LTC4211CS8 datasheet(28/36 Pages) LINER

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LTC4211

4211f

APPLICATIO S I FOR ATIO

voltage side of the MOSFET switch. An abrupt connection,

produced by inserting the board into a backplane connec-

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

of the LTC4211.

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.

In these applications, there are two methods that should

be applied together for eliminating these supply voltage

transients: using transient voltage suppressor 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 para-

sitic 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. For applica-

tions with supply voltages of 12V or higher the ringing and

overshoot during hot-swapping or when the output is

short-circuited can easily exceed the absolute maximum

specification of the LTC4211. To reduce the danger,

transient voltage suppressors and snubber networks are

highly recommended. For applications with lower supply

voltage such as 5V, usually a snubber is adequate to

reduce the supply ringing. Although, the need of a tran-

sient voltage suppressor arises for inductive and high

current application. Note that in all LTC4211 5V applica-

tions schematics, transient suppressor and snubber net-

works have been added for protection. The transient

suppressor is optional and a simple short-circuit test can

be performed to determine the need of it. These protection

networks should be mounted very close to the LTC4211’s

supply input rail using short lead lengths to minimize lead

inductance. This is shown schematically in Figure 21, and

a recommended layout of the transient protection devices

around the LTC4211 is shown in Figure 22.

VCC

SENSE

GATE

RESET

TIMER

GND

LTC4211

4211 F20

1N4148

1/2 Si4936DY

0.015

Ω

Z1*

0.1

µF

10nF

16V

10k

Ω

2.74k

TRIP POINT: 4.06V

1.2k

0.047

µF

25V

0.022

µF

25V

CLOAD

D3**

VOUT1

3.3V

VOUT2

5V OUT

3.3V OUT

CURRENT LIMIT: 3.3A

Ω

R10

10k

Z2*

0.1

µF

R11

10k

LONG

3.3V

GND

RESET

LONG

**D3 IS OPTIONAL AND HELPS DISCHARGE VOUT1 IF VOUT2 SHORTS

PCB EDGE

CONNECTOR

(MALE)

LONG

BACKPLANE

CONNECTOR

(FEMALE)

SHORT

Z1, Z2: 1SMA10A OR SMAJ10A

* OPTIONAL

Figure 20. Switching 5V and 3.3V

Figure 21. Placing Transient Protection Devices

Close to the LTC4211’s Input Rail

VCC

SENSE

LTC4211

COUT

VOUT

OUTPUT

GND

INPUT

GND

4211 F21

15k

GATE

GND

TIMER

CTIMER

RESET

Ω

VIN

RSENSE

0.007

Ω

Si4410DY

36k

RESET

Z1*

0.1

µF

Z1 = 1SMA10A OR SMAJ10A

* OPTIONAL

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