MAX4370 Просмотр технического описания (PDF) - Maxim Integrated

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MAX4370

Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection

Maxim Integrated 

Current-Regulating Hot-Swap Controller with

DualSpeed/BiLevel Fault Protection

Case A: Slow Turn-On (without overcurrent)

There are two ways to turn on the MOSFET without

reaching the fast comparator current limit:

1) If the board capacitance (CBOARD) is low, the

inrush current is low.

2) If the capacitance at GATE is high, the MOSFET

turns on slowly.

In both cases, the turn-on (tON) is determined only by

the charge required to enhance the MOSFET—effec-

tively, the small gate-charging current limits the output

voltage dv/dt. This time can be extended by connect-

ing an external capacitor between GATE and GND, as

shown in Figure 7. The turn-on time is dominated by the

external gate capacitance if its value is considerably

higher than MOSFET gate capacitance. Table 4 shows

the timing required to enhance the recommended

MOSFET with or without an external capacitor at GATE;

Figures 2 and 3 show the related waveforms and timing

diagrams (see Start-Up Time with CBOARD = 0 and

Start-Up Time with External CGATE in the Typical

Operating Characteristics). Remember that a high gate

capacitance also increases the turn-off time.

When using the MAX4370 without an external gate

capacitor, RS is not necessary. RS prevents MOSFET

source oscillations that can occur when CGATE is high

while CBOARD is low.

Case B: Fast Turn-On (with current limit)

In applications where the board capacitor (CBOARD) at

VOUT is high, the inrush current causes a voltage drop

across RSENSE that exceeds the fast comparator

threshold (VFC,TH = 200mV). 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 2 shows the waveforms and timing dia-

grams for a turn-on transient with current regulation (see

Start-Up Time with CBOARD = 470µF in the Typical

Operating Characteristics). When operating under this

condition, an external gate capacitor is not required.

Adding an external capacitor at GATE reduces the regu-

lated current ripple but increases the turn-off time by

increasing the gate delay (td) (Figure 3).

VIN

RSENSE

M1

VOUT

RPULL-UP

CBOARD

RS*

VIN

STAT

ON

VSEN GATE

MAX4370

CGATE

CSPD

CTIM

GND

CSPD

CTIM

*OPTIONAL (SEE TEXT)

Figure 7. Operation with External Gate Capacitor

Table 4. MOSFET Turn-On Time (start-up without current limit)

(CBOARD = 0, turn-on with no load current, turn-off with 2A fault current)

DEVICE

CGATE

(nF)

MOSFET TURN-ON (tON)

VIN = 3V

VIN = 5V

VIN = 12V

MOSFET TURN-OFF (tOFF)

VIN = 3V

VIN = 5V

VIN = 12V

0

Fairchild FDS6670A

22

220µs

2.3ms

160µs

2ms

190µs

3.2ms

70µs

540µs

130µs

1.1ms

145µs

1.95ms

International Rectifier

0

IRF7401

22

175µs

1.9ms

130µs

1.8ms

160µs

3.5ms

75µs

540µs

130µs

1.1ms

160µs

2ms

Motorola

MMSF5N03HD

0

101µs

74µs

73µs

33µs

67µs

85µs

22

2ms

1.8ms

3.2ms

470µs

1ms

1.95ms

Electrical characteristics as specified by the manufacturer’s data sheet:

FDS6670A: CISS = 3200pF, QT(MAX) = 50nC, RDS(ON) = 8.2mΩ

IRF7401: CISS = 1600pF, QT(MAX) = 48nC, RDS(ON) = 22mΩ

MMSF5N03HD: CISS = 1200pF, QT(MAX) = 21nC, RDS(ON) = 40mΩ

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