LTC4218IGN datasheet(13/16 Pages) LINER

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LTC4218

4218fb

APPLICATIONS INFORMATION

In this same ﬁgure the OV threshold is lowered from

15.05V to 13.5V. Decreasing the OV threshold requires

adding a resistor between VDD and OV. This resistor can

be calculated as follows:

SHUNT

OLD

NEW

OV TH

OLD

2 =

()

•

–

– NNEW

()

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟

()

15 05

1 235

13 5 1 235

•.

.–.

15 05 13 5

1 736

.–.

()

⎛

⎝⎜

⎞

⎠⎟

Use the equation for RSHUNT1 for increasing the OV and

FB thresholds. Likewise, use the equation for RSHUNT2 for

decreasing the UV and FB thresholds.

Design Example

Consider the following design example (Figure 5): VIN =

12V, IMAX = 7.5A. IINRUSH = 1A, CL = 330μF, VUVON = 9.88V,

VOVOFF = 15.05V, VPWRGD = 10.5V. A current limit fault

triggers an automatic restart of the power up sequence.

The selection of the sense resistor, (RS), is set by the

overcurrent threshold of 15mV:

RS = 15mV/IMAX = 15mV/7.5A = 0.002Ω

The MOSFET should be sized to handle the power dissi-

pation during the inrush charging of the output capacitor

COUT. The method used to determine the power in Q1 is

the principal:

EC = Energy in CL = Energy in Q1

Thus:

EC = ½ CV2 = ½ (330μF)(12)2 = 0.048J

Calculate the time it takes to charge up COUT:

µF

CHARGUP

LIN

INRUSH

•

330

The inrush current is set to 1A using CGATE:

µF

µA

µF

GATE

GATE UP

INRUSH

≅

()

330

001

The average power dissipated in the MOSFET:

PDISS = EC/tCHARGUP = 0.048J/4ms = 12W

The SOA (safe operating area) curves of candidate MOSFETs

must be evaluated to ensure that the heat capacity of the

package can stand 12W for 4ms. The SOA curves of the

Vishay Siliconix Si7108DN provide 1.5A at 10V (15W) for

100ms, satisfying the requirement.

Next,thepowerdissipatedintheMOSFETduringovercurrent

must be limited. The active current limit uses a timer to

prevent excessive energy dissipation in the MOSFET. The

worst-case power occurs when the voltage versus current

proﬁle of the foldback current limit is at the maximum. This

occurs when the current is 6A and the voltage is one half

of 12V or (6V). See the Current Limit Sense Voltage vs

FB Voltage in the Typical Performance curves to view this

proﬁle. In order to survive 36W, the MOSFET SOA dictates

a maximum time at this power level. The Si7108DN allows

60W for 10ms or less. Therefore, it is acceptable to set the

current limit timeout using CT to be 1.2ms:

CT = 1.2ms/12[ms/μF] = 0.1μF

12V

0.1

μF

ADC

0.1

μF

20k

4218 F05

10k

330

μF

SENSE–

GATE

SENSE+

VDD

SOURCE

GND

IMON

LTC4218DHC-12

INTVCC

TIMER

FLT

Si7108DN

12V

VOUT

12V

RGATE

CGATE

0.01

μF

Figure 5. 6A, 12V Card Resident Application

Figure 4. Adjusting LTC4218-12 Thresholds

4218 F04

LTC4218-12

RSHUNT1

RSHUNT2

VDD

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