LTC4230IGN datasheet(31/36 Pages) LINER

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LTC4230

4230f

APPLICATIO S I FOR ATIO

bias current 1N4691 zener diode is chosen to protect the

system. Here, the zener diode is connected from VCC to

the LTC4230’s FILTER pin. If the input voltage to the

system is greater than 6.8V during start-up, the voltage

on the FILTER pin is pulled higher than its 1.19V thresh-

old. As a result, the GATE

n pin is not allowed to ramp and

the second timing cycle will not commence until the

supply overvoltage condition is removed. Should the

supply overvoltage condition occur during normal op-

eration, internal control logic would trip the electronic

circuit breaker and the GATE would be pulled to ground,

shutting off the external pass transistor. If a lower supply

overvoltage threshold is desired, use a zener diode with

a smaller breakdown voltage.

A timing diagram for illustrating LTC4230 operation under

a high side overvoltage condition is shown in Figure 23.

The start-up sequence in this case (between Time Points

1 and 2) is identical to any other start-up sequence under

normal operating conditions. At Time Point 2, the input

supply voltage causes the zener diode to conduct thereby

forcing VFILTER > 1.19V. At Time Point 3, FAULT is asserted

low and the TIMER pin voltage ramps down. At Time

Point 4, the LTC4230 checks if VFILTER < 1.19V. FAULT is

asserted low (but not latched) to indicate a start-up failure.

Only if the input overvoltage condition is removed before

Time Point 5 does the start-up sequence resume at the

second timing cycle. At this point in time, the GATE

n pin

voltage is allowed to ramp up, FAULT is pulled to logic high

and the circuit breaker is armed. Should, at any time after

Time Point 5, a supply overvoltage condition develop

(VFILTER > 1.26V), the electronic circuit breaker will trip,

the GATE

n will be pulled low to turn off the external

MOSFET and FAULT will be asserted low and latched.

Low Side (Output) Overvoltage Protection

A zener diode can be used in a similar fashion to detect/

protect the system against a supply overvoltage condition

on the load (or low) side of the pass transistor. In this case,

the zener diode is connected from the load to the LTC4230’s

FILTER pin, as shown in Figure 24. An additional diode,

D1, prevents the FILTER pin from pulling low during

output short-circuit. Figure 25 illustrates the timing dia-

gram for a low side output overvoltage condition. In this

example, the LTC4230 can only sense the overvoltage

supply condition after Time Point 5 and the GATE

n pin has

ramped up to its nominal operating value. After Time

Point 5, a supply voltage fault occurs at the load and the

zener diode conducts, causing VFILTER to increase. At Time

Point 6, VFILTER is greater than 1.26V, the circuit breaker

trips, GATE pulls to ground and FAULT asserts low and is

latched.

In either case, the LTC4230 can be configured to auto-

matically initiate a start-up sequence. Please refer to the

section on AutoRetry After a Fault for additional

information.

PCB LAYOUT CONSIDERATIONS

For proper operation of the LTC4230’s circuit breaker

function, a 4-wire Kelvin connection to the sense resistors

is highly recommended. A recommended PCB layout for

the sense resistor, the power MOSFET and the GATE drive

components around the LTC4230 is illustrated in Fig-

ure 26. In Hot Swap applications where load currents can

reach 10A or more, narrow PCB tracks exhibit more

resistance than wider tracks and operate at more elevated

temperatures. Since the sheet resistance of 1 ounce

copper foil is approximately 0.54m

Ω/square, track resis-

tances add up quickly in high current applications. Thus,

to keep PCB track resistance and temperature rise to a

minimum, PCB track width must be appropriately sized.

Consult Appendix A of LTC Application Note 69 for details

on sizing and calculating trace resistances as a function of

copper thickness.

In the majority of applications, it will be necessary to use

plated-through vias to make circuit connections from

component layers to power and ground layers internal to

the PC board. For 1 ounce copper foil plating, a good

starting point is 1A of DC current per via, making sure the

via is properly dimensioned so that solder completely fills

any void. For other plating thicknesses, check with your

PCB fabrication facility.

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