LTC4213IDDB datasheet(14/20 Pages) LINER

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LTC4213

4213f

Operating temperature of 0° to 70°C.

RDSON @ 25°C = 100%

RDSON @ 0°C = 90%

RDSON @ 70°C = 120%

MOSFET resistance variation:

RDSON(NOM) = 15m • 0.82 = 12.3mΩ

RDSON(MAX) = 15m • 1.333 • 0.93 • 1.2 = 15m • 1.488

= 22.3mΩ

RDSON(MIN) = 15m • 0.667 • 0.80 • 0.90 = 15m • 0.480

= 7.2mΩ

VCB variation:

NOM VCB = 25mV = 100%

MIN VCB = 22.5mV = 90%

MAX VCB = 27.5mV = 110%

The current limits are:

ILIMIT(NOM) = 25mV/12.3mΩ = 2.03A

ILIMIT(MIN) = 22.5mV/22.3mΩ = 1.01A

ILIMIT(MAX) = 27.5mV/7.2mΩ = 3.82A

For proper operation, the minimum current limit must

exceed the circuit maximum operating load current with

margin. So this system is suitable for operating load

current up to 1A. From this calculation, we can start with

the general rule for MOSFET RDSON by assuming maxi-

mum operating load current is roughly half of the

ILIMIT(NOM). Equation 7 shows the rule of thumb.

OPMAX

CB NOM

DSON NOM

()

•

()

Note that the RDSON(NOM) is at the LTC4213 nominal

operating ∆VGSMAX rather than at typical vendor spec.

Table 1 gives the nominal operating ∆VGSMAX at the

various operating VCC. From this table users can refer to

the MOSFET’s data sheet to obtain the RDSON(NOM) value.

Table 1. Nominal Operating ∆VGSMAX for Typical Bias

Supply Voltage

VCC (V)

∆VGSMAX (V)

2.3

4.3

2.5

5.0

2.7

5.6

3.0

6.5

3.3

7.0

5.0

7.0

6.0

7.0

Load Supply Power-Up after Circuit Breaker Armed

Figure 4 shows a normal power-up sequence for the

circuit in Figure 1 where the VIN load supply power-up after

circuit breaker is armed. VCC is first powered up by an

auxiliary bias supply. VCC rises above 2.07V at time

point 1. VON exceeds 0.8V at time point 2. After a 60µs

debounce delay, the GATE pin starts ramping up at time

point 3. The external MOSFET starts conducting at time

point 4. At time point 5, VGATE exceed ∆VGSARM and the

circuit breaker is armed. After 50µs (tREADY delay), READY

pulls high by an external resistor at time point 6. READY

signals the VIN load supply module to start its ramp. The

load supply begins soft-start ramp at time point 7. The load

supply ramp rate must be slow to prevent circuit breaker

tripping as in equation (8).

∆

OPMAX

LOAD

−

()

Where IOPMAX is the maximum operating current defined

by equation 7.

For illustration, VCB = 25mV and RDSON = 3.5mΩ at the

nominal operating ∆VGSMAX. The maximum operating

current is 3.5A (refer to equation 7). Assuming the load

can draw a current of 2A at power-up, there is a margin of

1.5A available for CLOAD of 100µF and VIN ramp rate should

be <15V/ms. At time point 8, the current through the

MOSFET reduces after CLOAD is fully charged.

APPLICATIO S I FOR ATIO

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