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LTC4211

4211fb

OPERATION

The maximum load current that trips the circuit breaker

is given in Equation 12.

ITRIP(MAX) =

VCB(MAX)

RSENSE(MIN)

60mV

RSENSE(MIN)

(12)

where

RSENSE(MIN) =RSENSE(NOM) 1–

RTOL

100

⎛

⎝

⎜

⎞

⎠

⎟

⎡

⎣

⎢

⎤

⎦

⎥

For example:

If a sense resistor with 7mΩ ±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 trip-

ping 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 con-

nected 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),andthemaxi-

mum 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 maximum

junction temperature rating of the MOSFET.

Power MOSFETs are classified into three categories:

standard MOSFETs (RDS(ON) specified at VGS = 10V)

logic-level MOSFETs (RDS(ON) specified at VGS = 5V), and

sub-logic-level MOSFETs (RDS(ON)specifiedatVGS=2.5V).

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 depend-

ing upon the manufacturer and the specific part number.

The LTC4211’s GATE overdrive as a function of VCC is

illustrated in the Typical Performance curves. Logic-level

and sub-logic-level MOSFETs are recommended for low

supply voltage applications and standard MOSFETs can

be used for applications 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 nega-

tive VGS voltage on the external MOSFET. Usually, the

selected external MOSFET should have a ± VGS(MAX) rat-

ing 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 LTC4211 if the GATE overdrive is clamped to a

lower voltage. The circuit in Figure 12 illustrates the use

of Zener diodes to clamp the LTC4211’s GATE overdrive

signal if lower voltage MOSFETs are used.

Figure 12. Optional Gate Clamp for Lower VGS(MAX) MOSFETs

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

4211 F12

RSENSE

GATE

D2*

D1*

200Ω

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

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