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LTC3780

3780f

The Standby Mode (STBYMD) Pin Function

The Standby mode (STBYMD) pin provides several choices

for start-up and standby operational modes. If the pin is

pulled to ground, the SS pin is internally pulled to ground,

preventing start-up and thereby providing a single control

pin for turning off the controller. If the pin is left open or

decoupled with a capacitor to ground, the SS pin is

internally provided with a starting current, permitting

external control for turning on the controller. If the pin is

connected to a voltage greater than 1.25V, the internal

regulator (INTVCC) will be on even when the controller is

shut down (RUN pin voltage < 1.5V). In this mode, the

onboard 6V linear regulator can provide power to keep-

alive functions such as a keyboard controller.

FCB Pin Regulates Secondary Winding in Buck Mode

In Buck mode, the FCB pin can be used to regulate a

secondary winding or as a logic level input. Continuous

operation is forced when the FCB pin drops below 0.8V.

During continuous mode, current flows continuously in

the transformer primary. The secondary winding(s) draw

current only when Switch B and Switch D are on in Buck

mode. When primary load currents are low and/or the

VIN/VOUT ratio is low, the Synchronous Switch B may not

be on for a sufficient amount of time to transfer power

from the output capacitor to the secondary load. Forced

continuous operation will support secondary windings if

there is sufficient synchronous switch duty factor. Thus,

the FCB input pin removes the requirement that power

must be drawn from the auxiliary windings. With the loop

in continuous mode, the auxiliary outputs may nominally

be loaded without regard to the primary output load.

The secondary output voltage VSEC is normally set as

shown in Figure 10 by turns ratio N of the transformer:

VSEC ≈ (N + 1) • VOUT

However, if the controller goes into Burst Mode operation

and halts switching due to a light primary load current,

then VSEC will drop. An external resistive divider from VSEC

to the FCB pin sets a minimum voltage VSEC(MIN):

SEC MIN

()

.•

≈

⎛

⎝

⎜

⎞

⎠

⎟

If the VSEC drops below this level, the FCB voltage forces

temporary continuous switching operation until VSEC is

again above its minimum.

In order to prevent erratic operation if no external connec-

tions are made to FCB pin, the FCB pin has a 0.18µA

internal current source pulling the pin high. Include this

current when choosing resistor values R5 and R6.

Fault Conditions: Current Limit and Current Foldback

The maximum inductor current is inherently limited in a

current mode controller by the maximum sense voltage. In

Boost mode, maximum sense voltage and the sense

resistance determines the maximum allowed inductor

peak current, which is:

L MAX BOOST

SENSE

) =

160

APPLICATIO S I FOR ATIO

TG2

BG2

SW1

TG1

COUT

BG1

RSENSE

3780 F10

SW2

LTC3780

FCB

SGND

VIN

VSEC

1:N

VOUT

•

Figure 10. Secondary Output Loop

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