LTC1646IGN datasheet(13/20 Pages) LINER

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LTC1646

1646fa

APPLICATIO S I FOR ATIO

5VIN (Pin 12) and 5VSENSE (Pin 11) for the 5V supply. For

the 3.3V supply, a sense resistor is connected between

3VIN (Pin 8) and 3VSENSE (Pin 9). The current limit and the

current foldback current level are given by Equations 4

and 5:

LIMIT XVOUT

SENSE XVOUT

FOLDBACK XVOUT

SENSE XVOUT

()

(5)

where XVOUT = 5VOUT or 3VOUT.

As a design aid, the current limit and foldback level for

commonly used values for RSENSE is shown in Table 3.

Table 3. ILIMIT(XVOUT) and IFOLDBACK(XVOUT) vs RSENSE

RSENSE (Ω)ILIMIT(XVOUT)

IFOLDBACK(XVOUT)

0.005

11A

0.006

9.2A

3.3A

0.007

7.9A

2.9A

0.008

6.9A

2.5A

0.009

6.1A

2.2A

0.01

5.5A

where XVOUT = 3VOUT or 5VOUT.

5VIN –5VSENSE

100mV/DIV

GATE

10V/DIV

FAULT

5V/DIV

µs/DIV

1646 F06

(4)

Figure 6. Short-Circuit Fault on 5V

–

5VIN

RSENSE

5VSENSE

VCB

VCB(MAX) = 65mV

VCB(NOM) = 56mV

VCB(MIN) = 50mV

ILOAD(MAX)

LTC1646*

*ADDITIONAL DETAILS

OMITTED FOR CLARITY

1646 F07

–

Figure 7. Circuit Breaker Equivalent

Circuit for Calculating RSENSE

Calculating RSENSE

An equivalent circuit for one of the LTC1646’s circuit

breakers useful in calculating the value of the sense

resistor is shown in Figure 7. To determine the most

appropriate value for the sense resistor first requires the

maximum current required by the load under worst-case

conditions.

Two other parameters affect the value of the sense resis-

tor. First is the tolerance of the LTC1646’s circuit breaker

threshold. The LTC1646’s nominal circuit breaker

threshold is VCB(NOM) = 56mV; however, it exhibits a

–6mV/+9mV tolerance due to process variations. Second

is the tolerance (RTOL) in the sense resistor. Sense

resistors are available in RTOLs of

±1%, ±2% and ±5%

and exhibit temperature coefficients of resistance (TCRs)

between

±75ppm/°C and ±100ppm/°C. How the sense

resistor changes as a function of temperature depends on

the I2R power being dissipated by it.

The first step in calculating the value of RSENSE is based on

ITRIP(MAX) and the lower limit for the circuit breaker

threshold, VCB(MIN). The maximum value for RSENSE in this

case is expressed by Equation 6:

SENSE MAX

CB MIN

TRIP MAX

()

(6)

The second step is to determine the nominal value of the

sense resistor which is dependent on its tolerance

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