LTC4221IGN datasheet(22/28 Pages) LINER

22 / 28 page

LTC4221

4221f

supply voltages, the LTC4221 can drive any MOSFET rated

with 4.5V or 2.5V gate drive. For higher supply voltages up

to 13.5V, the LTC4221 can drive any MOSFET rated with

a 10V or 4.5V gate drive. The selected MOSFET should

fulfill two VGS criteria:

1. Positive VGS absolute maximum rating > LTC4221’s

maximum ∆VGATE.

2. Negative VGS absolute maximum rating > supply volt-

age. The gate of the MOSFET can discharge faster than

VOUT when shutting down the MOSFET with a large

CLOAD.

If one of the conditions cannot be met, an external zener

clamp shown on Figure 15 can be used. The clamp

network is connected from each channel’s GATE to the

VOUT pins. VGS is clamped in both directions and RG limits

the current flow into the GATE

n pin’s internal zener clamp

during transient events.

A MOSFET with a VGS absolute maximum rating of ±20V

meets the two criteria for all the LTC4221 application ranges

from 1V to 13.5V. Typically most 10V gate rated MOSFETs

have VGS absolute maximum ratings of ±20V or greater, so

no external VGS zener clamp is needed. There are 4.5V gate

rated MOSFETs with VGS absolute maximum ratings of

±20V. In addition to the MOSFET gate drive rating and VGS

absolute maximum rating, other criteria such as VBDSS,

ID(MAX), RDS(ON), PD, θJA, TJ(MAX) and maximum safe

operating area (SOA) should also be carefully reviewed.

VBDSS should exceed the maximum supply voltage inclu-

sive of spikes and ringing. ID(MAX) must exceed the maxi-

mum short-circuit current in the channel during a fault

condition. RDS(ON) determines the MOSFET VDS which to-

gether with VRSENSE yields an error in the VOUT voltage. For

example, at 1V VCC2, VDS + VRSENSE2 = 50mV gives a 5%

VOUT2error.AthigherVCC voltagestheVDS requirementcan

be relaxed in which case the MOSFET’s thermal require-

ments (PD, TJ(MAX), SOA) may limit the value of RDS(ON).

The power dissipated in the MOSFET is (ILOAD)2 • RDS(ON)

and this should be less than the maximum power dissipa-

tion, PD, allowed in that package. Given power dissipation,

the MOSFET junction temperature, TJ can be computed

from the operating temperature (TA) and the MOSFET

package thermal resistance (θJA). The operating TJ should

be less than the TJ(MAX) specification. The VDS • ILOAD

figure must also be well within the manufacturer’s recom-

mended safe operating area (SOA) with sufficient margin.

These three thermal parameters must not be exceeded for

all conditions in a channel including normal mode opera-

tion, start-up with or without current limit, fault and

autoretry after a fault. To ensure a reliable design, fault

tests should be evaluated in the laboratory.

VCC Transient Protection

Good engineering practice calls for bypassing the supply

rail of any analog circuit. Bypass capacitors are often

placed at the supply connection of every active device, in

addition to one or more large value bulk bypass capacitors

per supply rail. If power is connected abruptly, the large

bypass capacitors slow the rate of rise of the supply

voltage and heavily damp any parasitic resonance of lead

or PC track inductance working against the supply bypass

capacitors.

The opposite is true for LTC4221 Hot Swap circuits

mounted on plug-in cards since controlling the surge

current to bypass capacitors at plug-in is the primary

motivation for the Hot Swap controller. In most cases,

there is no supply bypass capacitor present on the pow-

ered supply voltage side of the MOSFET switch. Although

wire harness, backplane and PCB trace inductances are

usually small, these can create large spikes when large

currents are suddenly drawn, cut off or limited. Abrupt

intervention can prevent subsequent damage caused by a

catastrophic fault but it does cause a large supply tran-

sient. These ringing transients appear as a fast edge on

APPLICATIO S I FOR ATIO

*USER SELECTED VOLTAGE CLAMP

(A LOW BIAS CURRENT ZENER DIODE IS RECOMMENDED)

1N4688 (5V)

1N4692 (7V): LOGIC-LEVEL MOSFET

1N4695 (9V)

1N4702 (15V): STANDARD-LEVEL MOSFET

RSENSE

GATE

4221 F15

200Ω

D1*

D2*

VCC

VOUT

Figure 15. Gate Protection Zener Clamp

LTC4221IGN датащи(PDF) 22 Page - Linear Technology

LTC4221IGN датащи(HTML) 22 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

датащи скачать

ссылки URL