTIM

CAPACITANCE (nF)

10,000

0.1

0.01

100

1000

0.01

100

0.1

1000

tRETRY vs. CTIM (MAX4272/MAX4273)

tON vs. CTIM (MAX4271/MAX4272)

tON vs. CTON (MAX4273)

Figure 9. Operation with an External Capacitor

CSPD

*OPTIONAL (SEE TEXT)

CTIM

GATE

SENSE

GND

CSPD

RSENSE

VOUT

CGATE

CBOARD

VIN

RS*

MAX4271

MAX4272

MAX4273

RPULLUP

STAT

Table 6. “No-Overcurrent” Turn-On Timing (Startup Without Current Limit)

MOSFET tON (ms)

MOSFET tOFF (ms)

DEVICE

CGATE

(nF)

VIN = 3V

VIN = 5V

VIN = 12V

VIN = 3V

VIN = 5V

VIN = 12V

0.22

0.16

0.19

0.07

0.13

0.145

Fairchild FDS6670A

2.3

3.2

0.54

1.1

1.95

0.175

0.130

0.16

0.075

0.13

0.16

International Rectifier

IRF7401

1.9

1.8

3.5

0.54

1.1

2.0

0.101

0.074

0.073

0.033

0.067

0.085

Motorola MMSF5N03HD

2.0

1.8

3.2

0.470

1.0

1.95

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