LTC4449 datasheet(20/38 Pages) LINER

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LTC3774

3774fc

For more information www.linear.com/LTC3774

APPLICATIONS INFORMATION

Pre-Biased Output Start-Up

There may be situations that require the power supply to

start up with a pre-bias on the output capacitors. In this

case, it is desirable to start up without discharging that

output pre-bias. The LTC3774 can safely power up into a

pre-biased output without discharging it.

The LTC3774 accomplishes this by disabling both the top

and bottom MOSFETs until the TK/SS pin voltage and the

internal soft-start voltage are above the VOSNS+ pin volt-

age. When VOSNS+ is higher than TK/SS or the internal

soft-start voltage, the error amp output is railed low. The

control loop would like to turn the bottom MOSFET on,

which would discharge the output. Disabling both top and

bottom MOSFETs prevents the pre-biased output voltage

from being discharged. When TK/SS and the internal

soft-start both cross 500mV or VOSNS+, whichever is

lower, both top and bottom MOSFETs are enabled. If the

pre-bias is higher than the OV threshold, the bottom gate

is turned on immediately to pull the output back into the

regulation window.

Overcurrent Fault Recovery

When the output of the power supply is loaded beyond its

preset current limit, the regulated output voltage will col-

lapse depending on the load. The output may be shorted

to ground through a very low impedance path or it may

be a resistive short, in which case the output will collapse

partially, until the load current equals the preset current

limit. The controller will continue to source current into

the short. The amount of current sourced depends on

the ILIM pin setting and the VOSNS+ voltage as shown in

the Current Foldback graph in the Typical Performance

Characteristics section.

Upon removal of the short, the output soft starts using

the internal soft-start, thus reducing output overshoot. In

the absence of this feature, the output capacitors would

have been charged at current limit, and in applications

with minimal output capacitance this may have resulted

in output overshoot. Current limit foldback is not disabled

during an overcurrent recovery. The load must step below

the folded back current limit threshold in order to restart

from a hard short.

Phase Shedding/n+1 Redundancy (HIZB Pin)

Unlike the RUN pins, the HIZB pins cause the PWM to enter

its high impedance state while not pulling down on ITH or

TK/SS. This allows two possibilities: First, one can shed a

phasebasedonloadrequirementsviatheHIZBpin.Thisim-

proves low current efficiency in a single output multiphase

casebyreducingswitchinglosses.Second,forapplications

that require n+1 redundancy, it is now easy to disconnect

a channel with damaged MOSFETs or drivers. When com-

bined with a Hot Swap™ controller, such as the LTC4226,

the HIZB pin could be connected to the gate of the Hot

Swap switch. When a damaged MOSFET triggers the Hot

Swap controller, it also disables the corresponding chan-

nel’s power stage, disconnecting it. Since ITH and TK/SS

are unaffected, it does not affect the rest of the system.

ThepropagationdelayfromHIZBfallingtohighimpedance

on PWM is <200ns.

Inductor Value Calculation

Given the desired input and output voltages, the inductor

value and operating frequency, fOSC, directly determine

the inductor’s peak-to-peak ripple current:

IRIPPLE =

VOUT

VIN

VIN – VOUT

fOSC •L

⎛

⎝

⎜

⎞

⎠

⎟

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors, and output voltage

ripple. Thus, highest efficiency operation is obtained at

low frequency with a small ripple current. Achieving this,

however, requires a large inductor.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). Note that the largest ripple

current occurs at the highest input voltage. To guarantee

that ripple current does not exceed a specified maximum,

the inductor should be chosen according to:

L ≥

VIN – VOUT

fOSC •IRIPPLE

•

VOUT

VIN

Inductor Core Selection

Once the inductance value is determined, the type of in-

ductor must be selected. Core loss is independent of core

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