LTC4213 datasheet(14/22 Pages) AD

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LTC4213

Rev. A

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Operating temperature of 0° to 70°C.

RDS(ON) at 25°C = 100%

RDS(ON) at 0°C = 90%

RDS(ON) at 70°C = 120%

MOSFET resistance variation:

RDS(ON)(NOM) = 15m • 0.82 = 12.3mΩ

RDS(ON)(MAX) = 15m • 1.333 • 0.93 • 1.2 = 15m • 1.488

= 22.3mΩ

RDS(ON)(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 cur-

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

general rule for MOSFET RDS(ON) by assuming maximum

operating load current is roughly half of the ILIMIT(NOM).

Equation 7 shows the rule of thumb.

IOPMAX =

VCB(NOM)

2 •RDS(ON)(NOM)

(7)

Note that the RDS(ON)(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 RDS(ON)(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.

ΔVIN

Δt

IOPMAX −ILOAD

CLOAD

(8)

Where IOPMAX is the maximum operating current defined

by Equation 7.

For illustration, VCB = 25mV and RDS(ON) = 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.

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