LTC4282 datasheet(24/32 Pages) AD

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LTC4238

Rev. 0

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current limit comparator engages at 3× the current limit

threshold and has a propagation delay of 500ns. If the

supply inductance is less than 2000nH in a 48V applica-

tion, it is unlikely that the VDD UVLO threshold will be

breached and the fast di/dt rate allows the current to rise

to the 3× level long before the UV pin responds.

Once the fast current limit comparator begins to arrest

the short circuit current, the input voltage rapidly recovers

and even overshoots its DC value. The LTC4238 is safe

from damage up to 100V. In card-resident applications

clamp the VDD pin with a surge suppressor Z1, as shown

in Figure 1. To minimize spikes in backplane resident

applications, bypass the LTC4238 input supply with an

electrolytic capacitor between VDD and GND.

In the worst case, Z1 has to absorb the high current that

triggers the fast current limit comparator. Several surge

suppressors may be required to clamp this current for

high power applications. In applications where a solid

ground is not available to connect the surge suppressor,

it may be connected from input to output, allowing the

output capacitance to absorb spikes. In 12V applications,

many 20V to 30V MOSFETs enter avalanche breakdown

before 50V. In such a case, the MOSFET can also act as

a surge suppressor and protect the Hot Swap controller

from inductive input voltage surges.

Design Examples

The design flow starts with specifying the maximum load

power and the operating voltage limits. If redundant sup-

plies are used, the system usually has wide supply range

and can experience large input steps when switching. An

operation mode is then selected based on the approximate

guideline in Table 4.

APPLICATIONS INFORMATION

50k

INTVCC

LTC4238

GATE

COMM

48V

4238 F09

PROCHOTB

Figure 9. Current Limit/PROCHOT Indication Using the COMM Pin

faults. When using thermal networks, each individual part

has its own thermal network and may generate a thermal

fault regardless of the state of the COMM pin.

When multiple LTC4238s work together by having COMM

pins connected, care must be taken to make sure all the

LTC4238s see the same solid ground potential. Ground

bounce could corrupt COMM function or possibly damage

the LTC4238 if the absolute maximum rating is violated.

Putting a current limiting resistor in series with the COMM

pins, or putting a Schottky or capacitor between COMM

and ground next to LTC4238 may mitigate ground noise.

The COMM pin may also be used as a current limit or

processor hot (PROCHOTB) indicator for a single or mul-

tiple LTC4238s, if the group is in current limit, the pin is

pulled to INTVCC. A PNP and resistor circuit can convert

this signal to a logic signal, as shown in Figure 9.

Table 4. The Guideline for Mode Selection

Mode

Power Level

Supply Range

Single Driver

<800W

Narrow or Wide

Parallel

<1500W

Narrow or Wide

HSSS

>1500W

Wide

LSSS

>1500W

Narrow

Two or More LTC4238

Using the COMM Pin

>3000W

Supply Transients

In card-resident applications, output short circuits work-

ing against the inductive nature of the supply can easily

cause the input voltage to dip below the UV threshold.

In severe cases where the supply inductance is 500nH or

more, the input can dip below the VDD undervoltage lock-

out threshold of 5.5V. It is possible for the UV comparator

or, the VDD UVLO circuit to respond before the current

reaches the current limit threshold. Adding a 100nF filter

capacitor to the UV pin ensures that the UV compara-

tor responds after current limiting commences. The fast

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