LTC4373CMS8 datasheet(11/20 Pages) AD

11 / 20 page

LTC4372/LTC4373

Rev. 0

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APPLICATIONS INFORMATION

As the load current increases, GATE is driven higher and

higher until a point is reached where ∆VGATE reaches

the maximum overdrive that the internal charge pump is

capable of (∆VGATE(H)) but ∆VSD is still above 30mV. In

this situation, the internal charge pump will periodically

turn on to recharge GATE as needed to keep ∆VGATE

between ∆VGATE(H) and ∆VGATE(H) – 0.7V. ∆VSD is then

equal to RDS(ON) • ILOAD. There is now insignificant

ripple on OUT as the 0.7Vpk-pk ripple on ∆VGATE has

little effect on the MOSFET RON.

Achieving Low Average IQ

To lower average IQ in diode control mode when GATE

is high, the LTC4372/LTC4373 operate by turning on

the charge pump periodically. When in charge pump

sleep mode, the IQ is 3.5μA. Once the charge pump is

turned on to deliver a current pulse to GATE, IQ goes up

to 300μA. The average IQ will depend on how often the

charge pump is turned on and this is affected by GATE

leakage, GATE capacitance, OUT bypass capacitance and

ILOAD. To achieve the lowest possible average IQ, mini-

mize GATE leakage and ensure that GATE has a moderate

capacitance (>1nF). If the CGS of the MOSFET does not

already exceed this, add a 1nF capacitor between GATE

and SOURCE. CLOAD may be placed nearer to the load but

an OUT bypass capacitance of at least 10μF low ESR and

ESL electrolytic or ceramic is required close to the drain

pin of MOSFET M1 (see Figure 6a). Average IQ for Diode

Control mode can be estimated by Equation 1.

AVERAGEIQ = 3.5+

IGATE(LEAKAGE)

IGATE(UP)

• 300µA

(1)

The Typical Performance Characteristics section shows

relationship of IQ with IGATE(LEAKAGE) and ILOAD.

MOSFET Selection

The LTC4372/LTC4373 drive N-channel MOSFETs to

conduct the load current. The important character-

istics of the MOSFET are the gate threshold voltage

VGS(TH), the maximum drain-source voltage BVDSS and

on-resistance RDS(ON).

Gate drive is compatible with 4.5V logic-level MOSFETs

over the entire operating range of 2.5V to 80V. In applica-

tions with supply voltages above 5V, standard 10V thresh-

old MOSFETs may be used. An internal clamp limits the

gate drive to 16V maximum between GATE and SOURCE.

The maximum allowable drain-source voltage, BVDSS,

must be higher than the power supply voltage. If the input

is grounded, the full supply voltage will appear across the

MOSFET. If a reverse battery is possible and the output

is held up by a charged capacitor, battery or power sup-

ply, then the sum of the input and output voltages will

appear across the MOSFET and BVDSS must be higher

than VOUT+|VIN|.

The MOSFET’s on-resistance, RDS(ON), directly affects

the forward voltage drop and power dissipation during a

heavy load. Desired forward voltage drop (VFWD) should

be less than that of a diode for reduced power dissipa-

tion; 50mV is a good starting point. Since the LTC4372/

LTC4373 drop at least 30mV across the MOSFET, a very

low RDS(ON) may be wasted. Choose a MOSFET using

Equation 2.

RDS(ON) <

VFWD

ILOAD

(2)

The resulting power dissipation is shown in Equation 3.

Pd = ILOAD2 • RDS(ON)

(3)

Input Short-Circuit Faults

Input short-circuits that cause reverse current to flow can

occur in many ways. Some examples include PCB traces

getting accidentally shorted or bypass capacitors in the

upstream power supply failing shorted. The LTC4372/

LTC4373 utilize the external MOSFETs to add rugged input

short-circuit protection without utilizing large TVS clamps

or capacitors.

Figure 6a models a low impedance input short with a

switch. When the short-circuit switch closes, reverse cur-

rent builds up in LIN, LOUT and M1 in the direction shown.

The LTC4372/LTC4373 detect the reverse current quickly

and activate the internal 130mA GATE to SOURCE pull-

down current to turn M1 off. The reverse current build up

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