LT4220IGN datasheet(14/16 Pages) LINER

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LT4220

4220f

APPLICATIO S I FOR ATIO

Supply Tracking

If the TRACK pin (Pin 7) is high the supply power-up

tracking mode is enabled. This feature forces both sup-

plies to reach their final value at the same time, during

power-up and for faults that drive the output supplies to

zero. During this mode the GATE pins are controlled to

keep the differential magnitude of the FB pins to within

50mV. The FB pins are scaled versions of the output

voltages. Therefore, control of the FB pins, via the GATE

pins, will control the output voltages at the same scale.

∆VFB(TRK)| = |VFB+ – VFB–|

(6)

Supply tracking will continue until: either FB pin reaches

the associated PWRGD threshold. If any fault condition

occurs that turns the GATE pins off, supply tracking will be

reenabled. The GATE off conditions include: (1) either ON

pin detects undervoltage, (2) internal undervoltage lock-

out, (3) the fault latch is set by a current limit time-out.

VEE Bypassing

The VEE supply pin should be filtered with an RC network

to reduce high dV/dt slew rates from disturbing internal

circuits. Typical RC bypassing sufficient to prevent circuit

misbehavior is R14 = 10

Ω and C5 = 1µF. The GATE–,

SENSEK and SENSE– pins have been designed such that

they can be pulled below or above VEE for short periods of

time while the VEE pin is reaching its steady state voltage.

If desired, a higher R14 • C5 time constant may be used to

prevent short circuit transients from tripping the VEE

undervoltage lockout circuit at –2.45V. R14 should be

sufficient to decouple C5 from causing transients on VIN–

during live insertion.

Under the condition of a short circuit on VOUT–, parasitic

inductance and resistance in the VIN– path will cause VIN–

to collapse toward 0V causing the VEE pin voltage to also

discharge toward 0V before the external FET can be turned

off (typically 7

µs to 10µs). To prevent a UVLO condition

from occurring, the R14 • C5 time constant should be

sufficient to hold the VEE pin voltage out of the VEE UVLO

voltage range. If the VEE pin reaches its UVLO voltage,

GATE+ will also be pulled low. For the case where C3 is

large, causing an even slower N-channel FET turnoff,

higher RC bypassing may be necessary to prevent tripping

the VEE UVLO.

ON+, ON– Bypass Capacitors

Bypass capacitors are required from ON+ to ground and

ON– to ground. A typical time constant is:

TC (ON+) = (R1||R2)C7 = 44

µs

TC (ON–) = (R3||R4)C8 = 44

µs

Supply Ringing

Normal circuit design practice calls for capacitive bypass-

ing of the input supply to active devices. The opposite is

true for Hot Swap circuits that are connected into a

backplane, where capacitive loading would cause tran-

sients during an abrupt connection to the backplane. With

little or no capacitive decoupling on the powered side of

the N-channel FETs, connection transients or load tran-

sients will typically cause ringing on the supply leads due

to parasitic inductance. It is recommended to use a

snubber circuit comprising of a series 10

Ω and 0.1µF

capacitor to dampen transient ringing. The supply

decoupling circuit on the VEE pin also provides a snubber

for VIN–.

Additionally, if the supply voltage overshoot can exceed

the

±22V maximum rating on the part, a transient voltage

suppressor is recommended. Voltage transients can oc-

cur during load short-circuit conditions, where parasitic

inductance in the supply leads can build up energy before

the external N-channel FET can be turned off. This is

especially true for the negative side FET where a large C3

value slows the turn off of the N-channel FET. Subsequent

overshoot when the FET is finally turned off can be as

much as 2

× the supply voltage even with the snubber

circuit. Additional protection using a transient suppressor

may be needed to prevent exceeding the maximum supply

voltage rating.

Supply Reversal Protection

A variety of conditions on VOUT+ and VOUT– may result in

supply reversal. To protect devices connected to VOUT+

and VOUT– protection diodes should be used. 1N4001

diodes can be used for most aplications. Connection of

these diodes (D1, D2) are shown in the front page Typical

Application.

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