LTC4212CMS datasheet(21/24 Pages) LINER

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LTC4212

4212f

OPERATIO

For example:

If a sense resistor with 7m

Ω ±5% RTOL is used for current

limiting, the nominal trip current ITRIP(NOM) = 7.1A. From

Equations 11 and 12, ITRIP(MIN) = 5.4A and ITRIP(MAX) =

9.02A respectively.

For proper operation and to avoid the circuit breaker

tripping unnecessarily, the minimum trip current

(ITRIP(MIN)) must exceed the circuit’s maximum operating

load current. For reliability purposes, the operation at the

maximum trip current (ITRIP(MAX)) must be evaluated

carefully. If necessary, two resistors with the same RTOL

can be connected in parallel to yield an RSENSE(NOM) value

that fits the circuit requirements.

Power MOSFET Selection Criteria

To start the power MOSFET selection process, choose the

maximum drain-to-source voltage, VDS(MAX), and the

maximum drain current, ID(MAX) of the MOSFET. The

VDS(MAX) rating must exceed the maximum input supply

voltage (including surges, spikes, ringing, etc.) and the

ID(MAX) rating must exceed the maximum short-circuit

current in the system during a fault condition. In addition,

consider three other key parameters: 1) the required gate-

source (VGS) voltage drive, 2) the voltage drop across the

drain-to-source on resistance, RDS(ON) and 3) the maxi-

mum junction temperature rating of the MOSFET.

Power MOSFETs are classified into two categories: stan-

dard MOSFETs (RDS(ON) specified at VGS = 10V) and

logic-level MOSFETs (RDS(ON) specified at VGS = 5V). The

absolute maximum rating for VGS is typically ±20V for

standard MOSFETs. However, the VGS maximum rating

for logic-level MOSFETs ranges from

±8V to ±20V de-

pending upon the manufacturer and the specific part

number. The LTC4212’s GATE overdrive as a function of

VCC is illustrated in the Typical Performance curves. Logic-

level MOSFETs are recommended for low supply voltage

applications and standard MOSFETs can be used for appli-

cations where supply voltage is greater than 4.75V.

Note that in some applications, the gate of the external

MOSFET can discharge faster than the output voltage

when the circuit breaker is tripped. This causes a negative

VGS voltage on the external MOSFET. Usually, the selected

external MOSFET should have a

±VGS(MAX) rating that is

higher than the operating input supply voltage to ensure

that the external MOSFET is not destroyed by a negative

VGS voltage. In addition, the ±VGS(MAX) rating of the

MOSFET must be higher than the gate overdrive voltage.

Lower

±VGS(MAX) rating MOSFETs can be used with the

LTC4212 if the GATE overdrive is clamped to a lower

voltage. The circuit in Figure 8 illustrates the use of zener

diodes to clamp the LTC4212’s GATE overdrive signal if

lower voltage MOSFETs are used.

VCC

VOUT

*USER SELECTED VOLTAGE CLAMP

(A LOW BIAS CURRENT ZENER DIODE IS RECOMMENDED)

1N4688 (5V)

1N4692 (7V): LOGIC-LEVEL MOSFET

1N4695 (9V)

1N4702 (15V): STANDARD-LEVEL MOSFET

4212 F08

RSENSE

GATE

D2*

D1*

200

Ω

Figure 8. Optional Gate Clamp for Lower VGS(MAX) MOSFETs

The RDS(ON) of the external pass transistor should be low

to make its drain-source voltage (VDS) a small percentage

of VCC. At a VCC = 2.5V, VDS + VRSENSE = 0.1V yields 4%

error at the output voltage. This restricts the choice of

MOSFETs to very low RDS(ON). At higher VCC voltages, the

VDS requirement can be relaxed in which case MOSFET

package dissipation (PD and TJ) may limit the value of

RDS(ON). Table 6 lists some power MOSFETs that can be

used with the LTC4212.

For reliable circuit operation, the maximum junction tem-

perature (TJ(MAX)) for a power MOSFET should not exceed

the manufacturer’s recommended value. This includes

normal mode operation, start-up, current-limit and

autoretry mode in a fault condition. Under normal condi-

tions the junction temperature of a power MOSFET is given

by Equation 13:

MOSFET Junction Temperature,

TJ(MAX) ≤ TA(MAX) + θJA • PD

(13)

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