LTC4449 datasheet(24/38 Pages) LINER

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LTC3774

3774fc

For more information www.linear.com/LTC3774

APPLICATIONS INFORMATION

the case of tantalum, it is critical that the capacitors are

surge tested for use in switching power supplies. Several

excellent choices are the AVX TPS, AVX TPSV, the KEMET

T510 series of surface mount tantalums or the Panasonic

SP series of surface mount special polymer capacitors

availableincaseheightsrangingfrom2mmto4mm.Other

capacitor types include Sanyo POSCAP, Sanyo OS-CON,

Nichicon PL series and Sprague 595D series. Consult the

manufacturers for other specific recommendations.

Differential Amplifier

TheLTC3774hastrueremotevoltagesensecapability.The

sense connections should be returned from the load, back

to the differential amplifier’s inputs through a common,

tightly coupled pair of PC traces. The differential amplifier

rejects common mode signals capacitively or inductively

radiated into the feedback PC traces as well as ground

loop disturbances. The LTC3774 diffamp has high input

impedance on VOSNS+ pin. The output of the diffamp con-

nects to the inverting input of the error amplifier internally.

Setting Output Voltage

The LTC3774 output voltage is set by an external feed-

back resistive divider carefully placed across the output,

as shown in Figure 2. The regulated output voltage is

determined by:

VOUT = 0.6V • 1+

RD1

RD2

⎛

⎝

⎜

⎞

⎠

⎟

To improve the frequency response, a feedforward ca-

pacitor, CF1, may be used. Great care should be taken to

route the VOSNS+ line away from noise sources, such as

the inductor or the SW line.

To minimize the effect of the voltage drop caused by high

current flowing through board conductance; connect

VOSNS– and VOSNS+ sense lines close to the ground and

the load output respectively.

External Soft-Start and Tracking

The LTC3774 has the ability to either soft-start by itself

or track the output of another channel or external supply.

When the controller is configured to soft-start by itself, a

capacitor may be connected to its TK/SS pin or the internal

soft-start may be used. The controller is in the shutdown

state if its RUN pin voltage is below 1.22V and its TK/SS

pin is actively pulled to ground in this shutdown state. If

the RUN pin voltage is above 1.22V, the controller powers

up. A soft-start current of 1.25µA then starts to charge the

TK/SS soft-start capacitor. Note that soft-start or tracking

is achieved not by limiting the maximum output current of

the controller but by controlling the output ramp voltage

according to the ramp rate on the TK/SS pin. The soft-start

or tracking range is defined to be the voltage range from

0V to 0.6V on the TK/SS pin. The total soft-start time can

be calculated as:

tSOFTSTART = 0.6 •

CSS

1.25µA

Regardless of the mode selected by the MODE/PLLIN pin,

the controller always starts in discontinuous mode up to

TK/SS = 0.5V. Between TK/SS = 0.5V and 0.565V, it will

operate in forced continuous mode and revert to the

selected mode once TK/SS > 0.565V. The output ripple

is minimized during the 65mV forced continuous mode

window, ensuring a clean PGOOD signal. When the chan-

nel is configured to track another supply, the feedback

voltage of the other supply is duplicated by a resistor

divider and applied to the TK/SS pin. Therefore, the volt-

age ramp rate on this pin is determined by the ramp rate

of the other supply’s voltage. It is only possible to track

another supply that is slower than the internal soft-start

ramp. Note that the small soft-start capacitor charging

current is always flowing, producing a small offset error.

To minimize this error, select the tracking resistive divider

value to be small enough to make this error negligible.

In order to track down another channel or supply after

the soft-start phase expires, the LTC3774 is forced into

continuous mode of operation as soon as VOSNS+ is below

the power good lower threshold regardless of the setting

on the MODE/PLLIN pin. However, the LTC3774 should

always be set in forced continuous mode tracking down

when there is no load. After TK/SS drops below 0.1V, the

controller operates in discontinuous mode.

The LTC3774 allows the user to program how its output

ramps up and down by means of the TK/SS pin. Through

these pins, the output can be set up to either coincidentally

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