LTC4230IGN datasheet(29/36 Pages) LINER

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LTC4230

4230f

Figure 20. Placing Transient Protection

Devices Close to the LTC4230

VCCn

SENSE

LTC4230*

CLOADn

VOUT

4230 F20

GATE

GND

TIMER

CTIMER

RESET

Ω

VIN

RSENSE

0.007

Ω

Si4410DY

RESET

0.1

µF

SMAJ10

*ADDITIONAL DETAILS

OMITTED FOR CLARITY

The opposite is true for LTC4230 Hot Swap circuits

mounted on plug-in cards. In most cases, there is no

supply bypass capacitor present on the powered supply

voltage side of the MOSFET switch. An abrupt connection,

produced by inserting the board into a backplane connec-

tor, results in a fast rising edge applied on the supply line

of the LTC4230.

Since there is no bulk capacitance to damp the parasitic

track inductance, supply voltage transients excite

parasitic resonant circuits formed by the power MOSFET

capacitance and the combined parasitic inductance from

the wiring harness, the backplane and the circuit board

traces. These ringing transients appear as a fast edge on

the input supply line, exhibiting a peak overshoot to 2.5

times the steady-state value. This peak is followed by a

damped sinusoidal response whose duration and period

are dependent on the resonant circuit parameters. Since

the absolute maximum supply voltage of the LTC4230 is

17V, transient protection against VCC > 16.8V supply

voltage spikes and ringing is highly recommended.

In these applications, there are two methods for eliminat-

ing these supply voltage transients: using zener diodes to

clip the transient to a safe level and snubber networks.

Snubber networks are series RC networks whose time

constants are experimentally determined based on the

board’s parasitic resonance circuits. As a starting point,

the capacitors in these networks are chosen to be 10

× to

100

× the power MOSFET’s COSS under bias. The series

resistor is a value determined experimentally and ranges

from 1

Ω to 50Ω, depending on the parasitic resonance

circuit. Note that in all LTC4230 circuit schematics,

TransZorb

® diodes and snubber networks have been

added to each 3.3V and 5V supply rail. These protection

networks should be mounted very close to the LTC4230’s

supply voltage using short lead lengths to minimize lead

inductance. This is shown schematically in Figure 20, and

a recommended layout of the transient protection devices

around the LTC4230 is shown in Figure 21.

ADDITIONAL SUPPLY OVERVOLTAGE

DETECTION/PROTECTION

In addition to using external protection devices around the

LTC4230 for large scale transient protection, low power

zener diodes can be used with the LTC4230’s FILTER pin

to act as a supply overvoltage detection/protection circuit

on either the high side (input) or low side (output) of the

external pass transistor. Recall that internal control cir-

cuitry keeps the LTC4230 GATE

n voltage from ramping up

if VFILTER > 1.26V, or when an external fault condition

(VFAULT < 1.234V) causes FAULT to be asserted low.

High Side (Input) Overvoltage Protection

As shown in Figure 22, a low power zener diode can be

used to sense an overvoltage condition on the input

(high) side of the main 5V supply. In this example, a low

APPLICATIO S I FOR ATIO

VCC2

4230 F21

NOTE: DRAWING IS NOT TO SCALE!

USE SIMILAR TECHNIQUES FOR VCC1 AND VCC3

*ADDITIONAL DETAILS OMITTED FOR CLARITY

SNUBBER

NETWORK

VIAS TO

GND PLANE

VCC2

GND

LTC4230*

Figure 21. Recommended Layout

for Transient Protection Devices

TransZorb is a registered trademark of General Instruments, GSI.

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