LTC7130 datasheet(22/36 Pages) LINER

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LTC7130

7130fb

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APPLICATIONS INFORMATION

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

The LTC7130 has true remote voltage sense capability.

The sense connections should be returned from the load,

back to the differential amplifier’s inputs through a com-

mon, 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 LTC7130 diffamp has

80kΩ input impedance on DIFFP. It is designed to be con-

nected directly to the output. The output of the diffamp

connects to the VFB pin through a voltage divider, setting

the output voltage.

External Soft-Start and Tracking

The LTC7130 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 1.1V 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 volt-

age according to the ramp rate on the TK/SS pin. Current

foldback is disabled during this phase to ensure smooth

soft-start or tracking. 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 0.54V, it will

operate in forced continuous mode and revert to the

selected mode once TK/SS > 0.54V. The output ripple

is minimized during the 40mV 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 LTC7130 is forced into

continuous mode of operation as soon as VFB is below the

undervoltage threshold of 0.54V regardless of the setting

on the MODE/PLLIN pin. However, the LTC7130 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 LTC7130 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 coinciden-

tally or ratiometrically track another supply’s output, as

shown in Figure 10. In the following discussions, VOUT2

refers to the LTC7130’s output as a slave and VOUT1 refers

to another supply output as a master. To implement the

coincident tracking in Figure 10a, connect an additional

resistive divider to VOUT1 and connect its mid-point to the

TK/SS pin of the slave controller. The ratio of this divider

should be the same as that of the slave controller’s feed-

back divider shown in Figure 11a. In this tracking mode,

VOUT1 must be set higher than VOUT2. To implement the

ratiometric tracking in Figure 10b, the ratio of the VOUT2

divider should be exactly the same as the master control-

ler’s feedback divider shown in Figure 11b . By selecting

different resistors, the LTC7130 can achieve different

modes of tracking including the two in Figure 10.

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