LTC4216IDE datasheet(18/24 Pages) LINER

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LTC4216

4216f

1. Maximum drain-to-source voltage, VDS(MAX)

The VDS(MAX) rating must exceed the maximum load sup-

ply voltage including spikes and ringing.

2. Gate-to-source voltage, VGS, overdrive

The absolute maximum rating for VGS is typically ±8V for

“logic level” and “sub-logic level” MOSFETs.

3. Drain-to-source resistance, RDS(ON)

The RDS(ON) should be low for low-voltage applications

to allow its drain-to-source voltage, VDS(ON), to be a very

small percentage of the supply voltage.

To begin a design, ﬁrst specify the maximum operating load

current and load capacitance. Calculate the RSENSE value

from Equation (12). The minimum trip current, ITRIP(MIN),

given by Equation (14) should be set to accommodate the

maximum operating load current.

During the start-up cycle, the LTC4216 may operate the

MOSFET in analog current limit, forcing ΔVACL(TH) between

32mV to 48mV across RSENSE.Theminimuminrushcurrent

given by Equation (16) is calculated using the minimum

ΔVACL(TH) and maximum RSENSE value.

INRUSH MIN

ACL TH MIN

SENSE MAX

()

)

()

∆

(16)

The maximum short-circuit current given by Equation (17)

is calculated using the maximum ΔVACL(TH) and minimum

RSENSE value.

SHORT CIRCUIT MAX

ACL TH MAX

SENSE MIN

−

∆

()

)

()

(17)

Select the FILTER capacitor, C3, based on the slowest

expected charging rate; otherwise, FILTER might time-out

before the load capacitor is fully charged. A value for C3

is calculated based on the maximum time it takes the load

capacitor, CLOAD, to charge to its maximum value of load

supply (VIN(MAX)). That time is given by:

CHARGE LOAD

LOAD

IN MAX

INRUSH MIN

()

•

(18)

Rearranging Equation (2) for the circuit breaker response

time, the FILTER capacitor, C3, is given by:

CHARGE LOAD

1 253

µµ

(–

)•

()

(19)

Returning to Equation (2), the circuit breaker response time

is calculated with a chosen C3 and used in conjunction

with VIN(MAX) and ISHORT-CIRCUIT(MAX) to check the SOA

curves of a prospective MOSFET.

As a numerical design example for the Typical Application,

consider VIN(MAX) = 1.8V + 5%, maximum operating load

current = 5A, CLOAD = 1000µF. Equation (12) gives RSENSE

= 4.3mΩ. Choose RSENSE = 4mΩ ± 1% tolerance. From

Equations (14) and (16), ITRIP(MIN) = 5.3A (> ILOAD(MAX)

= 5A) and IINRUSH(MIN) = 7.9A respectively. Equation (19)

gives C3 = 10nF. To account for errors in C3, FILTER current

(60µA) and FILTER threshold (1.253V), the calculated value

should be multiplied by 1.5, giving the nearest standard

value of C3 = 18nF.

If a short-circuit occurs, a current of up to ISHORT-

CIRCUIT(MAX) = 12.1A will ﬂow through the MOSFET for

400µs as dictated by C3 = 18nF in Equation (2). The

MOSFET must be selected based on this criterion and

checked against the SOA curve.

VCC Supply RC Network

The LTC4216 has two separate pins, VCC and SENSEP, for

supply input and sensing:

1. VCC pin for powering the internal circuitry.

2. SENSEP pin, together with the SENSEN pin, for sens-

ing the current ﬂowing from the load supply through the

external sense resistor and N-channel MOSFET to the

output load.

In most hot swap devices, VCC and SENSEP are one

common pin, providing the device’s supply and external

MOSFET’s current sensing. However, supply dips due to

output-shorts can potentially trigger the device into an

undervoltage lockout condition, causing the device to

disable and its internal latches to reset.

As bypass capacitors are not allowed on the powered

supply side of the external MOSFET switch residing on

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