LTC3894 datasheet(13/32 Pages) LINER

13 / 32 page

LTC3894

Rev 0

For more information www.analog.com

The LTC3894 is a current mode, constant frequency

nonsynchronous step-down DC/DC controller with a

P-channel power MOSFET acting as the main switch

and a Schottky power diode acting as the commutat-

ing (catch) diode. The input range extends from 4.5V to

150V. The output range can be programmed from 0.8V

to 60V. The LTC3894 can transition from regulation to

100% duty cycle when the input voltage drops below the

programmed output voltage. Additionally, the LTC3894

offers Burst Mode operation with a very low quiescent

current, delivering outstanding efficiency in light load

operation not typically found in a controller. The LTC3894

is a low pin-count, robust and easy to use solution in

applications which require high efficiency and operate

with widely varying high voltage inputs.

OPERATION

APPLICATIONS INFORMATION

10% or more below the 0.8V internal voltage reference.

The PGOOD pin is also pulled low during an overtempera-

ture, RUN pin shutdown, VIN overvoltage or VIN under-

voltage lockout event. When the VFB pin voltage is less

than 0.88V (0.8V + 10%) and PGUV is above 0.72V (0.8V

– 10%), the internal N-channel MOSFET is turned off and

the PGOOD pin is allowed to be pulled up by an external

resistor to VOUT or another source no greater than 60V.

The PGOOD open-drain output has a 100μs delay before

it can transition states.

When the VFB voltage is higher than 0.88V (0.8V + 10%)

nominal, this is considered an overvoltage condition and

the external P-MOSFET is immediately turned off and

remains turned off until VFB falls below 0.88V with built-

in hysteresis of 20mV.

Current Limit Foldback

In the event of an output short-circuit or overcurrent con-

dition that causes the output voltage to fall to less than

72% of its nominal regulated level and the PGUV pin volt-

age is less than 0.72V, current limit foldback is activated,

progressively lowering the peak current limit in propor-

tion to the drop of VOUT until reaching a minimum cur-

rent limit of about 36% of full current limit. Current limit

foldback reduces the power dissipation in the Schottky

diode and assures robust operation during a continu-

ous short-circuit fault. Current limit foldback is disabled

during soft-start (as long as the VFB voltage is keeping

up with the TRACK/SS voltage). Note that the LTC3894

continuously monitors the inductor current and prevents

current runaway under all conditions.

LTC3894 has an internal overtemperature protection cir-

cuit that shuts off the controller and the external P-channel

MOSFET when internal die temperature exceeds 180°C.

The circuit also discharges the TRACK/SS pin to GND to

ensure a smooth restart.

Input Supply Overvoltage Lockout (OVLO Pin)

The LTC3894 implements a protection feature that inhibits

switching when the input voltage rises above a program-

mable operating range. By using a resistor divider from

the input supply to ground, the OVLO pin serves as a

precise input supply voltage monitor. Switching is dis-

abled when the OVLO pin rises above 0.8V, which can be

configured to limit switching to a specific range of input

supply voltage. An input supply overvoltage event triggers

a TRACK/SS reset, which results in a graceful recovery

from an input supply transient.

The typical application on the front page is a basic

LTC3894 application circuit where the inductor current

is sensed using a low value sense resistor, RSENSE, placed

between the power inductor and VOUT. Once the required

output voltage and operating frequency have been deter-

mined, external component selection is driven by load

requirements, and begins with the selection of inductor

and RSENSE. Next, the power MOSFET and catch diode are

selected. Finally, input and output capacitors are selected.

Output Voltage Programming

The output voltage is programmed by connecting a feed-

back resistor divider from the output to the VFB pin as

shown in Figure 1. The output voltage in steady state

LTC3894 Datasheet(PDF) 13 Page - Linear Technology

LTC3894 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL